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Updated: Thursday, June 13, 2019
 
 

switch (1t)

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

switch(1t)                   Tcl Built-In Commands                  switch(1t)



______________________________________________________________________________

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 pat-
       tern arguments in order.  As soon as it finds a  pattern  that  matches
       string  it  evaluates  the following body argument by passing it recur-
       sively to the Tcl interpreter and returns the result  of  that  evalua-
       tion.   If  the  last  pattern argument is default then it matches any-
       thing.  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 man-
                 ner.

       -matchvar varName
                 This option (only legal when -regexp is also specified) spec-
                 ifies  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 sub-
                 string 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  writ-
                 ten  to it.  This option may be specified at the same time as
                 the -indexvar option.

       -indexvar varName
                 This option (only legal when -regexp is also specified) spec-
                 ifies  the  name of a variable into which the list of indices
                 referring to matching substrings found by the regular expres-
                 sion  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  ele-
                 ment 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  argu-
       ments  are in braces in the second form, no command or variable substi-
       tutions 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 pat-
       tern should also be used as the body for this pattern (if the next pat-
       tern  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  sev-
       eral 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.

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       {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"
                  }
              }


ATTRIBUTES
       See attributes(7) for descriptions of the following attributes:


       +---------------+------------------+
       |ATTRIBUTE TYPE | ATTRIBUTE VALUE  |
       +---------------+------------------+
       |Availability   | runtime/tcl-8    |
       +---------------+------------------+
       |Stability      | Uncommitted      |
       +---------------+------------------+
SEE ALSO
       for(n), if(n), regexp(n)

KEYWORDS
       switch, match, regular expression



NOTES
       This    software    was    built    from    source     available     at
       https://github.com/oracle/solaris-userland.    The  original  community
       source was downloaded from  http://prdownloads.sourceforge.net/tcl/tcl-
       core8.6.7-src.tar.gz

       Further information about this software can be found on the open source
       community website at https://www.tcl.tk/.



Tcl                                   8.5                           switch(1t)