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Loop constructs |
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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.
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
intx=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 stuffif(someCondition)break;// more stuff}
These while loops will calculate the factorial of the number 5:
varcounter:int=5;varfactorial:int=1;while(counter>1){factorial*=counter;counter--;}Printf("Factorial = %d",factorial);
withAda.Integer_Text_IO;procedureFactorialisCounter:Integer:=5;Factorial:Integer:=1;beginwhileCounter>0loopFactorial:=Factorial*Counter;Counter:=Counter-1;endloop;Ada.Integer_Text_IO.Put(Factorial);endFactorial;
counter←5factorial←1:Whilecounter>0factorial×←countercounter-←1:EndWhile⎕←factorial
or simply
!5
counter:=5factorial:=1Whilecounter>0factorial*=counter--MsgBox%factorial
counter=5' Counter = 5factorial=1' initial value of variable "factorial"Whilecounter>0factorial=factorial*countercounter=counter-1TextWindow.WriteLine(counter)EndWhile
DimcounterAsInteger=5' init variable and set valueDimfactorialAsInteger=1' initialize factorial variableDoWhilecounter>0factorial=factorial*countercounter=counter-1Loop' program goes here, until counter = 0'Debug.Print factorial ' Console.WriteLine(factorial) in Visual Basic .NET
counter=5factorial=1while[$counter-gt0];dofactorial=$((factorial*counter))counter=$((counter-1))doneecho$factorial
intmain(){intcount=5;intfactorial=1;while(count>1){factorial*=count--;}printf("%d",factorial);}
counter=5;factorial=1;while(counter>1){factorial*=counter--;}writeOutput(factorial);
<cfsetcounter=5><cfsetfactorial=1><cfloopcondition="counter GT 1"><cfsetfactorial*=counter--></cfloop><cfoutput>#factorial#</cfoutput>
program FactorialProginteger::counter=5integer::factorial=1do while(counter>0)factorial=factorial*countercounter=counter-1end do print*,factorialend program FactorialProg
Go has no while statement, but it has the function of a for statement when omitting some elements of the for statement.
counter,factorial:=5,1forcounter>1{counter,factorial=counter-1,factorial*counter}
The code for the loop is the same for Java, C# and D:
intcounter=5;intfactorial=1;while(counter>1){factorial*=counter--;}
letcounter=5;letfactorial=1;while(counter>1)factorial*=counter--;console.log(factorial);
counter=5factorial=1whilecounter>0dofactorial=factorial*countercounter=counter-1endprint(factorial)
counter=5;factorial=1;while(counter>0)factorial=factorial*counter;%Multiplycounter=counter-1;%Decrementendfactorial
Block[{counter=5,factorial=1},(*localize counter and factorial*)While[counter>0,(*While loop*)factorial*=counter;(*Multiply*)counter--;(*Decrement*)];factorial]
MODULEFactorial;IMPORTOut;VARCounter,Factorial:INTEGER;BEGINCounter:=5;Factorial:=1;WHILECounter>0DOFactorial:=Factorial*Counter;DEC(Counter)END;Out.Int(Factorial,0)ENDFactorial.
int$counter=5;int$factorial=1;int$multiplication;while($counter>0){$multiplication=$factorial*$counter;$counter-=1;print("Counter is: "+$counter+", multiplication is: "+$multiplication+"\n");}
varcounter=5# Set counter value to 5factorial=1# Set factorial value to 1whilecounter>0:# While counter is greater than 0factorial*=counter# Set new value of factorial to counter.deccounter# Set the counter to counter - 1.echofactorial
Non-terminating while loop:
whiletrue:echo"Help! I'm stuck in a loop!"
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.
programFactorial1;varFv:integer;procedurefact(counter:integer);varFactorial:integer;beginFactorial:=1;whileCounter>0dobeginFactorial:=Factorial*Counter;Counter:=Counter-1end;WriteLn(Factorial)end;beginWrite('Enter a number to return its factorial: ');readln(fv);repeatfact(fv);Write('Enter another number to return its factorial (or 0 to quit): ');untilfv=0;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:
openIN,"<test.txt";while(<IN>){print;}closeIN;
$counter=5;$factorial=1;while($counter>0){$factorial*=$counter--;// Multiply, then decrement.}echo$factorial;
declarecounterfixedinitial(5); declarefactorialfixedinitial(1);dowhile(counter>0)factorial=factorial*counter;counter=counter-1;end;
counter=5# Set the value to 5factorial=1# Set the value to 1whilecounter>0:# While counter(5) is greater than 0factorial*=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:
whileTrue: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(definecounter5)(definefactorial1)(letloop()(when(>counter0)(set!factorial(*factorialcounter))(set!counter(sub1counter))(loop)))(displaylnfactorial)
Using a macro system, implementing a while loop is a trivial exercise (commonly used to introduce macros):
#lang racket(define-syntax-rule(whiletestbody...); implements a while loop(letloop()(whentestbody...(loop))))(definecounter5)(definefactorial1)(while(>counter0)(set!factorial(*factorialcounter))(set!counter(sub1counter)))(displaylnfactorial)
However, an imperative programming style is often discouraged in Scheme and Racket.
# Calculate the factorial of 5i=1factorial=1whilei<=5factorial*=ii+=1endputsfactorial
fnmain(){letmutcounter=5;letmutfactorial=1;whilecounter>1{factorial*=counter;counter-=1;}println!("{}",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].Transcriptshow:factorial
varcounter=5// Set the initial counter value to 5varfactorial=1// Set the initial factorial value to 1whilecounter>0{// While counter(5) is greater than 0factorial*=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.
setcounter5setfactorial1while{$counter>0}{setfactorial[expr$factorial*$counter]incrcounter-1}puts$factorial
intcounter=5;intfactorial=1;while(counter>1)factorial*=counter--;printf("%d",factorial);
$counter=5$factorial=1while($counter){$factorial*=$counter--}$factorial
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)doF:=F*C;C:=C-1;