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.
The while construct consists of a block of code and a condition. The condition is evaluated, and if the condition is true, the code within the block is executed. This repeats until the condition becomes false. Because while loops check the condition before the block is executed, the control structure is often also known as a pre-test loop. Compare with the do while loop, which tests the condition after the loop has executed.
For example, in the C programming language (as well as Java and C++, which use the same syntax in this case), the code fragment
x = 0;
while (x < 3)
{
printf("x = %d\n",x);
x++;
}
first checks whether x is less than 3, which it is, so it increments x by 1. It then checks the condition again, and executes again, repeating this process until the variable x has the value 3.
Note that 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.
Equivalent constructs
while (condition) {
statements;
}
is equivalent to
if (condition) {
do {
statements;
} while (condition);
}
or
while (true) {
if (!condition) break;
statements;
}
or
goto TEST;
LOOPSTART:
statements;
TEST:
if (condition) goto LOOPSTART;
Also, in C and its descendants, a while loop is a for loop with no initialization or counting expressions, i.e.,
for ( ; condition; )
{
statements;
}
Demonstrating while loops
These while loops will calculate the factorial of the number 5:
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 Main;
counter=5
factorial=1
while [ $counter -gt 0 ]; do
factorial=$((factorial * counter))
counter=$((counter - 1))
done
echo $factorial
' Initialize the variables
Dim counter As Integer : counter = 5
Dim factorial As Long : factorial = 1
Do While counter > 0
factorial = factorial * counter ' Multiply
counter = counter - 1 ' Decrement
Loop
Print factorial ' Prints out the result.
- include<stdio.h>
main() { int d;
unsigned int counter = 5;
unsigned long factorial = 1;
while (counter > 0)
{
factorial *= counter--; //Multiply and decrement
}
printf("%i", factorial);
The code for the loop is the same for Java, C# and D:
int counter = 5;
long factorial = 1;
while (counter > 1)
{
factorial *= counter--;
}
For Java the result is printed as follows:
System.out.println(factorial);
The same in C#
System.Console.WriteLine(factorial);
And finally in D
writefln(factorial);
var counter = 5;
var factorial = 1;
while ( --counter > 1 )
{
factorial *= counter;
}
document.body.appendChild(document.createTextNode(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 ]
program Factorial;
var
Counter, Factorial: integer;
begin
Counter := 5;
Factorial := 1;
while Counter > 0 do
begin
Factorial := Factorial * Counter;
Counter = Counter - 1
end;
WriteLn(Factorial)
end.
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.
my $counter = 5;
my $factorial = 1;
while ( $counter > 0 )
{
$factorial *= $counter--; # Multiply, then decrement
}
print $factorial;
Very similar to C and C++, but the while loop could also have been written on one line:
$factorial *= $counter-- while $counter > 0;
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 first.
$counter--; // then decrement.
}
print $factorial;
counter = 5
factorial = 1
while counter > 0:
factorial *= counter
counter -= 1
print factorial
Contrary to other languages, in Smalltalk a while loop is not a language construct but defined in the class BlockClosure
as a method with one parameter, the body as a closure, using self as the condition.
| count factorial |
count := 5.
factorial := 1.
[ count > 0 ] whileTrue:
[ factorial := factorial * (count := count - 1) ]
Transcript show: factorial
Tcl (Tool command language)
set counter 5
set factorial 1
while {$counter > 0} {
set factorial [expr $factorial * $counter]
incr counter -1
}
puts $factorial
$counter = 5 $factorial = 1 while ($counter -gt 0) { $factorial *= $counter-- # Multiply, then decrement. } Write-Output $factorial