1switch(n) Tcl Built-In Commands switch(n)
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8 switch - Evaluate one of several scripts, depending on a given value
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11 switch ?options? string pattern body ?pattern body ...?
12 switch ?options? string {pattern body ?pattern body ...?}
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17 The switch command matches its string argument against each of the pat‐
18 tern arguments in order. As soon as it finds a pattern that matches
19 string it evaluates the following body argument by passing it recur‐
20 sively to the Tcl interpreter and returns the result of that evalua‐
21 tion. If the last pattern argument is default then it matches any‐
22 thing. If no pattern argument matches string and no default is given,
23 then the switch command returns an empty string.
24 If the initial arguments to switch start with - then they are treated
26 as options unless there are exactly two arguments to switch (in which │
27 case the first must the string and the second must be the pattern/body │
28 list). The following options are currently supported:
29 -exact Use exact matching when comparing string to a pattern. This
31 is the default.
32 -glob When matching string to the patterns, use glob-style matching
34 (i.e. the same as implemented by the string match command).
35 -regexp When matching string to the patterns, use regular expression
37 matching (as described in the re_syntax reference page). │
38 -nocase │
40 Causes comparisons to be handled in a case-insensitive man‐ │
41 ner. │
42 -matchvar varName │
44 This option (only legal when -regexp is also specified) spec‐ │
45 ifies the name of a variable into which the list of matches │
46 found by the regular expression engine will be written. The │
47 first element of the list written will be the overall sub‐ │
48 string of the input string (i.e. the string argument to │
49 switch) matched, the second element of the list will be the │
50 substring matched by the first capturing parenthesis in the │
51 regular expression that matched, and so on. When a default │
52 branch is taken, the variable will have the empty list writ‐ │
53 ten to it. This option may be specified at the same time as │
54 the -indexvar option. │
55 -indexvar varName │
57 This option (only legal when -regexp is also specified) spec‐ │
58 ifies the name of a variable into which the list of indices │
59 referring to matching substrings found by the regular expres‐ │
60 sion engine will be written. The first element of the list │
61 written will be a two-element list specifying the index of │
62 the start and index of the first character after the end of │
63 the overall substring of the input string (i.e. the string │
64 argument to switch) matched, in a similar way to the -indices │
65 option to the regexp can obtain. Similarly, the second ele‐ │
66 ment of the list refers to the first capturing parenthesis in │
67 the regular expression that matched, and so on. When a │
68 default branch is taken, the variable will have the empty │
69 list written to it. This option may be specified at the same │
70 time as the -matchvar option.
71 -- Marks the end of options. The argument following this one
73 will be treated as string even if it starts with a -. This │
74 is not required when the matching patterns and bodies are │
75 grouped together in a single argument.
76 Two syntaxes are provided for the pattern and body arguments. The
78 first uses a separate argument for each of the patterns and commands;
79 this form is convenient if substitutions are desired on some of the
80 patterns or commands. The second form places all of the patterns and
81 commands together into a single argument; the argument must have proper
82 list structure, with the elements of the list being the patterns and
83 commands. The second form makes it easy to construct multi-line switch
84 commands, since the braces around the whole list make it unnecessary to
85 include a backslash at the end of each line. Since the pattern argu‐
86 ments are in braces in the second form, no command or variable substi‐
87 tutions are performed on them; this makes the behavior of the second
88 form different than the first form in some cases.
89 If a body is specified as “-” it means that the body for the next pat‐
91 tern should also be used as the body for this pattern (if the next pat‐
92 tern also has a body of “-” then the body after that is used, and so
93 on). This feature makes it possible to share a single body among sev‐
94 eral patterns.
95 Beware of how you place comments in switch commands. Comments should
97 only be placed inside the execution body of one of the patterns, and
98 not intermingled with the patterns.
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101 The switch command can match against variables and not just literals,
102 as shown here (the result is 2):
103 set foo "abc"
104 switch abc a - b {expr {1}} $foo {expr {2}} default {expr {3}}
105 Using glob matching and the fall-through body is an alternative to
107 writing regular expressions with alternations, as can be seen here
108 (this returns 1):
109 switch -glob aaab {
110 a*b -
111 b {expr {1}}
112 a* {expr {2}}
113 default {expr {3}}
114 }
115 Whenever nothing matches, the default clause (which must be last) is
117 taken. This example has a result of 3:
118 switch xyz {
119 a -
120 b {
121 # Correct Comment Placement
122 expr {1}
123 }
124 c {
125 expr {2}
126 }
127 default {
128 expr {3}
129 }
130 }
131 When matching against regular expressions, information about what │
133 exactly matched is easily obtained using the -matchvar option: │
134 switch -regexp -matchvar foo -- $bar { │
135 a(b*)c { │
136 puts "Found [string length [lindex $foo 1]] 'b's" │
137 } │
138 d(e*)f(g*)h { │
139 puts "Found [string length [lindex $foo 1]] 'e's and\ │
140 [string length [lindex $foo 2]] 'g's" │
141 } │
142 } │
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145 for(n), if(n), regexp(n)
146
148 switch, match, regular expression
149Tcl 8.5 switch(n)