KBK closed a follow-up to the comp.lang.tcl newsgroup with the advice that, "Also, it's important always, always to brace your expressions. Don't do
set a [expr $b / $c]
but rather
set a [expr { $b / $c }]Otherwise, your variables may undergo unexpected conversions numeric -> string -> numeric, you can lose precision, your expressions will be much slower, and you can even have security problems:
# don't run
set x { [format C:\] }
set y [expr $x + 3]"
RS: In the following cases braced expressions don't work:
set op + expr $x $op $y
expr [join $intlist +]
KPV: Braced expressions also don't work when you do negation in the lazy way:
set n1 -3
set n2 -$n1
expr {2 + $n2} ;# error
expr 2 + $n2 ;# okaricb Dec. 10 2005 -- As a bonus, braced expressions are generally much faster than their non-braced counterparts. Here's an attempt to illustrate the difference:
proc time_braces {} {
set expressions {
{1 + 1}
{16 - 7}
{(16 - 7)}
{7 * 12}
{81 / 3}
{81 / 5}
{16224308 + 123580329}
{6.3 + 1.2}
{6.3 - 1.2}
{6.3 * 1.2}
{6.3 / 1.2}
{log(765) * 3}
{(log(765) * 3) / 8.23}
}
puts [format " %-22s %-8s %-8s %-8s %-5s" "Expression" "-braces" "+braces" "ratio" "% speedup"]
puts " [string repeat - 22] [string repeat - 8] [string repeat - 8] [string repeat - 8] [string repeat - 5]"
foreach expression $expressions {
set time1 [lindex [time "expr $expression" 100000] 0]
set time2 [lindex [time [list expr $expression] 100000] 0]
set ratio [expr {$time1 / $time2}]
set percent [expr {100 * (1 - ($time2 / $time1))}]
puts [format " %-22s %8.6f %8.6f %6.3f:1 %5.2f" $expression $time1 $time2 $ratio $percent]
}
puts "\n -braces and +braces are in microseconds per iteration"
}Results:
Expression -braces +braces ratio % speedup ---------------------- -------- -------- -------- ----- 1 + 1 2.334690 0.366480 6.371:1 84.30 16 - 7 2.388280 0.359960 6.635:1 84.93 (16 - 7) 3.748160 0.356120 10.525:1 90.50 7 * 12 2.438530 0.368200 6.623:1 84.90 81 / 3 2.456060 0.395680 6.207:1 83.89 81 / 5 2.518490 0.397380 6.338:1 84.22 16224308 + 123580329 2.726690 0.369530 7.379:1 86.45 6.3 + 1.2 3.237970 0.497100 6.514:1 84.65 6.3 - 1.2 3.199230 0.486350 6.578:1 84.80 6.3 * 1.2 3.279100 0.480630 6.823:1 85.34 6.3 / 1.2 3.257540 0.494340 6.590:1 84.82 log(765) * 3 6.202120 0.975590 6.357:1 84.27 (log(765) * 3) / 8.23 7.951270 1.142210 6.961:1 85.63 -braces and +braces are in microseconds per iteration