problem with floats and calculations

Stefan Krah stefan-usenet at
Sun Feb 14 11:29:56 CET 2010

Karsten Goen <karsten.goen at> wrote:
> hey all,
> I got a problem with floats and calculations. I made an mini-application where
> you get random questions with some science calculations in it
> So the user can type in his result with the values given by random creation.
> And the user value is compared against the computer value... the problem is
> that the user input is only 2 numbers behind the '.' so like 1.42, 1.75
> here is the example:
> without decimal it would be very inaccurate. decimal is very accurate when I
> have to compare d with users calculations from a,b,c,var.
> But when I ask the user what is "a" the result gets inaccurate when calculating
> with the same values given before (b,c,d,var).
> Maybe anyone can help me with this problem, I don't want to generate for every
> possible user input a single formula. And also it should be possible for a
> computer, my calculator at home does the same and is much smaller and slower.

d = (a * b)/ (c * var)
d = Decimal(d).quantize(Decimal('0.01'))

By quantizing d, the above equality does not hold any longer. You've got
to drop that line (your calculator doesn't quantize either).

Stefan Krah

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