10-06-2008
Quote:
Originally Posted by
redstone
No, that's why I'm posting here. If you take look any commercial scripts, you will see that a lot in while, if statements.
I've never seen it used in while or if statments. I've seen it in test statements ([ is synonymous with test) and in arguments to expr.
When I have seen it, it has never been used the way you show it; the X is placed before the variable or string.
It is done to avoid ambiguity (e.g., when the value begins with a hyphen) or an empty string.
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for(3tcl) Tcl Built-In Commands for(3tcl)
__________________________________________________________________________________________________________________________________________________
NAME
for - 'For' loop
SYNOPSIS
for start test next body
_________________________________________________________________
DESCRIPTION
For is a looping command, similar in structure to the C for statement. The start, next, and body arguments must be Tcl command strings,
and test is an expression string. The for command first invokes the Tcl interpreter to execute start. Then it repeatedly evaluates test
as an expression; if the result is non-zero it invokes the Tcl interpreter on body, then invokes the Tcl interpreter on next, then repeats
the loop. The command terminates when test evaluates to 0. If a continue command is invoked within body then any remaining commands in
the current execution of body are skipped; processing continues by invoking the Tcl interpreter on next, then evaluating test, and so on.
If a break command is invoked within body or next, then the for command will return immediately. The operation of break and continue are
similar to the corresponding statements in C. For returns an empty string.
Note: test should almost always be enclosed in braces. If not, variable substitutions will be made before the for command starts execut-
ing, which means that variable changes made by the loop body will not be considered in the expression. This is likely to result in an
infinite loop. If test is enclosed in braces, variable substitutions are delayed until the expression is evaluated (before each loop iter-
ation), so changes in the variables will be visible. See below for an example:
EXAMPLES
Print a line for each of the integers from 0 to 10:
for {set x 0} {$x<10} {incr x} {
puts "x is $x"
}
Either loop infinitely or not at all because the expression being evaluated is actually the constant, or even generate an error! The
actual behaviour will depend on whether the variable x exists before the for command is run and whether its value is a value that is less
than or greater than/equal to ten, and this is because the expression will be substituted before the for command is executed.
for {set x 0} $x<10 {incr x} {
puts "x is $x"
}
Print out the powers of two from 1 to 1024:
for {set x 1} {$x<=1024} {set x [expr {$x * 2}]} {
puts "x is $x"
}
SEE ALSO
break, continue, foreach, while
KEYWORDS
for, iteration, looping
Tcl for(3tcl)