for — 'For' loop
for start test next body
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 that test should almost always be enclosed in
braces. If not, variable substitutions will be made before the
for command starts executing, 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
iteration), so changes in the variables will be visible. See below
for an example:
Print a line for each of the integers from 0 to 9:
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"
}
break, continue, foreach, while
boolean, for, iteration, loop
Copyright © 1993 The Regents of the University of
California.
Copyright © 1994-1997 Sun Microsystems, Inc.