- NAME
- switch — Evaluate one of several scripts, depending on a given value
- SYNOPSIS
- DESCRIPTION
- -exact
- -glob
- -regexp
- -nocase
- -matchvar varName
- -indexvar varName
- --
- EXAMPLES
- SEE ALSO
- KEYWORDS
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:
- -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 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, if, regexp
switch, match, regular expression
Copyright © 1993 The Regents of the University of California.
Copyright © 1994-1997 Sun Microsystems, Inc.