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. 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).
- --
- Marks the end of options. The argument following this one
will be treated as string even if it starts with a -.
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.
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
}
}
SEE ALSO¶
for(3tcl), if(3tcl), regexp(3tcl)
KEYWORDS¶
switch, match, regular expression