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switch(n) Tcl Built-In Commands switch(n)
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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 ...?}
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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 pat-
tern 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 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 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 pat- |
terns 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 pat-
terns 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 pat-
tern 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 expres-
sions 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 eas- |
ily 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)
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