|Just another Perl shrine|
Re: IEEE-754 calculation - best way?by BrowserUk (Pope)
|on Jan 30, 2013 at 22:40 UTC||Need Help??|
if I multiply 0.29 with 50, I get 14.5: Only with this line: my $num4 = sprintf ("%.64f", $num7); I get the exact result in "double" (14.999...).
The only difference between:
is what gets printed out; not what is calculated and stored internally.
All floating point calculations are done internally using IEEE754 semantics; if you want to see the full internal results use printf to display the results.
As for your perceived difference when gratuitously using eval:
The first thing to note is that the first two evals make no difference whatsoever:
And the reason for that difference is because (in this form) eval expects a string. So the result of the multiplication (eval( 0.29 ) * eval( 50 )) is converted to a string -- using the same rules as print -- before being passed to the final eval.
You get identically different results if you do:
Because the internal (IEEE754) representation has been lost by the conversion to a string.
Bottom line: Stop worrying about it. Do your floating point math in the usual way (no eval's; no stringifications), secure in the full IEEE754 semantics will be used internally; and then use printf/sprintf (*only*) when you need to display them.
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