[issue7704] Math calculation problem (1.6-1.0)>0.6, python said TRUE

[Tim Peters]

Tim Peters <tim.peters at gmail.com> added the comment:

You can use the comparison, provided you understand what it does, and that it does NOT do what you hoped it would do.  Here:

>>> 1.6 - 1.0
0.60000000000000009

That shows quite clearly that subtracting 1 from the binary approximation to 1.6 does not leave exactly 0.6.  If you need that to happen, use the decimal module (which, in return for being slower, emulates decimal floating-point arithmetic).

Your other case (8.6-8.0) does NOT equal (decimal) 0.6 exactly either, but fools you into thinking it's working the way you hope it works because the result is a little /less/ than 0.6:

>>> 8.6 - 8.0
0.59999999999999964

Do read the tutorial appendix Mark invited you to read.  If you don't, you're going to remain hopelessly confused ;-)

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nosy: +tim_one

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