While loop

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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.

The while construct consists of a block of code and a condition/expression.^[1] The condition/expression is evaluated, and if the condition/expression is true,^[1] 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#,^[2] Objective-C, and C++, (which use the same syntax in this case), the code fragment

int x = 0;  while (x < 5) {     printf ("x = %d\n", x);     x++; } 

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:

while (true) {     // do complicated stuff     if (someCondition)         break;     // more stuff } 

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

var counter: int = 5; var factorial: int = 1;  while (counter > 1) {     factorial *= counter;     counter--; }  Printf("Factorial = %d", factorial); 

The Wikibook Ada_Programming has a page on the topic of: Control

with Ada.Integer_Text_IO;  procedure Factorial is     Counter   : Integer := 5;     Factorial : Integer := 1; begin     while Counter > 0 loop         Factorial := Factorial * Counter;         Counter   := Counter - 1;     end loop;      Ada.Integer_Text_IO.Put (Factorial); end Factorial; 

counter ← 5 factorial ← 1  :While counter > 0     factorial ×← counter     counter -← 1 :EndWhile  ⎕ ← factorial 

or simply

!5 

counter := 5 factorial := 1  While counter > 0     factorial *= counter--  MsgBox % factorial 

counter = 5    ' Counter = 5 factorial = 1  ' initial value of variable "factorial"  While counter > 0     factorial = factorial * counter     counter = counter - 1     TextWindow.WriteLine(counter) EndWhile 

Dim counter As Integer = 5    ' init variable and set value Dim factorial As Integer = 1  ' initialize factorial variable  Do While counter > 0     factorial = factorial * counter     counter = counter - 1 Loop     ' program goes here, until counter = 0  'Debug.Print factorial         ' Console.WriteLine(factorial) in Visual Basic .NET 

counter=5 factorial=1 while [ $counter -gt 0 ]; do     factorial=$((factorial * counter))     counter=$((counter - 1)) done  echo $factorial 

int main() {     int count = 5;     int factorial = 1;      while (count > 1)         factorial *= count--;      printf("%d", factorial); } 

counter = 5; factorial = 1;  while (counter > 1) {     factorial *= counter--; }  writeOutput(factorial); 

<cfset counter = 5> <cfset factorial = 1> <cfloop condition="counter GT 1">     <cfset factorial *= counter--> </cfloop> <cfoutput>#factorial#</cfoutput> 

program FactorialProg     integer :: counter = 5     integer :: factorial = 1      do while (counter > 0)         factorial = factorial * counter         counter = counter - 1     end do      print *, factorial end program FactorialProg 

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

counter, factorial := 5, 1  for counter > 1 { 	counter, factorial = counter-1, factorial*counter } 

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

int counter = 5; int factorial = 1;  while (counter > 1)     factorial *= counter--; 

let counter = 5; let factorial = 1;  while (counter > 1)     factorial *= counter--;  console.log(factorial); 

counter = 5 factorial = 1  while counter > 0 do   factorial = factorial * counter   counter = counter - 1 end  print(factorial) 

counter = 5; factorial = 1;  while (counter > 0)     factorial = factorial * counter;      %Multiply     counter = counter - 1;                %Decrement end  factorial 

Block[{counter=5,factorial=1},  (*localize counter and factorial*)     While[counter>0,            (*While loop*)         factorial*=counter;     (*Multiply*)         counter--;              (*Decrement*)     ];      factorial ] 

MODULE Factorial; IMPORT Out; VAR     Counter, Factorial: INTEGER; BEGIN     Counter := 5;     Factorial := 1;      WHILE Counter > 0 DO         Factorial := Factorial * Counter;         DEC(Counter)     END;      Out.Int(Factorial,0) END Factorial. 

int $counter = 5; int $factorial = 1;  int $multiplication;  while ($counter > 0) {     $multiplication = $factorial * $counter;      $counter -= 1;      print("Counter is: " + $counter + ", multiplication is: " + $multiplication + "\n"); } 

var   counter = 5            # Set counter value to 5   factorial = 1          # Set factorial value to 1  while counter > 0:       # While counter is greater than 0     factorial *= counter # Set new value of factorial to counter.     dec counter          # Set the counter to counter - 1.  echo factorial 

Non-terminating while loop:

while true:   echo "Help! I'm stuck in a loop!" 

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.

program Factorial1; var     Fv: integer;      procedure fact(counter:integer);     var         Factorial: integer;      begin          Factorial := 1;           while Counter > 0 do          begin              Factorial := Factorial * Counter;              Counter := Counter - 1          end;           WriteLn(Factorial)      end;  begin     Write('Enter a number to return its factorial: ');     readln(fv);     repeat          fact(fv);          Write('Enter another number to return its factorial (or 0 to quit): ');      until fv=0; end. 

my $counter   = 5; my $factorial = 1;  while ($counter > 0) {     $factorial *= $counter--; # Multiply, then decrement }  print $factorial; 

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

open IN, "<test.txt";  while (<IN>) {     print; }  close IN; 

$counter = 5; $factorial = 1;  while ($counter > 0) {     $factorial *= $counter--; // Multiply, then decrement. }  echo $factorial; 

declare counter   fixed initial(5); declare factorial fixed initial(1);  do while(counter > 0)     factorial = factorial * counter;     counter = counter - 1; end; 

counter = 5                           # Set the value to 5 factorial = 1                         # Set the value to 1  while counter > 0:                    # While counter(5) is greater than 0     factorial *= counter              # Set new value of factorial to counter.     counter -= 1                      # Set the counter to counter - 1.  print(factorial)                      # Print the value of factorial. 

Non-terminating while loop:

while True:     print("Help! I'm stuck in a loop!") 

In Racket, as in other Scheme implementations, a named-let is a popular way to implement loops:

#lang racket (define counter 5) (define factorial 1) (let loop ()     (when (> counter 0)         (set! factorial (* factorial counter))         (set! counter (sub1 counter))         (loop))) (displayln factorial) 

Using a macro system, implementing a while loop is a trivial exercise (commonly used to introduce macros):

#lang racket (define-syntax-rule (while test body ...) ; implements a while loop     (let loop () (when test body ... (loop)))) (define counter 5) (define factorial 1) (while (> counter 0)     (set! factorial (* factorial counter))     (set! counter (sub1 counter))) (displayln factorial) 

However, an imperative programming style is often discouraged in Scheme and Racket.

# Calculate the factorial of 5 i = 1 factorial = 1  while i <= 5   factorial *= i   i += 1 end  puts factorial 

fn main() {     let mut counter = 5;     let mut factorial = 1;      while counter > 1 {         factorial *= counter;         counter -= 1;     }      println!("{}", factorial); } 

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

Smalltalk also has a corresponding whileFalse: method.

| count factorial | count := 5. factorial := 1. [count > 0] whileTrue:     [factorial := factorial * count.     count := count - 1]. Transcript show: factorial 

var counter = 5                 // Set the initial counter value to 5 var factorial = 1               // Set the initial factorial value to 1  while counter > 0 {             // While counter(5) is greater than 0     factorial *= counter        // Set new value of factorial to factorial x counter.     counter -= 1                // Set the new value of counter to  counter - 1. }  print(factorial)                // Print the value of factorial. 

set counter 5 set factorial 1  while {$counter > 0} {     set factorial [expr $factorial * $counter]     incr counter -1 }  puts $factorial 

int counter = 5; int factorial = 1;  while (counter > 1)     factorial *= counter--;  printf("%d", factorial); 

$counter = 5 $factorial = 1  while ($counter) {     $factorial *= $counter-- }  $factorial 

While^[3] is a simple programming language constructed from assignments, sequential composition, conditionals, and while statements, used in the theoretical analysis of imperative programming language semantics.^[4][5]

C := 5; F := 1;  while (C > 1) do     F := F * C;     C := C - 1; 

• Do while loop
• For loop
• Foreach
• Primitive recursive function
• General recursive function
• LOOP (programming language) – a programming language with the property that the functions it can compute are exactly the primitive recursive functions