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about_Hash_Tables

Updated: October 21, 2009

Applies To: Windows PowerShell 2.0

TOPIC
    about_Hash_Tables

SHORT DESCRIPTION
    Describes how to create, use, and sort hash tables in Windows PowerShell.


LONG DESCRIPTION
    A hash table, also known as a dictionary or associative array, is a
    compact data structure that stores one or more key/value pairs. For
    example, a hash table might contain a series of IP addresses and
    computer names, where the IP addresses are the keys and the computer
    names are the values, or vice versa.

    In Windows PowerShell, each hash table is a Hashtable 
    (System.Collections.Hashtable) object. You can use the properties and
    methods of Hashtable objects to manage hash tables in Windows PowerShell.

    The keys and value in hash tables are also .NET objects. They are most
    often strings or integers, but they can have any object type. You can also
    create nested hash tables, in which the value of a key is another hash table.

    Hash tables are frequently used because they are very efficient for finding
    and retrieving data. You can use hash tables to store lists and to create
    calculated properties in Windows PowerShell. And, Windows PowerShell has a
    cmdlet, ConvertFrom-StringData, that converts strings to a hash table.


  Syntax
     In Windows PowerShell, the syntax of a hash table is as follows:

 
          @{ <name> = <value>; [<name> = <value> ] ...}


  Creating Hash Tables
     To create a hash table, follow these guidelines:

          - Begin the hash table with an at sign (@).

          - Enclose the hash table in braces ({}).

          - Enter one or more key/value pairs for the content of the hash 
            table.

          - Use an equal sign (=) to separate each key from its value.

          - Use a semicolon (;) or a line break to separate the
            key/value pairs.

          - Key that contains spaces must be enclosed in quotation marks.
            Values must be valid Windows PowerShell expressions. Strings 
            must appear in quotation marks, even if they do not include
            spaces.

          - To manage the hash table, save it in a variable.


      To create an empty hash table in the value of $hash, type:

          $hash = @{}


      You can also add keys and values to a hash table when you create it. For
      example, the following statement creates a hash table with two keys.
 
          $hash = @{ a = 1; "blue" = "bl"}


  Displaying Hash Tables 
     To display a hash table that is saved in a variable, type the variable
     name. By default, a hash tables is displayed as a table with one column 
     for keys and one for values.

          C:\PS> $hash
        
          Name                           Value
          ----                           -----
          a                              1
          blue                           bl


     Hash tables have Keys and Values properties. Use dot notation to display
     all of the keys or all of the values.

          C:\PS> $hash.keys
          a
          blue
        
          C:\PS> $hash.values
	  1        
          bl

         

     Each key name is also a property of the hash table, and its value is 
     the value of the key-name property. Use the following format to display the
     property values.

          $hashtable.<key> = value
     
     For example:

          C:\PS> $hash.a
          1

          C:\PS> $hash.blue
          bl
     
    
     Hash tables have a Count property that indicates the number of key-value
     pairs in the hash table.

          C:\PS> $hash.count
          2


     Hash table tables are not arrays, so you cannot use an integer as an
     index into the hash table, but you can use a key name to index into the
     hash table. If the key is a string value, enclose the key name in quotation
     marks.

     For example:


          C:\PS> $hash["a"]
          1

     

  Adding and Removing Keys and Values
      To add keys and values to a hash table, use the Add method of the
      System.Collections.Hashtable object. The Add method has the following
      syntax:

          Add(Key, Value)

      For example, to add a "Time" key with a value of "Now" to the hash
      table, use the following statement format.

          $hash = $hash.Add("Time", "Now")

      You can also add keys and values to a hash table by using the addition 
      operator (+) to add a hash table to an existing hash table. For example,
      the following statement adds a "Time" key with a value of "Now" to the 
      hash table in the $hash variable.

          $hash = $hash + @{"Time", "Now"}


      You can also add values that are stored in variables.
      
          $t = "Today"
          $now = (get-date)
          
          $hash.Add($t, $now)      

      
      You cannot use a subtraction operator to remove a key/value pair from
      a hash table, but you can use the Remove method of the hashtable object.
      The Remove method takes the key as its value. 

      The Remove method has the following syntax:

          Remove(Key)          
     

      For example, to remove the Time=Now key/value pair from the hash table in
      the value of the $hash variable, type: 

          $hash.remove("Time")


      You can use all of the properties and methods of Hashtable objects in
      Windows PowerShell, including Contains, Clear, Clone, and CopyTo. For
      more information about Hashtable objects, see 
      "System.Collections.Hashtable" on MSDN.



  Object Types in HashTables  
      The keys and values in a hash table can have any .NET object type, 
      and a single hash table can have keys and values of multiple types.

      The following statement creates a hash table of process name strings
      and process object values and saves it in the $p variable.


          $p = @{"PowerShell" = (get-process powershell); 
          "Notepad" = (get-process notepad)}


      You can display the hash table in $p and use the key-name properties
      to display the values.


          C:\PS> $p

          Name                           Value
          ----                           -----
          PowerShell                     System.Diagnostics.Process (powershell)
          Notepad                        System.Diagnostics.Process (notepad)

     

          C:\PS> $p.powershell        
      
          Handles  NPM(K)    PM(K)      WS(K) VM(M)   CPU(s)     Id ProcessName
          -------  ------    -----      ----- -----   ------     -- -----------
              441      24    54196      54012   571     5.10   1788 powershell


          C:\PS> $p.keys | foreach {$p.$_.handles}
          441
          251


       The keys in a hash table can also be any .NET type. The following
       statement adds a key/value pair to the hash table in the $p variable.
       The key is a Service object that represents the WinRM service, and the
       value is the current status of the service.

        
           C:\PS> $p = $p + @{(get-service winrm) = ((get-service winrm).status)}


       You can display and access the new key/value pair by using the same
       methods that you use for other pairs in the hash table.

           C:\PS> $p

           Name                           Value
           ----                           -----
           PowerShell                     System.Diagnostics.Process (powershell)
           Notepad                        System.Diagnostics.Process (notepad)
           System.ServiceProcess.Servi... Running

       
           C:\PS> $p.keys
           PowerShell
           Notepad

           Status   Name               DisplayName
           ------   ----               -----------
           Running  winrm              Windows Remote Management (WS-Manag...

            
           C:\PS> $p.keys | foreach {$_.name}
           winrm        


       
       The keys and values in a hash table can also be Hashtable objects. The
       following statement adds key/value pair to the hash table in the $p 
       variable in which the key is a string, Hash2, and the value is a hash
       table with three key/value pairs.


          C:\PS> $p = $p + @{"Hash2"= @{a=1; b=2; c=3}}


       You can display and access the new values by using the same methods.

          C:\PS> $p


          Name                           Value
          ----                           -----
          PowerShell                     System.Diagnostics.Process (powershell)
          Notepad                        System.Diagnostics.Process (notepad)
          System.ServiceProcess.Servi... Running
          Hash2                          {a, b, c}


          C:\PS> $p.Hash2

          Name                           Value
          ----                           -----
          a                              1
          b                              2
          c                              3


          C:\PS> $p.Hash2.b
          2
     


  Sorting Keys and Values
      The items in a hash table are intrinsically unordered. The key/value
      pairs might appear in a different order each time that you display
      them.

      Although you cannot sort a hash table, you can use the GetEnumerator
      method of hash tables to enumerate the keys and values, and then use
      the Sort-Object cmdlet to sort the enumerated values for display.

      For example, the following commands enumerate the keys and values
      in the hash table in the $p variable and then sort the keys in 
      alphabetical order.

          C:\PS> $p.getenumerator() | sort-object -property key

          Name                           Value
          ----                           -----
          Notepad                        System.Diagnostics.Process (notepad)
          PowerShell                     System.Diagnostics.Process (powershell)
          System.ServiceProcess.Servi... Running


          
      The following command uses the same procedure to sort the hash values in 
      descending order.


          C:\PS> $p.getenumerator() | sort-object -property value -descending

          Name                           Value
          ----                           -----
          PowerShell                     System.Diagnostics.Process (powershell)
          Notepad                        System.Diagnostics.Process (notepad)
          System.ServiceProcess.Servi... Running



  ConvertFrom-StringData
      The ConvertFrom-StringData cmdlet converts a string or a here-string of 
      key/value pairs into a hash table. You can use the 
      ConvertFrom-StringData cmdlet safely in the Data section of a script, 
      and you can use it with the Import-LocalizedData cmdlet to display user
      messages in the user-interface (UI) culture of the current user.


      Here-strings are especially useful when the values in the hash table 
      include quotation marks. (For more information about here-strings, see
      about_Quoting_Rules.)


      The following example shows how to create a here-string of the user 
      messages in the previous example and how to use ConvertFrom-StringData
      to convert them from a string into a hash table.


      The following command creates a here-string of the key/value pairs and
      then saves it in the $string variable.


          C:\PS> $string = @"
          Msg1 = Type "Windows".
          Msg2 = She said, "Hello, World."
          Msg3 = Enter an alias (or "nickname").
          "@

   
    This command uses the ConvertFrom-StringData cmdlet to convert the 
    here-string into a hash table.


        C:\PS> convertfrom-stringdata $string

        Name                           Value
        ----                           -----
        Msg3                           Enter an alias (or "nickname").
        Msg2                           She said, "Hello, World."
        Msg1                           Type "Windows".

    For more information about here-strings, see about_Quoting_Rules.


SEE ALSO
    about_Arrays
    about_Quoting_Rules
    about_Script_Internationalization 
    ConvertFrom-StringData
    Import-LocalizedData
    "System.Collections.Hashtable" on MSDN
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