While loop

Loop constructs
Do while loop While loop For loop Foreach loop Infinite loop Control flow
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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.

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I said we didn't need any new constructs, but we are gonna introduce one and that's gonna make writing the code we need for a search engine much more convenient It's also gonna be very useful for lots and lots of procedures that we wanna write and the construct we are gonna introduce is called a loop It's a way to do things over and over again The kind of loop that we are gonna introduce first is called the while loop The syntax for the while loop is this We have the key word "while", followed by a test expression followed by a colon, and then inside we have a "block" and the "block" is a sequence of instructions This is very similar to the "if" statement So as a reminder, here's what the "if" statement look like we had an "if" followed by a test expression, followed by a colon and then an indented "block", which is a list of statements that executes whenever the test expression evaluates to a "True" value With "if", the block executes either zero or one times, depending on whether the test expression is "True" With "while", the block can execute any number of times It keeps going as long as the test expression is "True" So with the "if " statement, if the test expression is "True" we go to the block, run the block and then continue if the test expression was "False", we go right to the next expression With a "while", we also start by checking the test expression if it's "True", we go to the block But now, instead of going to the next statement, after the block, we go back we try the test expression again, if it's "True", we go back to the block we always go back to the test expression if it's "True", we do the block again, we go back to the test expression If it's "True" again, we do the block again, we go back to the test expression and we can keep going around as many times as we need as long as the test expression is "True" we'll keep executing the block and keep trying the test expression again At some point, maybe the test expression is "False" Once the test expression is "False", we go to the next instruction So this means a while loop can execute the block either zero times if the test expression was "False" at the beginning one time if it was "True" the first time, but "False" after that two times, three times...any number of times, it could keep going forever There's no requirement that guarantees the test expression eventually becomes "False" So here's an example of while loop. We start by initializing variable i, and we'll give it a value, zero then we have the "while", the test expression says i is less than ten So that means as long as this evaluates to "True", we'll evaluate the block and what the block does is print i and then add one to i So here's what happens when this executes So initially, the value of i is zero, i is less than ten So that means we'll enter the loop So that means the test expression is "True", so we'll enter the block, we'll print i So we'll see the value zero printed and then we'll do the assignment, so that will change the value of i We add one to i, so that's gonna be making the value of i now refers to one So if it was an "if", we would be done now, but because it's a "while", we keep going We go back, test again, if i is less than ten Now the value of i is one, which also is less than ten so we continue, go to the block again we are gonna print i, this time we'll see the value one Then we go to the next statement, increase the value of i by one that's gonna make the value of i two Now i refers to the number two Because it's a "while", we keep going, we go back to the test expression i is less than ten, still less than ten, now it's two, we are gonna print the two we are gonna add one that it will make the value of i three and we are gonna keep going Test again, and i is still less than ten, so we are gonna print i again we will print the value nine, then we add one, so that's gonna have one to nine, we'll get ten So that's the new value of i and we go back again and now we do i is less than ten, and now i has the value ten Ten is not less than ten, so that will be "False" and we are done with the while loop We'll continue with whatever statements here, in this case, there is none So then the loopÃ¢Â€Â”Ã¢Â€Â”what we've done, we've gone through it ten times we've printed the numbers from zero to nine The new value of i will be ten, if we do anything here that uses the value of i we'll see that the value of i is ten So to see that you understand while loops, we'll have a quiz So the question is what does this program do Here's the program We start by assigning zero to i We have a while loop, where the test is not equal to ten So the test is i is not equal to ten Then we have i equals i plus one, so we are assigning to i the value of i plus one and then we are printing i So this is similar to the example, but different in a couple of ways So it's up to you to see if you can figure out what the program does Try to figure out yourself You can certainly also try running this in the Python interpreter The choices are produce an error print out the numbers from zero to nine print out the numbers from one to nine print out the numbers from one to ten or the final choice is it runs forever or at least until our machine runs out of power So see if you can figure out what it does You can definitely try running it but try to figure out on your own before running it in the Python interpreter

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 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

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

ActionScript 3

var counter: int = 5;
var factorial: int = 1;

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

Printf("Factorial = %d", factorial);

Ada

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;

APL

counter ← 5
factorial ← 1

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

⎕ ← factorial

or simply

!5

AutoHotkey

counter := 5
factorial := 1

While counter > 0
    factorial *= counter--

MsgBox % factorial

Small Basic

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

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

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

echo $factorial

C, C++

int main() {
    int count = 5;
    int factorial = 1;

    while (count > 1)
        factorial *= count--;

    printf("%d", factorial);
}

ColdFusion Markup Language (CFML)

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>

Fortran

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

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

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

let counter = 5;
let factorial = 1;

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

console.log(factorial);

Lua

counter = 5
factorial = 1

while counter > 0 do
  factorial = factorial * counter
  counter = counter - 1
end

print(factorial)

MATLAB, Octave

counter = 5;
factorial = 1;

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

factorial

Mathematica

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

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

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

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

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

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

$counter = 5;
$factorial = 1;

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

echo $factorial;

PL/I

declare counter   fixed initial(5);
declare factorial fixed initial(1);

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

Python

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

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

# Calculate the factorial of 5
i = 1
factorial = 1

while i <= 5
  factorial *= i
  i += 1
end

puts factorial

Rust

fn main() {
    let mut counter = 5;
    let mut factorial = 1;

    while counter > 1 {
        factorial *= counter;
        counter -= 1;
    }

    println!("{}", factorial);
}

Smalltalk

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

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

set counter 5
set factorial 1

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

puts $factorial

VEX

int counter = 5;
int factorial = 1;

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

printf("%d", factorial);

PowerShell

$counter = 5
$factorial = 1

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

$factorial

While (language)

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;

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.
^ "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.

This page was last edited on 1 April 2024, at 12:11

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