while loop

 

C++

Language

Standard library headers

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

 

C++ language

 

Statements

Labels

label : statement

Expression statements

expression ;

Compound statements

{ statement... }

Selection statements

if

switch

Iteration statements

while

do-while

for

range for(C++11)

Jump statements

break

continue

return

goto

Declaration statements

declaration ;

Try blocks

try compound-statement handler-sequence

 

Executes a statement repeatedly, until the value of condition becomes false. The test takes place before each iteration.

Syntax

attr(optional) while ( condition ) statement

attr(C++11)	-	any number of attributes
condition	-	any expression which is contextually convertible to bool or a declaration of a single variable with a brace-or-equals initializer. This expression is evaluated before each iteration, and if it yields false, the loop is exited. If this is a declaration, the initializer is evaluated before each iteration, and if the value of the declared variable converts to false, the loop is exited.
statement	-	any statement, typically a compound statement, which is the body of the loop

Notes

If statement is a single statement (not a compound statement), the scope of variables declared in it is limited to the while loop as if it was a compound statement, in other words,

while(--x >= 0)
    int i;
// i goes out of scope

is the same as

while(--x >= 0) {
    int i;
} // i goes out of scope

If condition is a declaration such as T t = x, the declared variable is only in scope in the body of the loop, and is destroyed and recreated on every iteration, in other words, such while loop is equivalent to

label:
{ // start of condition scope
    T t = x;
    if (t) {
        statement
        goto label; // calls the destructor of t
    }
}

If the execution of the loop needs to be terminated at some point, break statement can be used as terminating statement.

If the execution of the loop needs to be continued at the end of the loop body, continue statement can be used as shortcut.

Keywords

while

Example

run this code

#include <iostream>
 
int main()
{
    // while loop with a single statement
    int i = 0;
    while (i < 10)
         i++;
    std::cout << i << '\n';
 
    // while loop with a compound statement
    int j = 2;
    while (j < 9) {
        std::cout << j << ' ';
        j += 2;
    }
    std::cout << '\n';
 
   // while loop with a declaration condition
   char cstr[] = "Hello";
   int k = 0;
   while(char c = cstr[k++])
       std::cout << c;
   std::cout << '\n';
}

Output:

10
2 4 6 8 
Hello