In Tcl, the use of square brackets tells the interpreter to "do this first" whereas curly braces are a request to defer execution of a block - perhaps suppressing it completely as in the action on an if statement.

So the if statement's action will NEVER be written in square brackets - [ ] - as that would cause it to be performed irrespective of the actual condition and would defeat the whole purpose of the condition! But it's a different matter with the condition itself. Have a look at this code:

set cost [gets stdin]
 
if [expr $cost > 20] {
    set gpt [expr $cost * 0.9]
    puts "Group ticket at $gpt"
 }
if {$cost > 20} {
    set gpt [expr $cost * 0.9]
    puts "Group ticket at $gpt"
 }
if {[expr $cost > 20]} {
    set gpt [expr $cost * 0.9]
    puts "Group ticket at $gpt"
 }

All three statements produce the same end result, but they do it differently.

The first - using [ ] - evaluates the expression before it ever runs the if comment, to which is passed just a true or false value and not the condition itself. This works well for an if but should never be used in a while, since the condition would never be rechecked and you would have an infinite loop.

The second - using { } - passes the string containing into if which assumes an expr command is to be used to evaluate it. You would usually want it to be expr-ed, so most of the time thats a good and fair assumption.

Using { } and the [ ] with it gives you the deferred block that you'll need when giving a condition for the while, but then also gives you the ability to embed any command - not JUST an expr - with it. And this is the structure you'll need to use for more complex while loops.

(written 2007-10-23, updated 2011-03-01)

 T203 - Tcl/Tk - Conditionals and Loops
  [210] Joining lists in Tcl. Indirect variables in Tcl. - (2005-02-12)
  [1477] Decisions - small ones, or big ones? - (2007-12-18)
  [1696] Saying NOT in Perl, PHP, Python, Lua ... - (2008-07-04)
  [2261] Tcl - nice and nasty - (2009-06-29)
  [2471] A short form of if ... then ... else - (2009-10-23)
  [2681] Tcl - a great engineering language - (2010-03-17)
  [3189] Tcl - the danger of square brackets in a while command - (2011-03-02)
  [3397] Does a for loop evaluate its end condition once, or on every iteration? - (2011-08-18)
  [3570] Trapping errors in Tcl - the safety net that catch provides - (2012-01-06)
  [3571] Comparing loop commands in Tcl - (2012-01-06)
  [4322] Learning to Program - the conditional statement (if) - (2014-11-21)
  [4323] Learning to program - Loop statements such as while - (2014-11-22)
  [4455] Working out distance between places, using OS grid references and a program in Tcl - (2015-03-11)

Back to Dark Dawn

Previous and next or Horse's mouth home

Forward to Tcl - append v lappend v concat

Sorting in Tcl - lists and arrays
Tcl - global, upvar and uplevel.
Square Bracket protection in Tcl
Tcl - append v lappend v concat
Tcl - using [] or {} for conditions in an if (and while)
Dark Dawn
Pictures Framed
Someone else's wedding
Perl - progress bar, supressing ^C and coping with huge data flows
Using PHP to upload images / Store on MySQL database - security questions