The words then and else are optional, and may be included if you
think they improve the readability of the code, or left out, as you prefer.
The test expression following the word if is evaluated in the same manner
as in the expr command. Hex strings 0xXX will be converted to their numeric
equivalent before evaluation.
The test expression following if should return one of:
False True
------- ------
a numeric value 0 all others
yes/no no yes
true/false false true
The case of a textual return is not checked.
The test expression following the word if is evaluated in
the same manner as in the expr command. Hex strings 0xXX will be
converted to their numeric equivalent before evaluation.
The test expression following if may be enclosed within
quotes, or braces. If it is enclosed within braces, it will be
evaluated within the if command, and if enclosed within quotes it will
be evaluated during the substitution phase, and then another round of
substitutions will be done within the if command.
:TEXT_END:
:LESSON_TEXT_START_LEVEL 1: Like most languages, Tcl supports an if command. The syntax is:
The words then and else are optional, and may be included if you
think they improve the readability or left out. If you are used to
programming in a shell language, you may want to leave them in. If you
are used to C, you will prefer leaving them out.
The test expression following if should return one of:
False True
------- ------
a numeric value 0 all others
yes/no no yes
true/false false true
If the test expression returns a string "yes"/"no" or "true"/"false", the
case of the return is not checked. True/FALSE or YeS/nO are legitimate
returns.
If the test expression evaluates to True, then body1 will be executed.
If the test expression evaluates to False, then the word after
body1 will be examined. If the next word is
elseif, then the next test expression will be tested as a
condition. If the next word is else then the final body
will be evaluated as a command.
The test expression following the word if is evaluated in the same manner
as in the expr command. Hex strings 0xXX will be converted to their numeric
equivalent before evaluation.
The test expression following if may be enclosed within
quotes, or braces. If it is enclosed within braces, it will be
evaluated within the if command, and if enclosed within quotes it will
be evaluated during the substitution phase, and then another round of
substitutions will be done within the if command.
:TEXT_END:
:LESSON_TEXT_START_LEVEL 2: The switch command allows you to compare a string to other strings and find a match. Another feature of most computer languages is the ability to choose between two courses of action based on a numeric test. Like most other languages, Tcl supports this functionality with the if command.
The syntax for the if command is:
The words then and else are optional, you
may include them if you
think they improve the readability of your code, or leave them out if you
think that looks better.
When an if command is encountered the Tcl interpreter
evaluates test1 using the expr command. If that test is
true, the Tcl interpreter executes body1. If the test expression
evaluates to
False, then the word after body1 will be examined. If the
next word is elseif, then the next expression will be
evaluated as a test condition. If the next word is else then the
final body will be evaluated as a command.
The test expression following if should return one of
these values:
False True
------- ------
a numeric value 0 all others
yes/no no yes
true/false false true
If the test expression returns a character string like "yes", "no", "true"
or "false", the case of the return is not checked. "True", "FALSE",
"YeS" or "nO" are legitimate returns.
The test following if may be enclosed within quotes, or
braces. If it is enclosed within braces, it will be parsed by expr
within the if command. If the test is enclosed within quotes
substitutions will be done during the Tcl parsing substitution phase,
and then another round of substitutions will be done within the if
command.
Since the test in an if is evaluated in the same manner
as in the expr command, hex strings 0xXX will be converted to their
numeric equivalent before evaluation. You can think of the test in
the if command as being executed by expr, and all the same rules
will apply.
if {$i == 1} {puts "MATCH}
is the same as:
if {[expr $i == 1]} {puts "MATCH}
The body code may be enclosed within quotes or braces. Since body will be evaluated just like any other Tcl command, body can be placed within braces and variable substitution will still occur. Note that the opening quote or bracket must be on the same line as the if command, however.
:TEXT_END: :CODE_START: set x 1; if {$x == 2} {puts "$x is 2"} else {puts "$x is not 2"} if {$x != 1} { puts "$x is != 1" } else { puts "$x is 1" } if $x==1 {puts "GOT 1"} ;# Two passes of substitution will be done on this test string ;# The initial pass replaces $y with x ;# Within the if statement, $x is replaced with 1 set y x; if "$$y != 1" { puts "$$y is != 1" } else { puts "$$y is 1" } :TEXT_END: