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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.
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
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 ; 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
Blockcounter>0, (*While loop*)
factorial*=counter; (*Multiply*)
counter--; (*Decrement*)
;
factorial
Oberon, Oberon-2, Oberon-07, Component Pascaledit
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 Languageedit
int $counter = 5;
int $factorial = 1;
int $multiplication;
while ($counter > 0) {
$multiplication = $factorial * $counter;
$counter -= 1;
print("Counter is: " + $counter + ", multiplication is: " + $multiplication + "\n");
}
Nimedit
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!"
Pascaledit
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.
Perledit
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;
PHPedit
$counter = 5;
$factorial = 1;
while ($counter > 0) {
$factorial *= $counter--; // Multiply, then decrement.
}
echo $factorial;
PL/Iedit
declare counter fixed initial(5);
declare factorial fixed initial(1);
do while(counter > 0)
factorial = factorial * counter;
counter = counter - 1;
end;
Pythonedit
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!")
Racketedit
In Racket, as in other Scheme implementations, a named-let is a popular way to implement loops:
#lang racket
Zdroj:https://en.wikipedia.org?pojem=While_loop
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