switch string pattern1 body1 ?pattern2 body2?...?patternN bodyN?
- or -
switch string { pattern1 body1 ?pattern2 body2?...?patternN bodyN? }
The switch command compares the argument string to each of the pattern arguments in order. When a pattern matches stringbody argument is evaluated by the Tcl interpreter and the result of that evaluation is returned as the return of the switch command.
The switch command stops comparing string and pattern arguments as soon as it finds one that matches. It will match only one pattern to a string.
If the last pattern argument to the switch command is the string default then it will match any string that was not previously matched. If no pattern argument matches the string and no default is given, then the switch command returns an empty string.
:TEXT_END:
:LESSON_TEXT_START_LEVEL 1: The switch command allows you to choose one of several options in your code based on the contents of a string. The string will be compared to a set of patterns, and when a pattern matches the string, the code associated with that pattern will be evaluated.
The syntax of the command is:
switch string pattern1 body1 ?pattern2 body2?...?patternN bodyN?
- or -
switch string { pattern1 body1 ?pattern2 body2?...?patternN bodyN? }
String is the string that you wish to test, and pattern1, pattern2, etc are the patterns that the string will be compared to. If string matches a pattern, then the code within the body associated with that pattern will be executed. The return from the body will be returned as the return of the switch statement, and the switch command will be completed. Only one pattern will be matched.
If the last pattern argument is the string default, that pattern will match to any string. This guarantees that some set of code will be executed no matter what the contents of string are.
If there is no default argument, and none of the patterns match string, then the switch command will return an empty string.
If you use the brace version of this command, there will be no substitutions done on the patterns. The body of the command, however, will be parsed and evaluated just like any other command, so there will be a pass of substitutions done on that, just as will be done in the first syntax. The advantage of the second form is that you can write multiple line commands more readably with the brackets.
Note that you can use braces to group the body argument when using the switch or if commands. This is because these commands pass their body argument to the Tcl interpreter for evaluation. This evaluation includes a pass of substitutions just as it does for code not within a command body argument.
If you haven't already run the example, do so, then try changing the
string assigned to x from
ONE to TWO and running it again.
:TEXT_END:
:LESSON_TEXT_START_LEVEL 2: When you are writing a program you sometimes want to choose one set of code to execute instead of another. One of the commands you can use to select which set of code to execute is the switch command.
The switch command allows you to choose one of several options in your code based on a string. The string will be compared to a set of patterns, and when the string matches a pattern, the code associated with that pattern will be evaluated.
The syntax of the switch command is:
switch string pattern1 body1 ?pattern2 body2?...?patternN bodyN?
- or -
switch string { pattern1 body1 ?pattern2 body2?...?patternN bodyN? }
String is the string that you wish to compare to various patterns, and pattern1 is the first pattern that the string will be compared to. If string matches pattern1, then the code within body1 will be executed, and the value returned by body1 will be the return of the switch command.
If pattern1 does not match string, then pattern2 will be checked, and so forth until either a pattern matches the string, or there are no more patterns. When a pattern matches string, then the switch command is complete, and the next line is executed. Only one pattern in a switch command will be matched.
If the last pattern argument is the string default, that pattern will match to any string. This guarantees that some set of code will be executed no matter what the contents of string are.
If there is no default argument, and none of the patterns match string, then the switch command will return an empty string.
The switch command comes in two flavors. Every pattern and body can be passed to the command as a separate argument (the first form), or they can all be grouped within brackets into one argument. If they are grouped into a single unit, the switch command will separate them into patterns and bodies, but it will not perform any variable substitutions on the patterns.
When a pattern matches the string the body argument will be evaluated by the Tcl interpreter just like any other line of code. If the body is grouped with braces, the braces will be stripped off before the evaluation occurs, allowing substitution to occur.
If you haven't already run the example, do so, then try changing the
string assigned to x from
ONE to TWO and running it again.
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:CODE_START:
;# Set the variables we'll be comparing
set x "ONE";
set y 1;
set z "ONE";
;# This is legal
switch $x "ONE" "puts ONE=1" "TWO" "puts TWO=2" "default" "puts NO_MATCH"
;# This is more readable, the patterns are to the left, and the body
;# associated with each pattern is to its right.
switch $x \
"ONE" "puts ONE=1" \
"TWO" "puts TWO=2" \
"default" "puts NO_MATCH";
;# These two examples show the difference between using the un-braced and
;# braced versions of the switch command.
;# You can also see how a switch command can become too complex to
;# be written on a single line.
;# This form allows substitution in the pattern arguments.
switch $x \
"$z" {set y1 [expr $y+1]; puts "MATCH \$z. $y + $z is $y1" } \
"ONE" {set y1 [expr $y+1]; puts "MATCH ONE. $y + one is $y1"} \
"TWO" {set y1 [expr $y+2]; puts "MATCH TWO. $y + two is $y1" } \
"THREE" {set y1 [expr $y+3]; puts "MATCH THREE. $y + three is $y1" } \
"default" {puts "$x does not match any of these choices";}
;# This form of the command disables variable substitution in the pattern
switch $x {
"$z" {set y1 [expr $y+1]; puts "MATCH \$z. $y + $z is $y1" }
"ONE" {set y1 [expr $y+1]; puts "MATCH ONE. $y + one is $y1"}
"TWO" {set y1 [expr $y+2]; puts "MATCH TWO. $y + two is $y1"}
"THREE" {set y1 [expr $y+3]; puts "MATCH THREE. $y + three is $y1"}
"default" {puts "$x is NOT A MATCH"}
}
:TEXT_END: