loop

Tclx documents the loop command as:

loop var first limit ?increment? body

Loop is a looping command, similar in behavior to the Tcl for statement, except that the loop statement achieves substantially higher performance and is easier to code when the beginning and ending values of a loop are known, and the loop variable is to be incremented by a known, fixed amount every time through the loop.

The var argument is the name of a Tcl variable that will contain the loop index. The loop index is set to the value specified by first. The Tcl interpreter is invoked upon body zero or more times, where var is incremented by increment every time through the loop, or by one if increment is not specified. Increment can be negative in which case the loop will count downwards.

When var reaches limit, the loop terminates without a subsequent execution of body. For instance, if the original loop parameters would cause loop to terminate, say first was one, limit was zero and increment was not specified or was non-negative, body is not executed at all and loop returns.

The first, limit and increment are integer expressions. They are only evaluated once at the beginning of the loop.

If a continue command is invoked within body then any remaining commands in the current execution of body are skipped, as in the for command. If a break command is invoked within body then the loop command will return immediately. Loop returns an empty string.

 proc loop {_var first limit args} {
    switch [llength $args] {
        1 {set body [lindex $args 0]; set increment 1}
        2 {foreach {increment body} $args break}
        default {error "usage: loop var first limit ?increment? body"}
    }
    upvar 1 $_var var
    set limit     [expr $limit]
    set increment [expr $increment]
    for {set var [expr $first]} {$var<=$limit} {incr var $increment} {
        uplevel 1 $body
    }
 }