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

__________________________________________________________________________________________________________________________________________________

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 varName															   |
		 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.				   |

       -indexvar varName															   |
		 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 unnec-
       essary to include a backslash at the end of each line.  Since the pattern arguments are in braces in the second form, no command  or  vari-
       able 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.

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" 										   |
		 }																   |
	      } 																   |

SEE ALSO

       for(n), if(n), regexp(n)

KEYWORDS

       switch, match, regular expression

Tcl									8.5								 switch(n)