Cover by Don Libes

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Control Structures

Control structures change the flow of control. Many of the control structures in Tcl are patterned directly after their C equivalents. Tcl gives you the power to write your own control structures, so if you do not like those of C, you may yet find happiness. I will not describe how to do it, but it is surprisingly easy. (The hard part is designing something that makes sense.)

The while Command

The while command loops until an expression is false. It looks very similar to a while in the C language. The following while loop computes the factorial of the number stored in the variable count.

set total 1

while {$count > 0} {
    set total [expr $total * $count]
    set count [expr $count−1]
}

The body of the loop is composed of the two set commands between the braces. The body is executed as long as $count is greater than 0.

Taking a step back, the while command has two arguments. The first is the controlling expression. The second is the body. Notice that both arguments are enclosed in braces. That means that no $ substitutions or bracket evaluations are made. For instance, this while command literally gets the string "$count > 0" as its first argument. Similarly, for the body. So how does anything happen?

The answer is that the while command itself evaluates the expression. If true (nonzero), the while command evaluates the body. The while command then re-evaluates the expression. As long as the expression keeps re-evaluating to a nonzero value, the while command keeps re-evaluating ...

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