distrib/share/tcl/help/tcl/control/while

   1 NAME
   2        while - Execute script repeatedly as long as a condition is met
   3
   4 SYNOPSIS
   5        while test body
   6
   7
   8 DESCRIPTION
   9        The while command evaluates test as an expression (in the same way that
  10        expr evaluates its argument).  The  value  of  the  expression  must  a
  11        proper  boolean  value;  if it is a true value then body is executed by
  12        passing it to the Tcl interpreter.  Once body has  been  executed  then
  13        test  is evaluated again, and the process repeats until eventually test
  14        evaluates to a false boolean value.  Continue commands may be  executed
  15        inside  body  to terminate the current iteration of the loop, and break
  16        commands may be executed inside body to cause immediate termination  of
  17        the while command.  The while command always returns an empty string.
  18
  19        Note:  test  should almost always be enclosed in braces.  If not, vari-
  20        able substitutions will be made before the while command starts execut-
  21        ing,  which  means that variable changes made by the loop body will not
  22        be considered in the expression.  This is likely to result in an  infi-
  23        nite  loop.   If test is enclosed in braces, variable substitutions are
  24        delayed until the expression is evaluated (before each loop iteration),
  25        so  changes  in the variables will be visible.  For an example, try the
  26        following script with and without the braces around $x<10:
  27               set x 0
  28               while {$x<10} {
  29                   puts "x is $x"
  30                   incr x
  31               }
  32
  33 EXAMPLE
  34        Read lines from a channel until we get to the end of  the  stream,  and
  35        print them out with a line-number prepended:
  36               set lineCount 0
  37               while {[gets $chan line] >= 0} {
  38                   puts "[incr lineCount]: $line"
  39               }
  40
  41
  42 SEE ALSO
  43        break(n), continue(n), for(n), foreach(n)
  44
  45
  46 KEYWORDS