Tcl Tutorial Lesson 7

Numeric Comparisons 101 - if

Like most languages, Tcl supports an if command. The syntax is:

    if {expr1} ?then? {
        body1
    } elseif {expr2} ?then? {
        body2
    } elseif {
        ...
    } ?else {
        bodyN
    }?

The words then and else are optional, although usually then is left out and else is used.

The test expression following if should return a value that can be interpreted as representing "true" or "false":

FalseTrue
a numeric value0all others
yes/nonoyes
true/falsefalsetrue

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.

The test expression following if should be enclosed within braces. This causes the expression to be evaluated within the if command.

Note: as was explained in the discussion of the expr command, you should always use braces around expressions.


Example

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"
}

  Resulting output
1 is not 2
1 is 1