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(3tcl),
if(3tcl),
regexp(3tcl)
switch, match, regular expression