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about_Switch

  • Artikkeli

Short description

Explains how to use a switch to handle multiple if statements.

Long description

To check a condition in a script or function, use an if statement. The if statement can check many types of conditions, including the value of variables and the properties of objects.

To check multiple conditions, use a switch statement. The switch statement is equivalent to a series of if statements, but it is simpler. The switch statement lists each condition and an optional action. If a condition obtains, the action is performed.

The switch statement can use the $_ and $switch automatic variables. For more information, see about_Automatic_Variables.

Syntax

A basic switch statement has the following format:

Switch (<test-expression>)
{
    <result1-to-be-matched> {<action>}
    <result2-to-be-matched> {<action>}
}

The equivalent if statements are:

if (<result1-to-be-matched> -eq (<test-expression>)) {<action>}
if (<result2-to-be-matched> -eq (<test-expression>)) {<action>}

The <test-expression> is single expression that is evaluated in expression mode to return a value.

The <result-to-be-matched> is an expression whose value is compared to the input value. Expressions include literal values (strings or numbers), variables, and scriptblocks that return a boolean value.

Any unquoted value that is not recognized as a number is treated as a string. To avoid confusion or unintended string conversion, you should always quote string values. Enclose any expressions in parentheses (), creating subexpressions, to ensure that the expression is evaluated correctly.

It is important to understand that the <result-to-be-matched> value is on the left-hand side of the comparison expression. That means the result of the <test-expression> is on the right-hand side, which can be converted to the type of the left-hand side value for comparison. For more information, see about_Comparison_Operators

The value default is reserved for the action used when there are no other matches.

The $_ automatic variable contains the value of the expression passed to the switch statement and is available for evaluation and use within the scope of the <result-to-be-matched> statements.

The complete switch statement syntax is as follows:

switch [-regex | -wildcard | -exact] [-casesensitive] (<test-expression>)
{
    "string" | number | variable | { <value-scriptblock> } { <action-scriptblock> }
    default { <action-scriptblock> } # optional
}

or

switch [-regex | -wildcard | -exact] [-casesensitive] -file filename
{
    "string" | number | variable | { <value-scriptblock> } { <action-scriptblock> }
    default { <action-scriptblock> }  # optional
}

If no parameters are used, switch behaves the same as using the Exact parameter. It performs a case-insensitive match for the value. If the value is a collection, each element is evaluated in the order in which it appears.

The switch statement must include at least one condition statement.

The default clause is triggered when the value does not match any of the conditions. It is equivalent to an else clause in an if statement. Only one default clause is permitted in each switch statement.

switch has the following parameters:

  • Wildcard - Indicates that the condition is a wildcard string. If the match clause is not a string, the parameter is ignored. The comparison is case-insensitive.
  • Exact - Indicates that the match clause, if it is a string, must match exactly. If the match clause is not a string, this parameter is ignored. The comparison is case-insensitive.
  • CaseSensitive - Performs a case-sensitive match. If the match clause is not a string, this parameter is ignored.
  • File- Takes input from a file rather than a <test-expression>. If multiple File parameters are included, only the last one is used. Each line of the file is read and evaluated by the switch statement. The comparison is case-insensitive.
  • Regex - Performs regular expression matching of the value to the condition. If the match clause is not a string, this parameter is ignored. The comparison is case-insensitive. The $matches automatic variable is available for use within the matching statement block.

Note

When specifying conflicting values, like Regex and Wildcard, the last parameter specified takes precedence, and all conflicting parameters are ignored. Multiple instances of parameters are also permitted. However, only the last parameter listed is used.

Examples

In the following example, the switch statement compares the test value, 3, to each of the conditions. When the test value matches the condition, the action is performed.

switch (3)
{
    1 {"It is one."}
    2 {"It is two."}
    3 {"It is three."}
    4 {"It is four."}
}

It is three.

In this simple example, the value is compared to each condition in the list, even though there is a match for the value 3. The following switch statement has two conditions for a value of 3. It demonstrates that, by default, all conditions are tested.

switch (3)
{
    1 {"It is one."}
    2 {"It is two."}
    3 {"It is three."}
    4 {"It is four."}
    3 {"Three again."}
}

It is three.
Three again.

To direct the switch to stop comparing after a match, use the break statement. The break statement terminates the switch statement.

switch (3)
{
    1 {"It is one."}
    2 {"It is two."}
    3 {"It is three."; Break}
    4 {"It is four."}
    3 {"Three again."}
}

It is three.

If the test value is a collection, such as an array, each item in the collection is evaluated in the order in which it appears. The following examples evaluates 4 and then 2.

switch (4, 2)
{
    1 {"It is one." }
    2 {"It is two." }
    3 {"It is three." }
    4 {"It is four." }
    3 {"Three again."}
}

It is four.
It is two.

Any break statements apply to the collection, not to each value, as shown in the following example. The switch statement is terminated by the break statement in the condition of value 4.

switch (4, 2)
{
    1 {"It is one."; Break}
    2 {"It is two." ; Break }
    3 {"It is three." ; Break }
    4 {"It is four." ; Break }
    3 {"Three again."}
}

It is four.

In this example, the switch statement is testing for the type of the value in the hashtable. You must use and expression that returns a boolean value to select the scriptblock to execute.

$var = @{A = 10; B = 'abc'}

foreach ($key in $var.Keys) {
    switch ($var[$key].GetType()) {
        { $_ -eq [int32]  }  { "$key + 10 = $($var[$key] + 10)" }
        { $_ -eq [string] }  { "$key = $($var[$key])"           }
    }
}

A + 10 = 20
B = abc

In this example, an object that's not a string or numerical data is passed to the switch. The switch performs a string coercion on the object and evaluates the outcome.

$test = @{
    Test  = 'test'
    Test2 = 'test2'
}

$test.ToString()

switch -Exact ($test)
{
    'System.Collections.Hashtable'
    {
        'Hashtable string coercion'
    }
    'test'
    {
        'Hashtable value'
    }
}

System.Collections.Hashtable
Hashtable string coercion

In this example, there is no matching case so there is no output.

switch ("fourteen")
{
    1 {"It is one."; Break}
    2 {"It is two."; Break}
    3 {"It is three."; Break}
    4 {"It is four."; Break}
    "fo*" {"That's too many."}
}

By adding the default clause, you can perform an action when no other conditions succeed.

switch ("fourteen")
{
    1 {"It is one."; Break}
    2 {"It is two."; Break}
    3 {"It is three."; Break}
    4 {"It is four."; Break}
    "fo*" {"That's too many."}
    Default {
        "No matches"
    }
}

No matches

For the word "fourteen" to match a case you must use the -Wildcard or -Regex parameter.

   PS> switch -Wildcard ("fourteen")
       {
           1 {"It is one."; Break}
           2 {"It is two."; Break}
           3 {"It is three."; Break}
           4 {"It is four."; Break}
           "fo*" {"That's too many."}
       }

That's too many.

The following example uses the -Regex parameter.

$target = 'https://bing.com'
switch -Regex ($target)
{
    '^ftp\://.*$' { "$_ is an ftp address"; Break }
    '^\w+@\w+\.com|edu|org$' { "$_ is an email address"; Break }
    '^(http[s]?)\://.*$' { "$_ is a web address that uses $($matches[1])"; Break }
}

https://bing.com is a web address that uses https

The following example demonstrates the use of script blocks as switch statement conditions.

switch ("Test")
{
    {$_ -is [String]} {
        "Found a string"
    }
    "Test" {
        "This $_ executes as well"
    }
}

Found a string
This Test executes as well

The following example processes an array containing two date values. The <value-scriptblock> compares the Year property of each date. The <action-scriptblock> displays a welcome message or the number of days until the beginning of the year 2022.

switch ((Get-Date 1-Jan-2022), (Get-Date 25-Dec-2021)) {
    { $_.Year -eq 2021 } {
        $days = ((Get-Date 1/1/2022) - $_).days
        "There are $days days until 2022."
    }
    { $_.Year -eq 2022 } { 'Welcome to 2022!' }
}

If the value matches multiple conditions, the action for each condition is executed. To change this behavior, use the break or continue keywords.

The break keyword stops processing and exits the switch statement.

The continue keyword stops processing the current value, but continues processing any subsequent values.

The following example processes an array of numbers and displays if they are odd or even. Negative numbers are skipped with the continue keyword. If a non-number is encountered, execution is terminated with the break keyword.

switch (1,4,-1,3,"Hello",2,1)
{
    {$_ -lt 0} { continue }
    {$_ -isnot [Int32]} { break }
    {$_ % 2} {
        "$_ is Odd"
    }
    {-not ($_ % 2)} {
        "$_ is Even"
    }
}

1 is Odd
4 is Even
3 is Odd

See also