Sarnold 2005-11-10 -- The purpose is to offer a simple let command that works very much like C assignements for basic computations. The goal is not to join the general thing, but rather to keep it simple for everyday work.
Here is how it works :
% let a = 10 10 % let a = $a - 1 9 % puts $a 9 % let a *= 2 18
Please note that arguments have to be distinct. You cannot write this:
let a = $a+2
or this:
let a= 3 * 4
Warning: You may encounter problems with precision on floating-point values. It is not designed for precision, as the following example shows:
% let a = 1.0 / 3
0.333333333333
% let a *= 3
0.999999999999
% set x [expr {1.0/3}]
0.333333333333
% set x [expr {$x*3}]
1.0Here it is :
proc let {varname assign args} {
upvar $varname leftvalue
if {[llength $args]==3} {
# args look like : 1 + 3, $a - $b, $c / $d
foreach {a op b} $args {break}
set args [expr [string map [list a $a op $op b $b] {a op b}]]
}
if {[llength $args]==1} {
switch -exact -- $assign {
+= {set leftvalue [expr {$leftvalue+$args}]}
-= {set leftvalue [expr {$leftvalue-$args}]}
*= {set leftvalue [expr {$leftvalue*$args}]}
/= {set leftvalue [expr {$leftvalue/$args}]}
%= {set leftvalue [expr {$leftvalue%$args}]}
= {set leftvalue $args}
default {error "invalid syntax : second argument is not an assignment"}
}
return $leftvalue
}
if {[llength $args]==0} {
error "invalid argument number, check you have not put arguments and operators together"
}
error "syntax error"
}