On Feb 22, 2012 1:16 PM, "Alec Taylor" <alec.tayl...@gmail.com> wrote: > > Simple mathematical problem, + and - only: > > >>> 1800.00-1041.00-555.74+530.74-794.95 > -60.950000000000045 > > That's wrong. > > Proof > http://www.wolframalpha.com/input/?i=1800.00-1041.00-555.74%2B530.74-794.95 > -60.95 aka (-(1219/20)) > > Is there a reason Python math is only approximated? - Or is this a bug? > > Thanks for all info, > > Alec Taylor > --
You aren't doing math with decimal numbers. you're using IEEE 754-compliant double precision floating point numbers. this isn't just a python thing. You'd get the same results in C, Java, VB, and pretty much every other general purpose language written in the last 40 years. Floats are represented in a form similar to scientific notation (a * 2^b), so just like scientific notation, there's a finite number of significant figures. And just like there are rational numbers that can't be represented in decimal, like 1/3, there are numbers that can't be represented in binary, like 1/10. Double-precision numbers are accurate to about 15 decimal digits. if you need more precision, there is the decimal module but it's way slower because your processor doesn't natively support it. > http://mail.python.org/mailman/listinfo/python-list
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