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Viewing file:     about_Foreach.help.txt (9.82 KB)      -rw-rw-rw-

TOPIC
    about_Foreach

SHORT DESCRIPTION
    Describes a language command you can use to traverse all the items in a 
    collection of items.


LONG DESCRIPTION
    The Foreach statement (also known as a Foreach loop) is a language 
    construct for stepping through (iterating) a series of values in a 
    collection of items. 


    The simplest and most typical type of collection to traverse is an array.
    Within a Foreach loop, it is common to run one or more commands against
    each item in an array. 


  Syntax        
      The following shows the Foreach syntax:

        
          foreach ($<item> in $<collection>){<statement list>}


  The Foreach Statement Outside a Command Pipeline
      The part of the Foreach statement enclosed in parenthesis represents a
      variable and a collection to iterate. Windows PowerShell creates the 
      variable ($<item>) automatically when the Foreach loop runs. Prior to 
      each iteration through the loop, the variable is set to a value in the 
      collection. The block following a Foreach statement {<statement list>}
      contains a set of commands to execute against each item in a collection.
 

  Examples
      For example, the Foreach loop in the following example displays the 
      values in the $letterArray array.

        
          $letterArray = "a","b","c","d"
          foreach ($letter in $letterArray)
          {
              Write-Host $letter
          }

      
      In this example, the $letterArray array is created and initialized with 
      the string values "a", "b", "c", and "d". The first time the Foreach 
      statement runs, it sets the $letter variable equal to the first item in 
      $letterArray ("a"). Then, it uses the Write-Host cmdlet to display the 
      letter a. The next time through the loop, $letter is set to "b", and so 
      on. After the Foreach loop displays the letter d, Windows PowerShell 
      exits the loop.


      The entire Foreach statement must appear on a single line to run it as a
      command at the Windows PowerShell command prompt. The entire Foreach 
      statement does not have to appear on a single line if you place the 
      command in a .ps1 script file instead. 


      Foreach statements can also be used together with cmdlets that 
      return a collection of items. In the following example, the Foreach 
      statement steps through the list of items that is returned by the 
      Get-ChildItem cmdlet.


          foreach ($file in Get-ChildItem)
          {
              Write-Host $file
          }


      You can refine the example by using an If statement to limit the results 
      that are returned. In the following example, the Foreach statement 
      performs the same looping operation as the previous example, but it adds
      an If statement to limit the results to files that are greater than 100 
      kilobytes (KB):


          foreach ($file in Get-ChildItem)
          {
              if ($file.length -gt 100k) 
              {
                  Write-Host $file
              }
          }


      In this example, the Foreach loop uses a property of the $file variable
      to perform a comparison operation ($file.length -gt 100k). The $file 
      variable contains all the properties in the object that is returned by 
      the Get-ChildItem cmdlet. Therefore, you can return more than just a 
      file name. In the next example, Windows PowerShell returns the length and
      the last access time inside the statement list:


          foreach ($file in Get-ChildItem)
          {
              if ($file.length -gt 100k) 
              {
                  Write-Host $file
                  Write-Host $file.length
                  Write-Host $file.lastaccesstime
              }
          }

    
      In this example, you are not limited to running a single command in a 
      statement list. 

      
      You can also use a variable outside a Foreach loop and increment the 
      variable inside the loop. The following example counts files over 100 KB
      in size: 
      
  
          $i = 0
          foreach ($file in Get-ChildItem)
          {
              if ($file.length -gt 100k) 
              {
                  Write-Host $file "file size:" ($file.length / 
          1024).ToString("F0") KB
                  $i = $i + 1
              }
          }

          if ($i -ne 0)
          {
              Write-Host
              Write-Host $i " file(s) over 100 KB in the current 
          directory."}
          else 
          {
              Write-Host "No files greater than 100 KB in the current 
          directory."
          }


      In the preceding example, the $i variable is set to 0 outside the loop,
      and the variable is incremented inside the loop for each file that is
      found that is larger than 100 KB. When the loop exits, an If statement
      evaluates the value of $i to display a count of all the files over 
      100 KB. Or, it displays a message stating that no files over 100 KB were
      found.


      The previous example also demonstrates how to format the file length 
      results:


          ($file.length / 1024).ToString("F0")


      The value is divided by 1,024 to show the results in kilobytes rather 
      than bytes, and the resulting value is then formatted using the 
      fixed-point format specifier to remove any decimal values from the 
      result. The 0 makes the format specifier show no decimal places. 


  The Foreach Statement Inside a Command Pipeline
      When Foreach appears in a command pipeline, Windows PowerShell uses the
      foreach alias, which calls the ForEach-Object command. When you use 
      the foreach alias in a command pipeline, you do not include 
      the ($<item> in $<collection>) syntax as you do with the Foreach 
      statement. This is because the prior command in the pipeline provides 
      this information. The syntax of the foreach alias when used in a command
      pipeline is as follows:
        

          <command> | foreach {<command_block>}
     

      For example, the Foreach loop in the following command pipeline displays 
      any processes whose working set (memory usage) is greater 
      than 20 megabytes (MB). Windows PowerShell pipes the output from the
      Get-Process command to the foreach alias. Inside the foreach alias 
      command block, the $_.WS variable contains the value of the WS (working
      set) property passed to it by the Get-Process cmdlet. (The $_ portion
      of the declaration is a Windows Script Host [WSH] automatic variable,
      and the WS portion is a property). The If statement uses a conditional
      statement to determine whether the working set is greater than 20 MB 
      (20,000,000 bytes). If so, the name of the process that is stored in 
      the $_.name variable and the working-set size in megabytes are displayed.
      If no process working set is over 20 MB, nothing is displayed.


          Write-Host "Processes with working-sets greater than 20 MB"
          Get-Process | foreach { 
              if ($_.WS -gt 20m)
              {
                  Write-Host $_.name ": " 
          ($_.WS/1m).ToString("F0") MB -Separator ""
              }
          }


      The foreach alias also supports beginning command blocks, middle command
      blocks, and end command blocks. The beginning and end command blocks run
      once, and the middle command block runs every time the Foreach loop steps
      through a collection or array. 
        

      The syntax of the foreach alias when used in a command pipeline with a 
      beginning, middle, and ending set of command blocks is as follows:
        

          <command> | foreach {<beginning command_block>}{<middle 
          command_block>}{<ending command_block>}

 
      The following example demonstrates the use of the beginning, middle, and
      end command blocks. 


          Get-ChildItem | foreach {
          $fileCount = $directoryCount = 0}{
          if ($_.PsIsContainer) {$directoryCount++} else {$fileCount++}}{
          "$directoryCount directories and $fileCount files"}


      The beginning block creates and initializes two variables to 0:


          {$fileCount = $directoryCount = 0}
 

      The middle block evaluates whether each item returned by Get-ChildItem
      is a directory or a file:


          {if ($_.PsIsContainer) {$directoryCount++} else {$fileCount++}}


      If the item that is returned is a directory, the $directoryCount 
      variable is incremented by 1. If the item is not a directory, 
      the $fileCount variable is incremented by 1. The ending block runs after
      the middle block completes its looping operation and then returns the 
      results of the operation: 

       
          {"$directoryCount directories and $fileCount files"}


      By using the beginning, middle, and ending command block structure and 
      the pipeline operator, you can rewrite the earlier example to find any 
      files that are greater than 100 KB, as follows:


          Get-ChildItem | foreach{
              $i = 0}{
              if ($_.length -gt 100k)
              {
                  Write-Host $_.name "file size:" ($_.length / 
          1024).ToString("F0") KB
                  $i++
              }
              }{
              if ($i -ne 0)
              {
                  Write-Host
                  Write-Host "$i file(s) over 100 KB in the current 
          directory."
              }
              else 
              {
              Write-Host "No files greater than 100 KB in the current 
          directory."}
              }


SEE ALSO
    about_Automatic_Variables
    about_If
    Foreach-Object