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
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 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 C programming language (as well as 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.
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. For example:
while (true)
{
//do complicated stuff
if (someCondition) break;
//more stuff
}
Equivalent constructs
In the C programming language,
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;
or
TEST:
if (!condition) goto LOOPEND;
statements
goto TEST;
LOOPEND:
Those last two are not recommended because the use of "goto" statements makes it hard for a programmer to understand the flow of control, and is generally regarded as a last resort.
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:
var counter:int = 5;
var factorial:int = 1;
while ( counter > 1 )
{
factorial *= counter;
counter--;
}
Printf ("Factorial =%d", factorial);
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;
Basic (Visual Basic)
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(void)
{
int counter = 5;
long factorial = 1;
while (counter > 1)
{
factorial *= counter--;
}
printf("%d", factorial);
return 0;
}
Script syntax
counter = 5;
factorial = 1;
while ( counter > 1 ){
factorial *= counter--;
}
writeOutput(factorial);
Tag syntax
<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
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.write(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");
}
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.
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 first.
$counter--; // then decrement.
}
print $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
# factorial x counter.
counter -= 1 # Set the new value of 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)
But note that an imperative programming style is often discouraged in Racket (as in Scheme).
# Calculate the factorial of 5
i = 1
factorial = 1
while i < 5
factorial *= i
i += 1
end
puts 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 value to 5
var 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 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);
$number = 5
$counter = $number
$factorial = 1
while ($counter) {
$factorial *= $counter--
}
$factorial
While programming language
The While programming language[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
- Do while loop
- For loop
- Foreach
- LOOP (programming language) – a programming language with the property that the functions it can compute are exactly the primitive recursive functions
References
- ^ a b "The while and do-while Statements (The Java™ Tutorials > Learning the Java Language > Language Basics)". Dosc.oracle.com. Retrieved 2016-10-21.
{{cite web}}: Cite has empty unknown parameter:|dead-url=(help) - ^ "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.