While loop

From Seo Wiki - Search Engine Optimization and Programming Languages

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:

Ada

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.

unsigned int counter = 5;
unsigned long factorial = 1;
 
while (counter > 0)
{
  factorial *= counter--;    //Multiply and decrement
} 
 
printf("%i", factorial);

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

System.out.println(factorial);

System.Console.WriteLine(factorial);

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 Factorial1;
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;

$factorial *= $counter-- while $counter > 0;

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

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

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