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switch(n)                                   Tcl Built-In Commands                                  switch(n)



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

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

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

DESCRIPTION
       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:

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

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

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

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

       -matchvar var-                                                                                        |
       Name                                                                                     |            |
                 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  sub- |
                 string  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.              |

       -indexvar var-                                                                                        |
       Name                                                                                     |            |
                 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 expres- |
                 sion  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 substitu-tions substitutions
       tions 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 exe-cution execution
       cution body of one of the patterns, and not intermingled with the patterns.

EXAMPLES
       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 a*bb
                 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 ab
                 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"                                              |
                 }                                                                                           |
              }                                                                                              |

SEE ALSO
       for(n), if(n), regexp(n)

KEYWORDS
       switch, match, regular expression



Tcl                                                  8.5                                           switch(n)