Practical Programming in Tcl & Tk, Third Edition

If Then Else

The if command is the basic conditional command. If an expression is true, then execute one command body; otherwise, execute another command body. The second command body (the else clause) is optional. The syntax of the command is:

if expression ?then? body1 ?else? ?body2?

The then and else keywords are optional. In practice, I omit then but use else as illustrated in the next example. I always use braces around the command bodies, even in the simplest cases:

Example 6-1 A conditional if then else command.

if {$x == 0} { puts stderr "Divide by zero!" } else { set slope [expr $y/$x] }

Curly brace positioning is important.

The ...