for - "For" loop

'''for''' ''start test next body''
'''for''' ''start test next body''
http://www.purl.org/tcl/home/man/tcl8.4/TclCmd/for.htm

'''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 [expr]ession 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. 
   [http://www.tcl.tk/man/tcl/TclCmd/for.htm%|%official reference]:   
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. For an example, try the following script with and without the braces around '''$x<10''': 

 for {set x 0} {$x<10} {incr x} {
   puts "x is $x"
 }

(From: [TclHelp])
See also: [TclHelp]
----

The bracing of ''test'' will result in '''expr''' parsing the contents. But in braced expressions, '''expr''' does not tolerate operators to be passed in as variables. If you want to do that, call an explicit '''[expr]''' without braced condition (but inside brackets and braces, as explained above), so the Tcl parser substitutes that in each loop repetition:

 set op "<"
 for {set x 0} {[expr $x $op 10]} {incr x} {puts "x is $x"} ;#RS

----

In some situations, using a '''[foreach]''' with a fixed list is more convenient than a '''for''', compare:

 foreach i {1 2 3 4 5} {...
 for {set i 1} {$i <= 5} {incr i} {...
or, instead of unrolling a range, you can wrap one ''for'' for sugaring:
 proc range {from "to:" to} {
        set res [list]
        for {set i $from} {$i<=$to} {incr i} {lappend res $i}
        set res
 }
 foreach i [range 1 .. 5] {...
This is, however, slower than `for`.


Foreach generally is better style in non-numeric looping.  Those accustomed to writing in C sometimes find this a difficult habit to learn, because they think of an indexed array where a Tcl coder writes simply

 set color_list "red blue umber mauve"
 foreach color $color_list {...}

---

See also:

   * [break]
   * [continue]
   * [expr]
   * [foreach]
   * [if]
   * [while]

----

[Tcl syntax help] -
[Arts and Crafts of Tcl-Tk Programming] -
[Category Command] from [Tcl]



[overlay]