While loop
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| Loop constructs |
|---|
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
[edit]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
}
Demonstrating while loops
[edit]These while loops will calculate the factorial of the number 5:
ActionScript 3
[edit]var counter: int = 5;
var factorial: int = 1;
while (counter > 1) {
factorial *= counter;
counter--;
}
Printf("Factorial = %d", factorial);
Ada
[edit]
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;
APL
[edit]counter ← 5
factorial ← 1
:While counter > 0
factorial ×← counter
counter -← 1
:EndWhile
⎕ ← factorial
or simply
!5
AutoHotkey
[edit]counter := 5
factorial := 1
While counter > 0
factorial *= counter--
MsgBox % factorial
Small Basic
[edit]counter = 5 ' Counter = 5
factorial = 1 ' initial value of variable "factorial"
While counter > 0
factorial = factorial * counter
counter = counter - 1
TextWindow.WriteLine(counter)
EndWhile
Visual Basic
[edit]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
Bourne (Unix) shell
[edit]counter=5
factorial=1
while [ $counter -gt 0 ]; do
factorial=$((factorial * counter))
counter=$((counter - 1))
done
echo $factorial
C, C++
[edit]int main() {
int count = 5;
int factorial = 1;
while (count > 1)
factorial *= count--;
printf("%d", factorial);
}
ColdFusion Markup Language (CFML)
[edit]Script syntax
[edit]counter = 5;
factorial = 1;
while (counter > 1) {
factorial *= counter--;
}
writeOutput(factorial);
Tag syntax
[edit]<cfset counter = 5>
<cfset factorial = 1>
<cfloop condition="counter GT 1">
<cfset factorial *= counter-->
</cfloop>
<cfoutput>#factorial#</cfoutput>
Fortran
[edit]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
[edit]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
}
Java, C#, D
[edit]The code for the loop is the same for Java, C# and D:
int counter = 5;
int factorial = 1;
while (counter > 1)
factorial *= counter--;
JavaScript
[edit]let counter = 5;
let factorial = 1;
while (counter > 1)
factorial *= counter--;
console.log(factorial);
Lua
[edit]counter = 5
factorial = 1
while counter > 0 do
factorial = factorial * counter
counter = counter - 1
end
print(factorial)
MATLAB, Octave
[edit]counter = 5;
factorial = 1;
while (counter > 0)
factorial = factorial * counter; %Multiply
counter = counter - 1; %Decrement
end
factorial
Mathematica
[edit]Block[{counter=5,factorial=1}, (*localize counter and factorial*)
While[counter>0, (*While loop*)
factorial*=counter; (*Multiply*)
counter--; (*Decrement*)
];
factorial
]
Oberon, Oberon-2, Oberon-07, Component Pascal
[edit]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.
Maya Embedded Language
[edit]int $counter = 5;
int $factorial = 1;
int $multiplication;
while ($counter > 0) {
$multiplication = $factorial * $counter;
$counter -= 1;
print("Counter is: " + $counter + ", multiplication is: " + $multiplication + "\n");
}
Nim
[edit]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
[edit]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.
Perl
[edit]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 $/ line separator) from open filehandles:
open IN, "<test.txt";
while (<IN>) {
print;
}
close IN;
PHP
[edit]$counter = 5;
$factorial = 1;
while ($counter > 0) {
$factorial *= $counter--; // Multiply, then decrement.
}
echo $factorial;
PL/I
[edit]declare counter fixed initial(5);
declare factorial fixed initial(1);
do while(counter > 0)
factorial = factorial * counter;
counter = counter - 1;
end;
Python
[edit]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!")
Racket
[edit]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.
Ruby
[edit]# Calculate the factorial of 5
i = 1
factorial = 1
while i <= 5
factorial *= i
i += 1
end
puts factorial
Rust
[edit]fn main() {
let mut counter = 5;
let mut factorial = 1;
while counter > 1 {
factorial *= counter;
counter -= 1;
}
println!("{}", factorial);
}
Smalltalk
[edit]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
Swift
[edit]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.
Tcl
[edit]set counter 5
set factorial 1
while {$counter > 0} {
set factorial [expr $factorial * $counter]
incr counter -1
}
puts $factorial
VEX
[edit]int counter = 5;
int factorial = 1;
while (counter > 1)
factorial *= counter--;
printf("%d", factorial);
PowerShell
[edit]$counter = 5
$factorial = 1
while ($counter) {
$factorial *= $counter--
}
$factorial
While (language)
[edit]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;
See also
[edit]- 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
References
[edit]- ^ a b "The while and do-while Statements (The Java Tutorials > Learning the Java Language > Language Basics)". Dosc.oracle.com. Retrieved 2016-10-21.
- ^ "while (C# reference)". Msdn.microsoft.com. Retrieved 2016-10-21.
- ^ "Chapter 3: The While programming language" (PDF). Profs.sci.univr.it. Retrieved 2016-10-21.
- ^ Flemming Nielson; Hanne R. Nielson; Chris Hankin (1999). Principles of Program Analysis. Springer. ISBN 978-3-540-65410-0. Retrieved 29 May 2013.
- ^ Illingworth, Valerie (11 December 1997). Dictionary of Computing. Oxford Paperback Reference (4th ed.). Oxford University Press. ISBN 9780192800466.