[Tcl-bugs] [ tcl-Bugs-410043 ] "if" does not evaluate same as "expr"

[<no...@so...>]

Bugs item #410043, was updated on 2001-03-20 08:17
You can respond by visiting: 
http://sourceforge.net/tracker/?func=detail&atid=110894&aid=410043&group_id=10894

Category: Parsing and Eval
Group: 8.3
>Status: Closed
Priority: 5
Submitted By: Nobody/Anonymous (nobody)
Assigned to: Nobody/Anonymous (nobody)
>Summary: "if" does not evaluate same as "expr"

Initial Comment:
The following script produces strange output.

Script:
#-----------------------------------------------
set vmin 1.3
set vmax 1.7
set idx  4
set incr 0.1

set newval [expr {$vmin+int($idx)*$incr}]

if { $newval > $vmax } {
    puts "Bare 'if' says $newval > $vmax"
} else {
    puts "Bare 'if' says $newval <= $vmax"
}

if { [expr $newval > $vmax] } {
    puts "'if' with expr says $newval > $vmax"
} else {
    puts "'if' with expr says $newval <= $vmax"
}
#-----------------------------------------------

Output:
Bare 'if' says 1.7 > 1.7
'if' with expr says 1.7 <= 1.7

As you see, the "if" command returns a different answer
depending on whether or not you use expr.

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>Comment By: Donal K. Fellows (dkf)
Date: 2001-03-20 08:43

Message:
Logged In: YES 
user_id=79902

This problem (a known feature of Tcl that is unlikely to
ever be altered for a host of deep reasons) stems from the
fact that 0.1 is not a precisely representable floating
point number and that arguments to [expr] are [concat]enated
before evaluation. Rerun the script with tcl_precision set
to 17, or with the expression to pass to [expr] in {braces},
and you'll get agreement between the "bare if" and the "if
with expr".

If this matters very much, use fixed-point arithmetic
instead.

----------------------------------------------------------------------

Comment By: Nobody/Anonymous (nobody)
Date: 2001-03-20 08:23

Message:
Logged In: NO 

The problem seems to be that the arguments to [expr] aren't braced.  I added:

if { [expr {$newval > $vmax}] } {
    puts "'if' with braced expr says $newval > $vmax"
} else {
    puts "'if' with braced expr says $newval <= $vmax"
}

and it agreed with the raw [if].  I imagine this has to do with shimmering but that's a guess.  You ought to 
brace [expr] arguments for performance anyway so this doesn't seem like a real issue.

-- Chris Nelson (ch...@pi...), who can't manage to log into SorceForge reliably

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You can respond by visiting: 
http://sourceforge.net/tracker/?func=detail&atid=110894&aid=410043&group_id=10894