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learning c,c+ language
| Instructions for use |
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To whom is this tutorial directed?
This tutorial is for those people who want to learn programming in C++ and do not necessarily have any previous knowledge of other programming languages. Of course any knowledge of other programming languages or any general computer skill can be useful to better understand this tutorial, although it is not essential.It is also suitable for those who need a little update on the new features the language has acquired from the latests standards.
If you are familiar with the C language, you can take the first 3 parts of this tutorial as a review of concepts, since they mainly explain the C part of C++. There are slight differences in the C++ syntax for some C features, so I recommend you its reading anyway.
The 4th part describes object-oriented programming.
The 5th part mostly describes the new features introduced by ANSI-C++ standard.
Structure of this tutorial
The tutorial is divided in 6 parts and each part is divided on its turn into different sections covering a topic each one. You can access any section directly from the section index available on the left side bar, or begin the tutorial from any point and follow the links at the bottom of each section.Many sections include examples that describe the use of the newly acquired knowledge in the chapter. It is recommended to read these examples and to be able to understand each of the code lines that constitute it before passing to the next chapter.
A good way to gain experience with a programming language is by modifying and adding new functionalities on your own to the example programs that you fully understand. Don't be scared to modify the examples provided with this tutorial, that's the way to learn!
Compatibility Notes
The ANSI-C++ standard acceptation as an international standard is relatively recent. It was first published in November 1997, and revised in 2003. Nevertheless, the C++ language exists from a long time before (1980s). Therefore there are many compilers which do not support all the new capabilities included in ANSI-C++, specially those released prior to the publication of the standard.This tutorial is thought to be followed with modern compilers that suport -at least on some degree- ANSI-C++ specifications. I encourage you to get one if yours is not adapted. There are many options, both commercial and free.
Compilers
The examples included in this tutorial are all console programs. That means they use text to communicate with the user and to show their results.All C++ compilers support the compilation of console programs. Check the user's manual of your compiler for more info on how to compile them.
Probably the best way to start learning a programming language is by writing a program. Therefore, here is our first program:
| // my first program in C++#include <iostream>using namespace std;int main (){ cout << "Hello World!"; return 0;} | Hello World! |
The previous program is the typical program that programmer apprentices write for the first time, and its result is the printing on screen of the "Hello World!" sentence. It is one of the simplest programs that can be written in C++, but it already contains the fundamental components that every C++ program has. We are going to look line by line at the code we have just written:
// my first program in C++
This is a comment line. All lines beginning with two slash signs (//) are considered comments and do not have any effect on the behavior of the program. The programmer can use them to include short explanations or observations within the source code itself. In this case, the line is a brief description of what our program is.
#include <iostream>
Lines beginning with a pound sign (#) are directives for the preprocessor. They are not regular code lines with expressions but indications for the compiler's preprocessor. In this case the directive #include <iostream> tells the preprocessor to include the iostream standard file. This specific file (iostream) includes the declarations of the basic standard input-output library in C++, and it is included because its functionality is going to be used later in the program.
using namespace std;
All the elements of the standard C++ library are declared within what is called a namespace, the namespace with the name std. So in order to access its functionality we declare with this expression that we will be using these entities. This line is very frequent in C++ programs that use the standard library, and in fact it will be included in most of the source codes included in these tutorials.
int main ()
This line corresponds to the beginning of the definition of the main function. The main function is the point by where all C++ programs start their execution, independently of its location within the source code. It does not matter whether there are other functions with other names defined before or after it - the instructions contained within this function's definition will always be the first ones to be executed in any C++ program. For that same reason, it is essential that all C++ programs have a main function.
The word main is followed in the code by a pair of parentheses (()). That is because it is a function declaration: In C++, what differentiates a function declaration from other types of expressions are these parentheses that follow its name. Optionally, these parentheses may enclose a list of parameters within them.
Right after these parentheses we can find the body of the main function enclosed in braces ({}). What is contained within these braces is what the function does when it is executed.
cout << "Hello World";
This line is a C++ statement. A statement is a simple or compound expression that can actually produce some effect. In fact, this statement performs the only action that generates a visible effect in our first program.
cout represents the standard output stream in C++, and the meaning of the entire statement is to insert a sequence of characters (in this case the Hello World sequence of characters) into the standard output stream (which usually is the screen).
cout is declared in the iostream standard file within the std namespace, so that's why we needed to include that specific file and to declare that we were going to use this specific namespace earlier in our code.
Notice that the statement ends with a semicolon character (;). This character is used to mark the end of the statement and in fact it must be included at the end of all expression statements in all C++ programs (one of the most common syntax errors is indeed to forget to include some semicolon after a statement).
return 0;
The return statement causes the main function to finish. return may be followed by a return code (in our example is followed by the return code 0). A return code of 0 for the main function is generally interpreted as the program worked as expected without any errors during its execution. This is the most usual way to end a C++ program.
You may have noticed that not all the lines of this program perform actions when the code is executed. There were lines containing only comments (those beginning by //). There were lines with directives for the compiler's preprocessor (those beginning by #). Then there were lines that began the declaration of a function (in this case, the main function) and, finally lines with statements (like the insertion into cout), which were all included within the block delimited by the braces ({}) of the main function.
The program has been structured in different lines in order to be more readable, but in C++, we do not have strict rules on how to separate instructions in different lines. For example, instead of
| int main (){ cout << " Hello World "; return 0;} |
| int main () { cout << "Hello World"; return 0; } |
In C++, the separation between statements is specified with an ending semicolon (;) at the end of each one, so the separation in different code lines does not matter at all for this purpose. We can write many statements per line or write a single statement that takes many code lines. The division of code in different lines serves only to make it more legible and schematic for the humans that may read it.
Let us add an additional instruction to our first program:
| // my second program in C++#include <iostream>using namespace std;int main (){ cout << "Hello World! "; cout << "I'm a C++ program"; return 0;} | Hello World! I'm a C++ program |
| int main () { cout << " Hello World! "; cout << " I'm a C++ program "; return 0; } |
| int main (){ cout << "Hello World!"; cout << "I'm a C++ program"; return 0;} |
Preprocessor directives (those that begin by #) are out of this general rule since they are not statements. They are lines read and discarded by the preprocessor and do not produce any code by themselves. Preprocessor directives must be specified in their own line and do not have to end with a semicolon (;).
Comments
Comments are parts of the source code disregarded by the compiler. They simply do nothing. Their purpose is only to allow the programmer to insert notes or descriptions embedded within the source code.C++ supports two ways to insert comments:
| // line comment/* block comment */ |
We are going to add comments to our second program:
| /* my second program in C++ with more comments */#include <iostream>using namespace std;int main (){ cout << "Hello World! "; // prints Hello World! cout << "I'm a C++ program"; // prints I'm a C++ program return 0;} | Hello World! I'm a C++ program |
| | |||
| Variables. Data Types. |
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Let us think that I ask you to retain the number 5 in your mental memory, and then I ask you to memorize also the number 2 at the same time. You have just stored two different values in your memory. Now, if I ask you to add 1 to the first number I said, you should be retaining the numbers 6 (that is 5+1) and 2 in your memory. Values that we could now for example subtract and obtain 4 as result.
The whole process that you have just done with your mental memory is a simile of what a computer can do with two variables. The same process can be expressed in C++ with the following instruction set:
| a = 5;b = 2;a = a + 1;result = a - b; |
Therefore, we can define a variable as a portion of memory to store a determined value.
Each variable needs an identifier that distinguishes it from the others, for example, in the previous code the variable identifiers were a, b and result, but we could have called the variables any names we wanted to invent, as long as they were valid identifiers.
Identifiers
A valid identifier is a sequence of one or more letters, digits or underline characters (_). Neither spaces nor punctuation marks or symbols can be part of an identifier. Only letters, digits and underline characters are valid. In addition, variable identifiers always have to begin with a letter. They can also begin with an underline character (_ ), but this is usually reserved for compiler specific keywords or external identifiers. In no case they can begin with a digit.Another rule that you have to consider when inventing your own identifiers is that they cannot match any keyword of the C++ language or your compiler's specific ones since they could be confused with these. The standard reserved keywords are:
asm, auto, bool, break, case, catch, char, class, const, const_cast, continue, default, delete, do, double, dynamic_cast, else, enum, explicit, export, extern, false, float, for, friend, goto, if, inline, int, long, mutable, namespace, new, operator, private, protected, public, register, reinterpret_cast, return, short, signed, sizeof, static, static_cast, struct, switch, template, this, throw, true, try, typedef, typeid, typename, union, unsigned, using, virtual, void, volatile, wchar_t, while
Additionally, alternative representations for some operators cannot be used as identifiers since they are reserved words under some circumstances:
and, and_eq, bitand, bitor, compl, not, not_eq, or, or_eq, xor, xor_eq
Your compiler may also include some additional specific reserved keywords.
Very important: The C++ language is a "case sensitive" language. That means that an identifier written in capital letters is not equivalent to another one with the same name but written in small letters. Thus, for example, the RESULT variable is not the same as the result variable or the Result variable. These are three different variable identifiers.
Fundamental data types
When programming, we store the variables in our computer's memory, but the computer has to know what we want to store in them, since it is not going to occupy the same amount of memory to store a simple number than to store a single letter or a large number, and they are not going to be interpreted the same way.The memory in our computers is organized in bytes. A byte is the minimum amount of memory that we can manage in C++. A byte can store a relatively small amount of data: one single character or a small integer (generally an integer between 0 and 255). In addition, the computer can manipulate more complex data types that come from grouping several bytes, such as long numbers or non-integer numbers.
Next you have a summary of the basic fundamental data types in C++, as well as the range of values that can be represented with each one:
| Name | Description | Size* | Range* |
| char | Character or small integer. | 1byte | signed: -128 to 127 unsigned: 0 to 255 |
| short int (short) | Short Integer. | 2bytes | signed: -32768 to 32767 unsigned: 0 to 65535 |
| int | Integer. | 4bytes | signed: -2147483648 to 2147483647 unsigned: 0 to 4294967295 |
| long int (long) | Long integer. | 4bytes | signed: -2147483648 to 2147483647 unsigned: 0 to 4294967295 |
| bool | Boolean value. It can take one of two values: true or false. | 1byte | true or false |
| float | Floating point number. | 4bytes | 3.4e +/- 38 (7 digits) |
| double | Double precision floating point number. | 8bytes | 1.7e +/- 308 (15 digits) |
| long double | Long double precision floating point number. | 8bytes | 1.7e +/- 308 (15 digits) |
| wchar_t | Wide character. | 2bytes | 1 wide character |
Declaration of variables
In order to use a variable in C++, we must first declare it specifying which data type we want it to be. The syntax to declare a new variable is to write the specifier of the desired data type (like int, bool, float...) followed by a valid variable identifier. For example:| int a;float mynumber; |
If you are going to declare more than one variable of the same type, you can declare all of them in a single statement by separating their identifiers with commas. For example:
| int a, b, c; |
| int a;int b;int c; |
| unsigned short int NumberOfSisters;signed int MyAccountBalance; |
| int MyAccountBalance; |
An exception to this general rule is the char type, which exists by itself and is considered a different fundamental data type from signed char and unsigned char, thought to store characters. You should use either signed or unsigned if you intend to store numerical values in a char-sized variable.
short and long can be used alone as type specifiers. In this case, they refer to their respective integer fundamental types: short is equivalent to short int and long is equivalent to long int. The following two variable declarations are equivalent:
| short Year;short int Year; |
| unsigned NextYear;unsigned int NextYear; |
| // operating with variables#include <iostream>using namespace std;int main (){ // declaring variables: int a, b; int result; // process: a = 5; b = 2; a = a + 1; result = a - b; // print out the result: cout << result; // terminate the program: return 0;} | 4 |
Scope of variables
All the variables that we intend to use in a program must have been declared with its type specifier in an earlier point in the code, like we did in the previous code at the beginning of the body of the function main when we declared that a, b, and result were of type int.A variable can be either of global or local scope. A global variable is a variable declared in the main body of the source code, outside all functions, while a local variable is one declared within the body of a function or a block.
Global variables can be referred from anywhere in the code, even inside functions, whenever it is after its declaration.
The scope of local variables is limited to the block enclosed in braces ({}) where they are declared. For example, if they are declared at the beginning of the body of a function (like in function main) their scope is between its declaration point and the end of that function. In the example above, this means that if another function existed in addition to main, the local variables declared in main could not be accessed from the other function and vice versa.
Initialization of variables
When declaring a regular local variable, its value is by default undetermined. But you may want a variable to store a concrete value at the same moment that it is declared. In order to do that, you can initialize the variable. There are two ways to do this in C++:The first one, known as c-like, is done by appending an equal sign followed by the value to which the variable will be initialized:
type identifier = initial_value ;
For example, if we want to declare an int variable called a initialized with a value of 0 at the moment in which it is declared, we could write:
| int a = 0; |
type identifier (initial_value) ;
For example:
| int a (0); |
| // initialization of variables#include <iostream>using namespace std;int main (){ int a=5; // initial value = 5 int b(2); // initial value = 2 int result; // initial value undetermined a = a + 3; result = a - b; cout << result; return 0;} | 6 |
Introduction to strings
Variables that can store non-numerical values that are longer than one single character are known as strings.The C++ language library provides support for strings through the standard string class. This is not a fundamental type, but it behaves in a similar way as fundamental types do in its most basic usage.
A first difference with fundamental data types is that in order to declare and use objects (variables) of this type we need to include an additional header file in our source code: <string> and have access to the std namespace (which we already had in all our previous programs thanks to the using namespace statement).
| // my first string#include <iostream>#include <string>using namespace std;int main (){ string mystring = "This is a string"; cout << mystring; return 0;} | This is a string |
| string mystring = "This is a string";string mystring ("This is a string"); |
| // my first string#include <iostream>#include <string>using namespace std;int main (){ string mystring; mystring = "This is the initial string content"; cout << mystring << endl; mystring = "This is a different string content"; cout << mystring << endl; return 0;} | This is the initial string contentThis is a different string content |
| Constants |
|
Literals
Literals are used to express particular values within the source code of a program. We have already used these previously to give concrete values to variables or to express messages we wanted our programs to print out, for example, when we wrote:| a = 5; |
Literal constants can be divided in Integer Numerals, Floating-Point Numerals, Characters, Strings and Boolean Values. Integer Numerals
| 1776707-273 |
In addition to decimal numbers (those that all of us are used to use every day) C++ allows the use as literal constants of octal numbers (base 8) and hexadecimal numbers (base 16). If we want to express an octal number we have to precede it with a 0 (zero character). And in order to express a hexadecimal number we have to precede it with the characters 0x (zero, x). For example, the following literal constants are all equivalent to each other:
| 75 // decimal0113 // octal0x4b // hexadecimal |
Literal constants, like variables, are considered to have a specific data type. By default, integer literals are of type int. However, we can force them to either be unsigned by appending the u character to it, or long by appending l:
| 75 // int75u // unsigned int75l // long75ul // unsigned long |
| 3.14159 // 3.141596.02e23 // 6.02 x 10231.6e-19 // 1.6 x 10-193.0 // 3.0 |
The default type for floating point literals is double. If you explicitly want to express a float or long double numerical literal, you can use the f or l suffixes respectively:
| 3.14159L // long double6.02e23f // float |
| 'z''p'"Hello world""How do you do?" |
When writing both single character and string literals, it is necessary to put the quotation marks surrounding them to distinguish them from possible variable identifiers or reserved keywords. Notice the difference between these two expressions:
| x'x' |
Character and string literals have certain peculiarities, like the escape codes. These are special characters that are difficult or impossible to express otherwise in the source code of a program, like newline (\n) or tab (\t). All of them are preceded by a backslash (\). Here you have a list of some of such escape codes:
| \n | newline |
| \r | carriage return |
| \t | tab |
| \v | vertical tab |
| \b | backspace |
| \f | form feed (page feed) |
| \a | alert (beep) |
| \' | single quote (') |
| \" | double quote (") |
| \? | question mark (?) |
| \\ | backslash (\) |
| '\n''\t'"Left \t Right""one\ntwo\nthree" |
String literals can extend to more than a single line of code by putting a backslash sign (\) at the end of each unfinished line.
| "string expressed in \two lines" |
| "this forms" "a single" "string" "of characters" |
| L"This is a wide character string" |
Defined constants (#define)
You can define your own names for constants that you use very often without having to resort to memory-consuming variables, simply by using the #define preprocessor directive. Its format is:#define identifier value
For example:
| #define PI 3.14159265#define NEWLINE '\n' |
| // defined constants: calculate circumference#include <iostream>using namespace std;#define PI 3.14159#define NEWLINE '\n';int main (){ double r=5.0; // radius double circle; circle = 2 * PI * r; cout << circle; cout << NEWLINE; return 0;} | 31.4159 |
The #define directive is not a C++ statement but a directive for the preprocessor; therefore it assumes the entire line as the directive and does not require a semicolon (;) at its end. If you append a semicolon character (;) at the end, it will also be appended in all occurrences within the body of the program that the preprocessor replaces.
Declared constants (const)
With the const prefix you can declare constants with a specific type in the same way as you would do with a variable:| const int pathwidth = 100;const char tabulator = '\t';const zipcode = 12440; |
| Previous: Variables. Data Types. | index | Next: | |
| Operators |
| ||
You do not have to memorize all the content of this page. Most details are only provided to serve as a later reference in case you need it.
Assignment (=)
The assignment operator assigns a value to a variable.| a = 5; |
The most important rule when assigning is the right-to-left rule: The assignment operation always takes place from right to left, and never the other way:
| a = b; |
Consider also that we are only assigning the value of b to a at the moment of the assignment operation. Therefore a later change of b will not affect the new value of a.
For example, let us have a look at the following code - I have included the evolution of the content stored in the variables as comments:
| // assignment operator#include <iostream>using namespace std;int main (){ int a, b; // a:?, b:? a = 10; // a:10, b:? b = 4; // a:10, b:4 a = b; // a:4, b:4 b = 7; // a:4, b:7 cout << "a:"; cout << a; cout << " b:"; cout << b; return 0;} | a:4 b:7 |
A property that C++ has over other programming languages is that the assignment operation can be used as the rvalue (or part of an rvalue) for another assignment operation. For example:
| a = 2 + (b = 5); |
| b = 5;a = 2 + b; |
The following expression is also valid in C++:
| a = b = c = 5; |
Arithmetic operators ( +, -, *, /, % )
The five arithmetical operations supported by the C++ language are:| + | addition |
| - | subtraction |
| * | multiplication |
| / | division |
| % | modulo |
| a = 11 % 3; |
Compound assignment (+=, -=, *=, /=, %=, >>=, <<=, &=, ^=, |=)
When we want to modify the value of a variable by performing an operation on the value currently stored in that variable we can use compound assignment operators:| expression | is equivalent to |
| value += increase; | value = value + increase; |
| a -= 5; | a = a - 5; |
| a /= b; | a = a / b; |
| price *= units + 1; | price = price * (units + 1); |
| // compound assignment operators#include <iostream>using namespace std;int main (){ int a, b=3; a = b; a+=2; // equivalent to a=a+2 cout << a; return 0;} | 5 |
Increase and decrease (++, --)
Shortening even more some expressions, the increase operator (++) and the decrease operator (--) increase or reduce by one the value stored in a variable. They are equivalent to +=1 and to -=1, respectively. Thus:| c++;c+=1;c=c+1; |
In the early C compilers, the three previous expressions probably produced different executable code depending on which one was used. Nowadays, this type of code optimization is generally done automatically by the compiler, thus the three expressions should produce exactly the same executable code.
A characteristic of this operator is that it can be used both as a prefix and as a suffix. That means that it can be written either before the variable identifier (++a) or after it (a++). Although in simple expressions like a++ or ++a both have exactly the same meaning, in other expressions in which the result of the increase or decrease operation is evaluated as a value in an outer expression they may have an important difference in their meaning: In the case that the increase operator is used as a prefix (++a) the value is increased before the result of the expression is evaluated and therefore the increased value is considered in the outer expression; in case that it is used as a suffix (a++) the value stored in a is increased after being evaluated and therefore the value stored before the increase operation is evaluated in the outer expression. Notice the difference:
| Example 1 | Example 2 |
| B=3; A=++B; // A contains 4, B contains 4 | B=3; A=B++; // A contains 3, B contains 4 |
Relational and equality operators ( ==, !=, >, <, >=, <= )
In order to evaluate a comparison between two expressions we can use the relational and equality operators. The result of a relational operation is a Boolean value that can only be true or false, according to its Boolean result.We may want to compare two expressions, for example, to know if they are equal or if one is greater than the other is. Here is a list of the relational and equality operators that can be used in C++:
| == | Equal to |
| != | Not equal to |
| > | Greater than |
| < | Less than |
| >= | Greater than or equal to |
| <= | Less than or equal to |
| (7 == 5) // evaluates to false.(5 > 4) // evaluates to true.(3 != 2) // evaluates to true.(6 >= 6) // evaluates to true.(5 < 5) // evaluates to false. |
| (a == 5) // evaluates to false since a is not equal to 5.(a*b >= c) // evaluates to true since (2*3 >= 6) is true. (b+4 > a*c) // evaluates to false since (3+4 > 2*6) is false. ((b=2) == a) // evaluates to true. |
Logical operators ( !, &&, || )
The Operator ! is the C++ operator to perform the Boolean operation NOT, it has only one operand, located at its right, and the only thing that it does is to inverse the value of it, producing false if its operand is true and true if its operand is false. Basically, it returns the opposite Boolean value of evaluating its operand. For example:| !(5 == 5) // evaluates to false because the expression at its right (5 == 5) is true. !(6 <= 4) // evaluates to true because (6 <= 4) would be false. !true // evaluates to false!false // evaluates to true. |
&& OPERATOR
| a | b | a && b |
| true | true | true |
| true | false | false |
| false | true | false |
| false | false | false |
|| OPERATOR
| a | b | a || b |
| true | true | true |
| true | false | true |
| false | true | true |
| false | false | false |
| ( (5 == 5) && (3 > 6) ) // evaluates to false ( true && false ).( (5 == 5) || (3 > 6) ) // evaluates to true ( true || false ). |
Conditional operator ( ? )
The conditional operator evaluates an expression returning a value if that expression is true and a different one if the expression is evaluated as false. Its format is:condition ? result1 : result2
If condition is true the expression will return result1, if it is not it will return result2.
| 7==5 ? 4 : 3 // returns 3, since 7 is not equal to 5.7==5+2 ? 4 : 3 // returns 4, since 7 is equal to 5+2.5>3 ? a : b // returns the value of a, since 5 is greater than 3.a>b ? a : b // returns whichever is greater, a or b. |
| // conditional operator#include <iostream>using namespace std;int main (){ int a,b,c; a=2; b=7; c = (a>b) ? a : b; cout << c; return 0;} | 7 |
Comma operator ( , )
The comma operator (,) is used to separate two or more expressions that are included where only one expression is expected. When the set of expressions has to be evaluated for a value, only the rightmost expression is considered.For example, the following code:
| a = (b=3, b+2); |
Bitwise Operators ( &, |, ^, ~, <<, >> )
Bitwise operators modify variables considering the bit patterns that represent the values they store.| operator | asm equivalent | description |
| & | AND | Bitwise AND |
| | | OR | Bitwise Inclusive OR |
| ^ | XOR | Bitwise Exclusive OR |
| ~ | NOT | Unary complement (bit inversion) |
| << | SHL | Shift Left |
| >> | SHR | Shift Right |
Explicit type casting operator
Type casting operators allow you to convert a datum of a given type to another. There are several ways to do this in C++. The simplest one, which has been inherited from the C language, is to precede the expression to be converted by the new type enclosed between parentheses (()):| int i;float f = 3.14;i = (int) f; |
| i = int ( f ); |
sizeof()
This operator accepts one parameter, which can be either a type or a variable itself and returns the size in bytes of that type or object:| a = sizeof (char); |
The value returned by sizeof is a constant, so it is always determined before program execution.
Other operators
Later in these tutorials, we will see a few more operators, like the ones referring to pointers or the specifics for object-oriented programming. Each one is treated in its respective section.Precedence of operators
When writing complex expressions with several operands, we may have some doubts about which operand is evaluated first and which later. For example, in this expression:| a = 5 + 7 % 2 |
| a = 5 + (7 % 2) // with a result of 6, ora = (5 + 7) % 2 // with a result of 0 |
| Level | Operator | Description | Grouping |
| 1 | :: | scope | Left-to-right |
| 2 | () [] . -> ++ -- dynamic_cast static_cast reinterpret_cast const_cast typeid | postfix | Left-to-right |
| 3 | ++ -- ~ ! sizeof new delete | unary (prefix) | Right-to-left |
| * & | indirection and reference (pointers) | ||
| + - | unary sign operator | ||
| 4 | (type) | type casting | Right-to-left |
| 5 | .* ->* | pointer-to-member | Left-to-right |
| 6 | * / % | multiplicative | Left-to-right |
| 7 | + - | additive | Left-to-right |
| 8 | << >> | shift | Left-to-right |
| 9 | < > <= >= | relational | Left-to-right |
| 10 | == != | equality | Left-to-right |
| 11 | & | bitwise AND | Left-to-right |
| 12 | ^ | bitwise XOR | Left-to-right |
| 13 | | | bitwise OR | Left-to-right |
| 14 | && | logical AND | Left-to-right |
| 15 | || | logical OR | Left-to-right |
| 16 | ?: | conditional | Right-to-left |
| 17 | = *= /= %= += -= >>= <<= &= ^= != | assignment | Right-to-left |
| 18 | , | comma | Left-to-right |
All these precedence levels for operators can be manipulated or become more legible by removing possible ambiguities using parentheses signs ( and ), as in this example:
| a = 5 + 7 % 2; |
| a = 5 + (7 % 2); |
| a = (5 + 7) % 2; |
So if you want to write complicated expressions and you are not completely sure of the precedence levels, always include parentheses. It will also become a code easier to read.
| Prev | |||
| Basic Input/Output |
| ||
C++ uses a convenient abstraction called streams to perform input and output operations in sequential media such as the screen or the keyboard. A stream is an object where a program can either insert or extract characters to/from it. We do not really need to care about many specifications about the physical media associated with the stream - we only need to know it will accept or provide characters sequentialy.
The standard C++ library includes the header file iostream, where the standard input and output stream objects are declared.
Standard Output (cout)
By default, the standard output of a program is the screen, and the C++ stream object defined to access it is cout.cout is used in conjunction with the insertion operator, which is written as << (two "less than" signs).
| cout << "Output sentence"; // prints Output sentence on screencout << 120; // prints number 120 on screencout << x; // prints the content of x on screen |
| cout << "Hello"; // prints Hellocout << Hello; // prints the content of Hello variable |
| cout << "Hello, " << "I am " << "a C++ statement"; |
| cout << "Hello, I am " << age << " years old and my zipcode is " << zipcode; |
| Hello, I am 24 years old and my zipcode is 90064 |
| cout << "This is a sentence.";cout << "This is another sentence."; |
This is a sentence.This is another sentence.
even though we had written them in two different insertions into cout. In order to perform a line break on the output we must explicitly insert a new-line character into cout. In C++ a new-line character can be specified as \n (backslash, n):
| cout << "First sentence.\n ";cout << "Second sentence.\nThird sentence."; |
First sentence.
Second sentence.
Third sentence.
Additionally, to add a new-line, you may also use the endl manipulator. For example:
| cout << "First sentence." << endl;cout << "Second sentence." << endl; |
First sentence.
Second sentence.
The endl manipulator produces a newline character, exactly as the insertion of '\n' does, but it also has an additional behavior when it is used with buffered streams: the buffer is flushed. Anyway, cout will be an unbuffered stream in most cases, so you can generally use both the \n escape character and the endl manipulator in order to specify a new line without any difference in its behavior.
Standard Input (cin).
The standard input device is usually the keyboard. Handling the standard input in C++ is done by applying the overloaded operator of extraction (>>) on the cin stream. The operator must be followed by the variable that will store the data that is going to be extracted from the stream. For example:| int age;cin >> age; |
cin can only process the input from the keyboard once the RETURN key has been pressed. Therefore, even if you request a single character, the extraction from cin will not process the input until the user presses RETURN after the character has been introduced.
You must always consider the type of the variable that you are using as a container with cin extractions. If you request an integer you will get an integer, if you request a character you will get a character and if you request a string of characters you will get a string of characters.
| // i/o example#include <iostream>using namespace std;int main (){ int i; cout << "Please enter an integer value: "; cin >> i; cout << "The value you entered is " << i; cout << " and its double is " << i*2 << ".\n"; return 0;} | Please enter an integer value: 702The value you entered is 702 and its double is 1404. |
You can also use cin to request more than one datum input from the user:
| cin >> a >> b; |
| cin >> a;cin >> b; |
cin and strings
We can use cin to get strings with the extraction operator (>>) as we do with fundamental data type variables:| cin >> mystring; |
In order to get entire lines, we can use the function getline, which is the more recommendable way to get user input with cin:
| // cin with strings#include <iostream>#include <string>using namespace std;int main (){ string mystr; cout << "What's your name? "; getline (cin, mystr); cout << "Hello " << mystr << ".\n"; cout << "What is your favorite team? "; getline (cin, mystr); cout << "I like " << mystr << " too!\n"; return 0;} | What's your name? Juan SouliéHello Juan Soulié.What is your favorite team? The IsotopesI like The Isotopes too! |
stringstream
The standard header file <sstream> defines a class called stringstream that allows a string-based object to be treated as a stream. This way we can perform extraction or insertion operations from/to strings, which is especially useful to convert strings to numerical values and vice versa. For example, if we want to extract an integer from a string we can write:| string mystr ("1204");int myint;stringstream(mystr) >> myint; |
After this piece of code, the variable myint will contain the numerical value 1204.
| // stringstreams#include <iostream>#include <string>#include <sstream>using namespace std;int main (){ string mystr; float price=0; int quantity=0; cout << "Enter price: "; getline (cin,mystr); stringstream(mystr) >> price; cout << "Enter quantity: "; getline (cin,mystr); stringstream(mystr) >> quantity; cout << "Total price: " << price*quantity << endl; return 0;} | Enter price: 22.25Enter quantity: 7Total price: 155.75 |
Using this method, instead of direct extractions of integer values, we have more control over what happens with the input of numeric values from the user, since we are separating the process of obtaining input from the user (we now simply ask for lines) with the interpretation of that input. Therefore, this method is usually preferred to get numerical values from the user in all programs that are intensive in user input.
| Control Structures |
|
With the introduction of control structures we are going to have to introduce a new concept: the compound-statement or block. A block is a group of statements which are separated by semicolons (;) like all C++ statements, but grouped together in a block enclosed in braces: { }:
{ statement1; statement2; statement3; }
Most of the control structures that we will see in this section require a generic statement as part of its syntax. A statement can be either a simple statement (a simple instruction ending with a semicolon) or a compund statement (several instructions grouped in a block), like the one just described. In the case that we want the statement to be a simple statement, we do not need to enclose it in braces ({}). But in the case that we want the statement to be a compund statement it must be enclosed between braces ({}), forming a block.
Conditional structure: if and else
The if keyword is used to execute a statement or block only if a condition is fulfilled. Its form is:if (condition) statement
Where condition is the expression that is being evaluated. If this condition is true, statement is executed. If it is false, statement is ignored (not executed) and the program continues right after this conditional structure.
For example, the following code fragment prints x is 100 only if the value stored in the x variable is indeed 100:
| if (x == 100) cout << "x is 100"; |
| if (x == 100){ cout << "x is "; cout << x;} |
if (condition) statement1 else statement2
For example:
| if (x == 100) cout << "x is 100";else cout << "x is not 100"; |
The if + else structures can be concatenated with the intention of verifying a range of values. The following example shows its use telling if the value currently stored in x is positive, negative or none of them (i.e. zero):
| if (x > 0) cout << "x is positive";else if (x < 0) cout << "x is negative";else cout << "x is 0"; |
Iteration structures (loops)
Loops have as purpose to repeat a statement a certain number of times or while a condition is fulfilled. The while loop Its format is:while (expression) statement
and its functionality is simply to repeat statement while the condition set in expression is true.
For example, we are going to make a program to countdown using a while-loop:
| // custom countdown using while#include <iostream>using namespace std;int main (){ int n; cout << "Enter the starting number > "; cin >> n; while (n>0) { cout << n << ", "; --n; } cout << "FIRE!\n"; return 0;} | Enter the starting number > 88, 7, 6, 5, 4, 3, 2, 1, FIRE! |
The whole process of the previous program can be interpreted according to the following script (beginning in main):
- User assigns a value to n
- The while condition is checked (n>0). At this point there are two posibilities:
* condition is true: statement is executed (to step 3)
* condition is false: ignore statement and continue after it (to step 5) - Execute statement:
cout << n << ", ";
--n;
(prints the value of n on the screen and decreases n by 1) - End of block. Return automatically to step 2
- Continue the program right after the block: print FIRE! and end program.
Of course this is such a simple action for our computer that the whole countdown is performed instantly without any practical delay between numbers. The do-while loop Its format is:
do statement while (condition);
Its functionality is exactly the same as the while loop, except that condition in the do-while loop is evaluated after the execution of statement instead of before, granting at least one execution of statement even if condition is never fulfilled. For example, the following example program echoes any number you enter until you enter 0.
| // number echoer#include <iostream>using namespace std;int main (){ unsigned long n; do { cout << "Enter number (0 to end): "; cin >> n; cout << "You entered: " << n << "\n"; } while (n != 0); return 0;} | Enter number (0 to end): 12345You entered: 12345Enter number (0 to end): 160277You entered: 160277Enter number (0 to end): 0You entered: 0 |
for (initialization; condition; increase) statement;
and its main function is to repeat statement while condition remains true, like the while loop. But in addition, the for loop provides specific locations to contain an initialization statement and an increase statement. So this loop is specially designed to perform a repetitive action with a counter which is initialized and increased on each iteration.
It works in the following way:
- initialization is executed. Generally it is an initial value setting for a counter variable. This is executed only once.
- condition is checked. If it is true the loop continues, otherwise the loop ends and statement is skipped (not executed).
- statement is executed. As usual, it can be either a single statement or a block enclosed in braces { }.
- finally, whatever is specified in the increase field is executed and the loop gets back to step 2.
| // countdown using a for loop#include <iostream>using namespace std;int main (){ for (int n=10; n>0; n--) { cout << n << ", "; } cout << "FIRE!\n"; return 0;} | 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, FIRE! |
Optionally, using the comma operator (,) we can specify more than one expression in any of the fields included in a for loop, like in initialization, for example. The comma operator (,) is an expression separator, it serves to separate more than one expression where only one is generally expected. For example, suppose that we wanted to initialize more than one variable in our loop:
| for ( n=0, i=100 ; n!=i ; n++, i-- ){ // whatever here...} |
n starts with a value of 0, and i with 100, the condition is n!=i (that n is not equal to i). Because n is increased by one and i decreased by one, the loop's condition will become false after the 50th loop, when both n and i will be equal to 50.
Jump statements.
The break statement Using break we can leave a loop even if the condition for its end is not fulfilled. It can be used to end an infinite loop, or to force it to end before its natural end. For example, we are going to stop the count down before its natural end (maybe because of an engine check failure?):| // break loop example#include <iostream>using namespace std;int main (){ int n; for (n=10; n>0; n--) { cout << n << ", "; if (n==3) { cout << "countdown aborted!"; break; } } return 0;} | 10, 9, 8, 7, 6, 5, 4, 3, countdown aborted! |
| // continue loop example#include <iostream>using namespace std;int main (){ for (int n=10; n>0; n--) { if (n==5) continue; cout << n << ", "; } cout << "FIRE!\n"; return 0;} | 10, 9, 8, 7, 6, 4, 3, 2, 1, FIRE! |
The destination point is identified by a label, which is then used as an argument for the goto statement. A label is made of a valid identifier followed by a colon (:).
Generally speaking, this instruction has no concrete use in structured or object oriented programming aside from those that low-level programming fans may find for it. For example, here is our countdown loop using goto:
| // goto loop example#include <iostream>using namespace std;int main (){ int n=10; loop: cout << n << ", "; n--; if (n>0) goto loop; cout << "FIRE!\n"; return 0;} | 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, FIRE! |
The purpose of exit is to terminate the current program with a specific exit code. Its prototype is:
| void exit (int exitcode); |
The selective structure: switch.
The syntax of the switch statement is a bit peculiar. Its objective is to check several possible constant values for an expression. Something similar to what we did at the beginning of this section with the concatenation of several if and else if instructions. Its form is the following:switch (expression){ case constant1: group of statements 1; break; case constant2: group of statements 2; break; . . . default: default group of statements} It works in the following way: switch evaluates expression and checks if it is equivalent to constant1, if it is, it executes group of statements 1 until it finds the break statement. When it finds this break statement the program jumps to the end of the switch selective structure.
If expression was not equal to constant1 it will be checked against constant2. If it is equal to this, it will execute group of statements 2 until a break keyword is found, and then will jump to the end of the switch selective structure.
Finally, if the value of expression did not match any of the previously specified constants (you can include as many case labels as values you want to check), the program will execute the statements included after the default: label, if it exists (since it is optional).
Both of the following code fragments have the same behavior:
| switch example | if-else equivalent |
| switch (x) { case 1: cout << "x is 1"; break; case 2: cout << "x is 2"; break; default: cout << "value of x unknown"; } | if (x == 1) { cout << "x is 1"; }else if (x == 2) { cout << "x is 2"; }else { cout << "value of x unknown"; } |
For example, if we did not include a break statement after the first group for case one, the program will not automatically jump to the end of the switch selective block and it would continue executing the rest of statements until it reaches either a break instruction or the end of the switch selective block. This makes unnecessary to include braces { } surrounding the statements for each of the cases, and it can also be useful to execute the same block of instructions for different possible values for the expression being evaluated. For example:
| switch (x) { case 1: case 2: case 3: cout << "x is 1, 2 or 3"; break; default: cout << "x is not 1, 2 nor 3"; } |
If you need to check ranges or values that are not constants, use a concatenation of if and else if statements.
| Functions (I) |
| ||
A function is a group of statements that is executed when it is called from some point of the program. The following is its format:
type name ( parameter1, parameter2, ...) { statement }
where:
- type is the data type specifier of the data returned by the function.
- name is the identifier by which it will be possible to call the function.
- parameters (as many as needed): Each parameter consists of a data type specifier followed by an identifier, like any regular variable declaration (for example: int x) and which acts within the function as a regular local variable. They allow to pass arguments to the function when it is called. The different parameters are separated by commas.
- statements is the function's body. It is a block of statements surrounded by braces { }.
| // function example#include <iostream>using namespace std;int addition (int a, int b){ int r; r=a+b; return (r);}int main (){ int z; z = addition (5,3); cout << "The result is " << z; return 0;} | The result is 8 |
We can see how the main function begins by declaring the variable z of type int. Right after that, we see a call to a function called addition. Paying attention we will be able to see the similarity between the structure of the call to the function and the declaration of the function itself some code lines above:
The parameters and arguments have a clear correspondence. Within the main function we called to addition passing two values: 5 and 3, that correspond to the int a and int b parameters declared for function addition.
At the point at which the function is called from within main, the control is lost by main and passed to function addition. The value of both arguments passed in the call (5 and 3) are copied to the local variables int a and int b within the function.
Function addition declares another local variable (int r), and by means of the expression r=a+b, it assigns to r the result of a plus b. Because the actual parameters passed for a and b are 5 and 3 respectively, the result is 8.
The following line of code:
| return (r); |
So being the value returned by a function the value given to the function call itself when it is evaluated, the variable z will be set to the value returned by addition (5, 3), that is 8. To explain it another way, you can imagine that the call to a function (addition (5,3)) is literally replaced by the value it returns (8).
The following line of code in main is:
| cout << "The result is " << z; |
| // function example#include <iostream>using namespace std;int subtraction (int a, int b){ int r; r=a-b; return (r);}int main (){ int x=5, y=3, z; z = subtraction (7,2); cout << "The first result is " << z << '\n'; cout << "The second result is " << subtraction (7,2) << '\n'; cout << "The third result is " << subtraction (x,y) << '\n'; z= 4 + subtraction (x,y); cout << "The fourth result is " << z << '\n'; return 0;} | The first result is 5The second result is 5The third result is 2The fourth result is 6 |
Nevertheless, if we examine function main we will see that we have made several calls to function subtraction. We have used some different calling methods so that you see other ways or moments when a function can be called.
In order to fully understand these examples you must consider once again that a call to a function could be replaced by the value that the function call itself is going to return. For example, the first case (that you should already know because it is the same pattern that we have used in previous examples):
| z = subtraction (7,2);cout << "The first result is " << z; |
| z = 5;cout << "The first result is " << z; |
| cout << "The second result is " << subtraction (7,2); |
| cout << "The second result is " << 5; |
In the case of:
| cout << "The third result is " << subtraction (x,y); |
The fourth case is more of the same. Simply note that instead of:
| z = 4 + subtraction (x,y); |
| z = subtraction (x,y) + 4; |
| z = 4 + 2;z = 2 + 4; |
Functions with no type. The use of void.
If you remember the syntax of a function declaration:type name ( argument1, argument2 ...) statement
you will see that the declaration begins with a type, that is the type of the function itself (i.e., the type of the datum that will be returned by the function with the return statement). But what if we want to return no value?
Imagine that we want to make a function just to show a message on the screen. We do not need it to return any value. In this case we should use the void type specifier for the function. This is a special specifier that indicates absence of type.
| // void function example#include <iostream>using namespace std;void printmessage (){ cout << "I'm a function!";}int main (){ printmessage (); return 0;} | I'm a function! |
| void printmessage (void){ cout << "I'm a function!";} |
What you must always remember is that the format for calling a function includes specifying its name and enclosing its parameters between parentheses. The non-existence of parameters does not exempt us from the obligation to write the parentheses. For that reason the call to printmessage is:
| printmessage (); |
| printmessage; | |||
| Functions (II) |
| ||
Arguments passed by value and by reference.
Until now, in all the functions we have seen, the arguments passed to the functions have been passed by value. This means that when calling a function with parameters, what we have passed to the function were copies of their values but never the variables themselves. For example, suppose that we called our first function addition using the following code:| int x=5, y=3, z;z = addition ( x , y ); |
This way, when the function addition is called, the value of its local variables a and b become 5 and 3 respectively, but any modification to either a or b within the function addition will not have any effect in the values of x and y outside it, because variables x and y were not themselves passed to the function, but only copies of their values at the moment the function was called.
But there might be some cases where you need to manipulate from inside a function the value of an external variable. For that purpose we can use arguments passed by reference, as in the function duplicate of the following example:
| // passing parameters by reference#include <iostream>using namespace std;void duplicate (int& a, int& b, int& c){ a*=2; b*=2; c*=2;}int main (){ int x=1, y=3, z=7; duplicate (x, y, z); cout << "x=" << x << ", y=" << y << ", z=" << z; return 0;} | x=2, y=6, z=14 |
When a variable is passed by reference we are not passing a copy of its value, but we are somehow passing the variable itself to the function and any modification that we do to the local variables will have an effect in their counterpart variables passed as arguments in the call to the function.
To explain it in another way, we associate a, b and c with the arguments passed on the function call (x, y and z) and any change that we do on a within the function will affect the value of x outside it. Any change that we do on b will affect y, and the same with c and z.
That is why our program's output, that shows the values stored in x, y and z after the call to duplicate, shows the values of all the three variables of main doubled.
If when declaring the following function:
| void duplicate (int& a, int& b, int& c) |
| void duplicate (int a, int b, int c) |
Passing by reference is also an effective way to allow a function to return more than one value. For example, here is a function that returns the previous and next numbers of the first parameter passed.
| // more than one returning value#include <iostream>using namespace std;void prevnext (int x, int& prev, int& next){ prev = x-1; next = x+1;}int main (){ int x=100, y, z; prevnext (x, y, z); cout << "Previous=" << y << ", Next=" << z; return 0;} | Previous=99, Next=101 |
Default values in parameters.
When declaring a function we can specify a default value for each parameter. This value will be used if the corresponding argument is left blank when calling to the function. To do that, we simply have to use the assignment operator and a value for the arguments in the function declaration. If a value for that parameter is not passed when the function is called, the default value is used, but if a value is specified this default value is ignored and the passed value is used instead. For example:| // default values in functions#include <iostream>using namespace std;int divide (int a, int b=2){ int r; r=a/b; return (r);}int main (){ cout << divide (12); cout << endl; cout << divide (20,4); return 0;} | 65 |
| divide (12) |
In the second call:
| divide (20,4) |
Overloaded functions.
In C++ two different functions can have the same name if their parameter types or number are different. That means that you can give the same name to more than one function if they have either a different number of parameters or different types in their parameters. For example:| // overloaded function#include <iostream>using namespace std;int operate (int a, int b){ return (a*b);}float operate (float a, float b){ return (a/b);}int main (){ int x=5,y=2; float n=5.0,m=2.0; cout << operate (x,y); cout << "\n"; cout << operate (n,m); cout << "\n"; return 0;} | 102.5 |
In the first call to operate the two arguments passed are of type int, therefore, the function with the first prototype is called; This function returns the result of multiplying both parameters. While the second call passes two arguments of type float, so the function with the second prototype is called. This one has a different behavior: it divides one parameter by the other. So the behavior of a call to operate depends on the type of the arguments passed because the function has been overloaded.
Notice that a function cannot be overloaded only by its return type. At least one of its parameters must have a different type.
inline functions.
The inline specifier is an optimization specifier. It does not change the behavior of a function, but serves to indicate the compiler that the code the function body generates shall be inserted at the point of each call to the function, instead of doing a regular call that generally involves stacking variables and jumping to other memory locations. This is equivalent to declaring a macro. It only represents an overhead advantage for very short functions, in which the resulting code from compiling the program may be faster when the overhead required to call a function is avoided.The format for its declaration is:
inline type name ( arguments ... ) { instructions ... }
and the call is just like the call to any other function. You do not have to include the inline keyword when calling the function, only in its declaration.
Most compilers already optimize code to generate inline functions when it is more convenient. This specifier only indicates the compiler that inline is preferred for this function.
Recursivity.
Recursivity is the property that functions have to be called by themselves. It is useful for many tasks, like sorting or calculate the factorial of numbers. For example, to obtain the factorial of a number (n!) the mathematical formula would be:n! = n * (n-1) * (n-2) * (n-3) ... * 1 more concretely, 5! (factorial of 5) would be:
5! = 5 * 4 * 3 * 2 * 1 = 120 and a recursive function to calculate this in C++ could be:
| // factorial calculator#include <iostream>using namespace std;long factorial (long a){ if (a > 1) return (a * factorial (a-1)); else return (1);}int main (){ long number; cout << "Please type a number: "; cin >> number; cout << number << "! = " << factorial (number); return 0;} | Please type a number: 99! = 362880 |
This function has a limitation because of the data type we used in its design (long) for more simplicity. The results given will not be valid for values much greater than 10! or 15!, depending on the system you compile it.
Declaring functions.
Until now, we have defined all of the functions before the first appearance of calls to them in the source code. These calls were generally in function main which we have always left at the end of the source code. If you try to repeat some of the examples of functions described so far, but placing the function main before any of the other functions that were called from within it, you will most likely obtain compiling errors. The reason is that to be able to call a function it must have been declared in some earlier point of the code, like we have done in all our examples.But there is an alternative way to avoid writing the whole code of a function before it can be used in main or in some other function. This can be achieved by declaring just a prototype of the function before it is used, instead of the entire definition. This declaration is shorter than the entire definition, but significant enough for the compiler to determine its return type and the types of its parameters.
Its form is:
type name ( argument_type1, argument_type2, ...);
It is identical to a function definition, except that it does not include the body of the function itself (i.e., the function statements that in normal definitions are enclosed in braces { }) and instead of that we end the prototype declaration with a mandatory semicolon (;).
The parameter enumeration does not need to include the identifiers, but only the type specifiers. The inclusion of a name for each parameter as in the function definition is optional in the prototype declaration. For example, we can declare a function called protofunction with two int parameters with any of the following declarations:
| int protofunction (int first, int second);int protofunction (int, int); |
| // declaring functions prototypes#include <iostream>using namespace std;void odd (int a);void even (int a);int main (){ int i; do { cout << "Type a number (0 to exit): "; cin >> i; odd (i); } while (i!=0); return 0;}void odd (int a){ if ((a%2)!=0) cout << "Number is odd.\n"; else even (a);}void even (int a){ if ((a%2)==0) cout << "Number is even.\n"; else odd (a);} | Type a number (0 to exit): 9Number is odd.Type a number (0 to exit): 6Number is even.Type a number (0 to exit): 1030Number is even.Type a number (0 to exit): 0Number is even. |
The first things that we see are the declaration of functions odd and even:
| void odd (int a);void even (int a); |
Anyway, the reason why this program needs at least one of the functions to be declared before it is defined is because in odd there is a call to even and in even there is a call to odd. If none of the two functions had been previously declared, a compilarion error would happen, since either odd would not not be visible from even (because it has still not been declared), or even would not be visible from odd (for the same reason).
Having the prototype of all functions together in the same place within the source code is found practical by some programmers, and this can be easily achieved by declaring all functions prototypes at the beginning of a program.
| Arrays |
|
That means that, for example, we can store 5 values of type int in an array without having to declare 5 different variables, each one with a different identifier. Instead of that, using an array we can store 5 different values of the same type, int for example, with a unique identifier.
For example, an array to contain 5 integer values of type int called billy could be represented like this:
where each blank panel represents an element of the array, that in this case are integer values of type int. These elements are numbered from 0 to 4 since in arrays the first index is always 0, independently of its length.
Like a regular variable, an array must be declared before it is used. A typical declaration for an array in C++ is:
type name [elements];
where type is a valid type (like int, float...), name is a valid identifier and the elements field (which is always enclosed in square brackets []), specifies how many of these elements the array has to contain.
Therefore, in order to declare an array called billy as the one shown in the above diagram it is as simple as:
| int billy [5]; |
Initializing arrays.
When declaring a regular array of local scope (within a function, for example), if we do not specify otherwise, its elements will not be initialized to any value by default, so their content will be undetermined until we store some value in them. The elements of global and static arrays, on the other hand, are automatically initialized with their default values, which for all fundamental types this means they are filled with zeros.In both cases, local and global, when we declare an array, we have the possibility to assign initial values to each one of its elements by enclosing the values in braces { }. For example:
| int billy [5] = { 16, 2, 77, 40, 12071 }; |
The amount of values between braces { } must not be larger than the number of elements that we declare for the array between square brackets [ ]. For example, in the example of array billy we have declared that it has 5 elements and in the list of initial values within braces { } we have specified 5 values, one for each element.
When an initialization of values is provided for an array, C++ allows the possibility of leaving the square brackets empty [ ]. In this case, the compiler will assume a size for the array that matches the number of values included between braces { }:
| int billy [] = { 16, 2, 77, 40, 12071 }; |