While loop



In most computer programming languages, a while loop is a control flow statement that allows code to be executed repeatedly based on a given Boolean condition. The while loop can be thought of as a repeating if statement.

Overview
The while construct consists of a block of code and a condition/expression. The condition/expression is evaluated, and if the condition/expression is true, the code within all of their following in the block is executed. This repeats until the condition/expression becomes false. Because the while loop checks the condition/expression before the block is executed, the control structure is often also known as a pre-test loop. Compare this with the do while loop, which tests the condition/expression after the loop has executed.

For example, in the languages C, Java, C#, Objective-C, and C++, (which use the same syntax in this case), the code fragment

first checks whether x is less than 5, which it is, so then the {loop body} is entered, where the printf function is run and x is incremented by 1. After completing all the statements in the loop body, the condition, (x < 5), is checked again, and the loop is executed again, this process repeating until the variable x has the value 5.

It is possible, and in some cases desirable, for the condition to always evaluate to true, creating an infinite loop. When such a loop is created intentionally, there is usually another control structure (such as a break statement) that controls termination of the loop. For example:

Demonstrating while loops
These while loops will calculate the factorial of the number 5:

APL
or simply

Go
Go has no while statement, but it has the function of a for statement when omitting some elements of the for statement.

Java, C#, D
The code for the loop is the same for Java, C# and D:

Nim
Non-terminating while loop:

Pascal
Pascal has two forms of the while loop, while and repeat. While repeats one statement (unless enclosed in a begin-end block) as long as the condition is true. The repeat statement repetitively executes a block of one or more statements through an until statement and continues repeating unless the condition is false. The main difference between the two is the while loop may execute zero times if the condition is initially false, the repeat-until loop always executes at least once.

Perl
While loops are frequently used for reading data line by line (as defined by the  line separator) from open filehandles:

Python
Non-terminating while loop:

Racket
In Racket, as in other Scheme implementations, a named-let is a popular way to implement loops: Using a macro system, implementing a while loop is a trivial exercise (commonly used to introduce macros): However, an imperative programming style is often discouraged in Scheme and Racket.

Smalltalk
Contrary to other languages, in Smalltalk a while loop is not a language construct but defined in the class  as a method with one parameter, the body as a closure, using self as the condition.

Smalltalk also has a corresponding whileFalse: method.

While (language)
While is a simple programming language constructed from assignments, sequential composition, conditionals, and while statements, used in the theoretical analysis of imperative programming language semantics.