DOKK / manpages / debian 11 / tcl8.6-doc / switch.3tcl.en
switch(3tcl) Tcl Built-In Commands switch(3tcl) 







switch - Evaluate one of several scripts, depending on a given
    value








switch ?options? string pattern body
    ?pattern body ...?


switch ?options? string {pattern body
    ?pattern body ...?}










The switch command matches its string argument
    against each of the pattern arguments in order. As soon as it finds a
    pattern that matches string it evaluates the following
    body argument by passing it recursively to the Tcl interpreter and
    returns the result of that evaluation. If the last pattern argument
    is default then it matches anything. If no pattern argument
    matches string and no default is given, then the switch
    command returns an empty string.


If the initial arguments to switch start with - then
    they are treated as options unless there are exactly two arguments to
    switch (in which case the first must the string and the second
    must be the pattern/body list). The following options are
    currently supported:



  

  
Use exact matching when comparing string to a pattern. This is the
      default.

  

  
When matching string to the patterns, use glob-style matching (i.e.
      the same as implemented by the string match command).

  

  
When matching string to the patterns, use regular expression
      matching (as described in the re_syntax reference page).

  

  
Causes comparisons to be handled in a case-insensitive manner.

  

  
This option (only legal when -regexp is also specified) specifies
      the name of a variable into which the list of matches found by the regular
      expression engine will be written. The first element of the list written
      will be the overall substring of the input string (i.e. the string
      argument to switch) matched, the second element of the list will be
      the substring matched by the first capturing parenthesis in the regular
      expression that matched, and so on. When a default branch is taken,
      the variable will have the empty list written to it. This option may be
      specified at the same time as the -indexvar option.

  

  
This option (only legal when -regexp is also specified) specifies
      the name of a variable into which the list of indices referring to
      matching substrings found by the regular expression engine will be
      written. The first element of the list written will be a two-element list
      specifying the index of the start and index of the first character after
      the end of the overall substring of the input string (i.e. the
      string argument to switch) matched, in a similar way to the
      -indices option to the regexp can obtain. Similarly, the
      second element of the list refers to the first capturing parenthesis in
      the regular expression that matched, and so on. When a default
      branch is taken, the variable will have the empty list written to it. This
      option may be specified at the same time as the -matchvar
    option.

  
--

  
Marks the end of options. The argument following this one will be treated
      as string even if it starts with a -. This is not required
      when the matching patterns and bodies are grouped together in a single
      argument.




Two syntaxes are provided for the pattern and body
    arguments. The first uses a separate argument for each of the patterns and
    commands; this form is convenient if substitutions are desired on some of
    the patterns or commands. The second form places all of the patterns and
    commands together into a single argument; the argument must have proper list
    structure, with the elements of the list being the patterns and commands.
    The second form makes it easy to construct multi-line switch commands, since
    the braces around the whole list make it unnecessary to include a backslash
    at the end of each line. Since the pattern arguments are in braces in
    the second form, no command or variable substitutions are performed on them;
    this makes the behavior of the second form different than the first form in
    some cases.


If a body is specified as “-” it means
    that the body for the next pattern should also be used as the body
    for this pattern (if the next pattern also has a body of
    “-” then the body after that is used, and so on). This
    feature makes it possible to share a single body among several
    patterns.


Beware of how you place comments in switch commands.
    Comments should only be placed inside the execution body of one of
    the patterns, and not intermingled with the patterns.








The switch command can match against variables and not just
    literals, as shown here (the result is 2):






set foo "abc"
switch abc a - b {expr {1}} $foo {expr {2}} default {expr {3}}




Using glob matching and the fall-through body is an alternative to
    writing regular expressions with alternations, as can be seen here (this
    returns 1):






switch -glob aaab {


    a*b     -


    b       {expr {1}}


    a*      {expr {2}}


    default {expr {3}}
}




Whenever nothing matches, the default clause (which must be
    last) is taken. This example has a result of 3:






switch xyz {


    a -


    b {


        # Correct Comment Placement


        expr {1}


    }


    c {


        expr {2}


    }


    default {


        expr {3}


    }
}




When matching against regular expressions, information about what
    exactly matched is easily obtained using the -matchvar option:






switch -regexp -matchvar foo -- $bar {


    a(b*)c {


        puts "Found [string length [lindex $foo 1]] 'b's"


    }


    d(e*)f(g*)h {


        puts "Found [string length [lindex $foo 1]] 'e's and\


                [string length [lindex $foo 2]] 'g's"


    }
}










for(3tcl), if(3tcl), regexp(3tcl)








switch, match, regular expression







  

    8.5
    Tcl