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about_Break

  • Artikkeli

Short description

Describes the break statement, which provides a way to exit the current control block.

Long description

The break statement provides a way to exit the current control block. Execution continues at the next statement after the control block. The statement supports labels. A label is a name you assign to a statement in a script.

Using break in loops

When a break statement appears in a loop, such as a foreach, for, do, or while loop, PowerShell immediately exits the loop.

A break statement can include a label that lets you exit embedded loops. A label can specify any loop keyword, such as foreach, for, or while, in a script.

The following example shows how to use a break statement to exit a for statement:

for($i=1; $i -le 10; $i++) {
   Write-Host $i
   break
}

In this example, the break statement exits the for loop when the $i variable equals 1. Even though the for statement evaluates to True until $i is greater than 10, PowerShell reaches the break statement the first time the for loop is run.

It is more common to use the break statement in a loop where an inner condition must be met. Consider the following foreach statement example:

$i=0
$varB = 10,20,30,40
foreach ($val in $varB) {
  if ($val -eq 30) {
    break
  }
  $i++
}
Write-Host "30 was found in array index $i"

In this example, the foreach statement iterates the $varB array. The if statement evaluates to False the first two times the loop is run and the variable $i is incremented by 1. The third time the loop is run, $i equals 2, and the $val variable equals 30. At this point, the break statement runs, and the foreach loop exits.

Using a labeled break in a loop

A break statement can include a label. If you use the break keyword with a label, PowerShell exits the labeled loop instead of exiting the current loop. The label is a colon followed by a name that you assign. The label must be the first token in a statement, and it must be followed by the looping keyword, such as while.

break moves execution out of the labeled loop. In embedded loops, this has a different result than the break keyword has when it is used by itself. This example has a while statement with a for statement:

:myLabel while (<condition 1>) {
  for ($item in $items) {
    if (<condition 2>) {
      break myLabel
    }
    $item = $x   # A statement inside the For-loop
  }
}
$a = $c  # A statement after the labeled While-loop

If condition 2 evaluates to True, the execution of the script skips down to the statement after the labeled loop. In the example, execution starts again with the statement $a = $c.

You can nest many labeled loops, as shown in the following example.

:red while (<condition1>) {
  :yellow while (<condition2>) {
    while (<condition3>) {
      if ($a) {break}
      if ($b) {break red}
      if ($c) {break yellow}
    }
    Write-Host "After innermost loop"
  }
  Write-Host "After yellow loop"
}
Write-Host "After red loop"

If the $b variable evaluates to True, execution of the script resumes after the loop that is labeled "red". If the $c variable evaluates to True, execution of the script control resumes after the loop that is labeled "yellow".

If the $a variable evaluates to True, execution resumes after the innermost loop. No label is needed.

PowerShell does not limit how far labels can resume execution. The label can even pass control across script and function call boundaries.

Using break in a switch statement

In a switchconstruct, break causes PowerShell to exit the switch code block.

The break keyword is used to leave the switch construct. For example, the following switch statement uses break statements to test for the most specific condition:

$var = "word2"
switch -regex ($var) {
    "word2" {
      Write-Host "Exact" $_
      break
    }

    "word.*" {
      Write-Host "Match on the prefix" $_
      break
    }

    "w.*" {
      Write-Host "Match on at least the first letter" $_
      break
    }

    default {
      Write-Host "No match" $_
      break
    }
}

In this example, the $var variable is created and initialized to a string value of word2. The switch statement uses the Regex class to match the variable value first with the term word2. Because the variable value and the first test in the switch statement match, the first code block in the switch statement runs.

When PowerShell reaches the first break statement, the switch statement exits. If the four break statements are removed from the example, all four conditions are met. This example uses the break statement to display results when the most specific condition is met.

Using break in a trap statement

If the final statement executed in the body of a trap statement is break, the error object is suppressed and the exception is re-thrown.

The following example creates a DivideByZeroException exception that is trapped using the trap statement.

function test {
  trap [DivideByZeroException] {
    Write-Host 'divide by zero trapped'
    break
  }

  $i = 3
  'Before loop'
  while ($true) {
     "1 / $i = " + (1 / $i--)
  }
  'After loop'
}
test

Notice that execution stops at the exception. The After loop is never reached. The exception is re-thrown after the trap executes.

Before loop
1 / 3 = 0.333333333333333
1 / 2 = 0.5
1 / 1 = 1
divide by zero trapped
Attempted to divide by zero.
At line:10 char:6
+      "1 / $i = " + (1 / $i--)
+      ~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [], ParentContainsErrorRecordException
    + FullyQualifiedErrorId : RuntimeException

Do not use break outside of a loop, switch, or trap

When break is used outside of a construct that directly supports it (loops, switch, trap), PowerShell looks up the call stack for an enclosing construct. If it can't find an enclosing construct, the current runspace is quietly terminated.

This means that functions and scripts that inadvertently use a break outside of an enclosing construct that supports it can inadvertently terminate their callers.

Using break inside a pipeline break, such as a ForEach-Object script block, not only exits the pipeline, it potentially terminates the entire runspace.

See also