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:
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(3tcl),
continue(3tcl),
foreach(3tcl),
while(3tcl)
boolean, for, iteration, loop