Daddy Clanger (imc) wrote,
Daddy Clanger
imc

And finally…

A tcl stupidity.

Let c be an arbritrary number. When does c2 not equal c2?

In tcl 8.0:
% set c 64.5165238678
64.5165238678
% set a [expr $c * $c]
4162.38185198
% set b $a
4162.38185198
% expr $b - $c * $c
0.0


So far, if we set b to c2 then b-c2 is zero. Makes sense, yes?

% $a
invalid command name "4162.38185198"
% expr $b - $c * $c
-4.40741132479e-09


And now it's magically not zero!

In tcl 8.4.4 the rules are slightly different: as soon as you set b=c2, the value of b-c2 is not zero - you don't need the $a trick*. And that's why my program dies (invalid argument to the sqrt function) when it tries to compute the standard deviation of a set which happens to contain only one element.

* Any command which treats $a as a list will also do, such as llength $a.
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