34,333
edits
Changes
New page: The Windows PowerShell ''for'' and ''foreach'' loops described in the chapter entitled Windows PowerShell 1.0 Looping with the for and foreach Statements works well when you know in ad...
The Windows PowerShell ''for'' and ''foreach'' loops described in the chapter entitled [[Windows PowerShell 1.0 Looping with the for and foreach Statements]] works well when you know in advance how many times a particular task needs to be repeated in a program. There will, however, be instances where code needs to be repeated until a certain condition is met, with no way of knowing in advance how many repetitions are going to be needed to meet that criteria. To address this, Windows PowerShell provides the ''while'' and ''do ... while'' loops. Those familiar with other programming and scripting languages will find the Windows PowerShell implementations of these loops to be quite familiar.
== The Windows PowerShell while Loop ==
Essentially, the ''while'' loop repeats a set of tasks until a specified condition is met. The ''while'' loop syntax is defined follows:
<pre>
while (''condition'')
{
# PowerShell statements go here
}
</pre>
where ''condition'' is an expression that will return either ''true'' or ''false'' and the ''# Windows PowerShell statements go here'' comment represents the PowerShell code to be executed while the ''condition'' expression is ''true''. For example:
<pre>
$myCount = 0;
while ( $myCount -lt 100 )
{
$myCount++
"$myCount"
}
</pre>
In the above example, the ''while'' expression will evaluate whether the ''$myCount'' variable is less than 100. If it is already greater than 100 the code in the braces is skipped and the loop exits without performing any tasks. This is essentially an "upfront" evaluation process. If, on the other hand, ''$myCount'' is not greater than 100 the code in the braces is executed and the loop returns to the ''while'' statement and repeats the evaluation of ''$myCount''. This process repeats until the value of ''$myCount'' is greater than 100, at which point the loop exits.
== Windows PowerShell do ... while loops ==
It is often helpful to think of the ''do ... while'' loop as an inverted ''while'' loop. The ''while'' loop evaluates an expression before executing the code contained in the body of the loop. If the expression evaluates to ''false'' on the first check then the code is not executed. The ''do .. while'' loop, on the other hand, is provided for situations where you know that the code contained in the body of the loop will ''always'' need to be executed at least once. For example, you may want to keep stepping through the items in an array until a specific item is found. You know that you have to at least check the first item in the array to have any hope of finding the entry you need. The syntax for the ''do ... while'' loop is as follows:
<pre>
do
{
// Windows PowerShell statements here
} while (''conditional expression'')
</pre>
In the ''do ... while'' example below the loop will continue until the value of a variable named $i equals 0:
<pre>
$i = 10;
do
{
$i--
"$i"
} while ($i -gt 0)
</pre>
== Breaking from while Loops ==
As with the ''for'' and ''foreach'' loop, it is also possible to exit from a ''while'' or ''do ... while'' loop at any time through the use of the ''break'' statement. When the execution path encounters a ''break'' statement, the looping will stop and execution will proceed to the statements immediately following the loop. In the following example, the loop is designed to exit when the value of variable ''$i'' matches the value of variable ''$j'':
<pre>
$i = 0;
$j = 5;
while ($i -lt 100)
{
$i++
if ($i -eq $j)
{
break
}
"$i"
}
</pre>
It is important to note that in the case of nested loops the ''break'' statement only exists the current loop leaving the outer loop to continue executing (and most likely once again executing the inner loop). To break from all outer loops in a nested loop it is necessary to use a ''label'' as described in the chapter entitled [[Windows PowerShell 1.0 Looping with the for and foreach Statements]].
== The continue Statement ==
The ''continue'' statement causes all remaining PowerShell statements in a loop to be skipped, and execution to be returned to the top of the loop. In the following example, the ?????? cmdlet is only called when the value of variable ''$i'' is an even number (i.e divisible by 2 with no remainder):
<pre>
$j = 0;
while ($j -lt 20)
{
$j++
if (($j % 2) -ne 0)
{
continue
}
write-host "j = $j"
}
</pre>
The ''continue'' statement in the above example will cause the WriteLine call to be skipped unless the value of ''i'' can be divided by 2 with no remainder. If the ''continue'' statement is triggered, execution will skip to the top of the while loop and the statements in the body of the loop will be repeated (until the value of ''i'' exceeds 19), resulting in the following output:
<pre>
j = 2
j = 4
j = 6
j = 8
j = 10
j = 12
j = 14
j = 16
j = 18
j = 20
</pre>
== The Windows PowerShell while Loop ==
Essentially, the ''while'' loop repeats a set of tasks until a specified condition is met. The ''while'' loop syntax is defined follows:
<pre>
while (''condition'')
{
# PowerShell statements go here
}
</pre>
where ''condition'' is an expression that will return either ''true'' or ''false'' and the ''# Windows PowerShell statements go here'' comment represents the PowerShell code to be executed while the ''condition'' expression is ''true''. For example:
<pre>
$myCount = 0;
while ( $myCount -lt 100 )
{
$myCount++
"$myCount"
}
</pre>
In the above example, the ''while'' expression will evaluate whether the ''$myCount'' variable is less than 100. If it is already greater than 100 the code in the braces is skipped and the loop exits without performing any tasks. This is essentially an "upfront" evaluation process. If, on the other hand, ''$myCount'' is not greater than 100 the code in the braces is executed and the loop returns to the ''while'' statement and repeats the evaluation of ''$myCount''. This process repeats until the value of ''$myCount'' is greater than 100, at which point the loop exits.
== Windows PowerShell do ... while loops ==
It is often helpful to think of the ''do ... while'' loop as an inverted ''while'' loop. The ''while'' loop evaluates an expression before executing the code contained in the body of the loop. If the expression evaluates to ''false'' on the first check then the code is not executed. The ''do .. while'' loop, on the other hand, is provided for situations where you know that the code contained in the body of the loop will ''always'' need to be executed at least once. For example, you may want to keep stepping through the items in an array until a specific item is found. You know that you have to at least check the first item in the array to have any hope of finding the entry you need. The syntax for the ''do ... while'' loop is as follows:
<pre>
do
{
// Windows PowerShell statements here
} while (''conditional expression'')
</pre>
In the ''do ... while'' example below the loop will continue until the value of a variable named $i equals 0:
<pre>
$i = 10;
do
{
$i--
"$i"
} while ($i -gt 0)
</pre>
== Breaking from while Loops ==
As with the ''for'' and ''foreach'' loop, it is also possible to exit from a ''while'' or ''do ... while'' loop at any time through the use of the ''break'' statement. When the execution path encounters a ''break'' statement, the looping will stop and execution will proceed to the statements immediately following the loop. In the following example, the loop is designed to exit when the value of variable ''$i'' matches the value of variable ''$j'':
<pre>
$i = 0;
$j = 5;
while ($i -lt 100)
{
$i++
if ($i -eq $j)
{
break
}
"$i"
}
</pre>
It is important to note that in the case of nested loops the ''break'' statement only exists the current loop leaving the outer loop to continue executing (and most likely once again executing the inner loop). To break from all outer loops in a nested loop it is necessary to use a ''label'' as described in the chapter entitled [[Windows PowerShell 1.0 Looping with the for and foreach Statements]].
== The continue Statement ==
The ''continue'' statement causes all remaining PowerShell statements in a loop to be skipped, and execution to be returned to the top of the loop. In the following example, the ?????? cmdlet is only called when the value of variable ''$i'' is an even number (i.e divisible by 2 with no remainder):
<pre>
$j = 0;
while ($j -lt 20)
{
$j++
if (($j % 2) -ne 0)
{
continue
}
write-host "j = $j"
}
</pre>
The ''continue'' statement in the above example will cause the WriteLine call to be skipped unless the value of ''i'' can be divided by 2 with no remainder. If the ''continue'' statement is triggered, execution will skip to the top of the while loop and the statements in the body of the loop will be repeated (until the value of ''i'' exceeds 19), resulting in the following output:
<pre>
j = 2
j = 4
j = 6
j = 8
j = 10
j = 12
j = 14
j = 16
j = 18
j = 20
</pre>
