1for(n) Tcl Built-In Commands for(n)
2______________________________________________________________________________
6
8 for - ``For'' loop
9
11 for start test next body
12_________________________________________________________________
13
16 For is a looping command, similar in structure to the C for statement.
17 The start, next, and body arguments must be Tcl command strings, and
18 test is an expression string. The for command first invokes the Tcl
19 interpreter to execute start. Then it repeatedly evaluates test as an
20 expression; if the result is non-zero it invokes the Tcl interpreter on
21 body, then invokes the Tcl interpreter on next, then repeats the loop.
22 The command terminates when test evaluates to 0. If a continue command
23 is invoked within body then any remaining commands in the current exe‐
24 cution of body are skipped; processing continues by invoking the Tcl
25 interpreter on next, then evaluating test, and so on. If a break com‐
26 mand is invoked within body or next, then the for command will return
27 immediately. The operation of break and continue are similar to the
28 corresponding statements in C. For returns an empty string.
29 Note: test should almost always be enclosed in braces. If not, vari‐
31 able substitutions will be made before the for command starts execut‐
32 ing, which means that variable changes made by the loop body will not
33 be considered in the expression. This is likely to result in an infi‐
34 nite loop. If test is enclosed in braces, variable substitutions are
35 delayed until the expression is evaluated (before each loop iteration),
36 so changes in the variables will be visible. See below for an example:
37
39 Print a line for each of the integers from 0 to 10:
40 for {set x 0} {$x<10} {incr x} {
41 puts "x is $x"
42 }
43 Either loop infinitely or not at all because the expression being eval‐
45 uated is actually the constant, or even generate an error! The actual
46 behaviour will depend on whether the variable x exists before the for
47 command is run and whether its value is a value that is less than or
48 greater than/equal to ten, and this is because the expression will be
49 substituted before the for command is executed.
50 for {set x 0} $x<10 {incr x} {
51 puts "x is $x"
52 }
53 Print out the powers of two from 1 to 1024:
55 for {set x 1} {$x<=1024} {set x [expr {$x * 2}]} {
56 puts "x is $x"
57 }
58
61 break, continue, foreach, while
62
65 for, iteration, looping
66Tcl for(n)