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 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:

EXAMPLES

       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"
              }

SEE ALSO

       break(3tcl), continue(3tcl), foreach(3tcl), while(3tcl)

KEYWORDS

       boolean, for, iteration, loop