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man pages section 1: User Commands

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Updated: Thursday, June 13, 2019

for (1t)


for - 'For' loop


for start test next body


for(1t)                      Tcl Built-In Commands                     for(1t)


       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  exe-
       cution  of  body  are skipped; processing continues by invoking the Tcl
       interpreter on next, then evaluating test, and so on.  If a break  com-
       mand  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, vari-
       able 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  infi-
       nite  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 eval-
       uated 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 attributes(7) for descriptions of the following attributes:

       |Availability   | runtime/tcl-8    |
       |Stability      | Uncommitted      |
       break(n), continue(n), foreach(n), while(n)

       boolean, for, iteration, loop

       This    software    was    built    from    source     available     at
       https://github.com/oracle/solaris-userland.    The  original  community
       source was downloaded from  http://prdownloads.sourceforge.net/tcl/tcl-

       Further information about this software can be found on the open source
       community website at https://www.tcl.tk/.

Tcl                                                                    for(1t)