[Python-Dev] math.areclose ...? (original) (raw)

Georg Brandl g.brandl at gmx.net
Sun Feb 5 19:02:34 CET 2006

Alex Martelli wrote:

When teaching some programming to total newbies, a common frustration is how to explain why a==b is False when a and b are floats computed by different routes which ``should'' give the same results (if arithmetic had infinite precision). Decimals can help, but another approach I've found useful is embodied in Numeric.allclose(a,b) -- which returns True if all items of the arrays are ``close'' (equal to within certain absolute and relative tolerances):

>>> (1.0/3.0)==(0.1/0.3) False >>> Numeric.allclose(1.0/3.0, 0.1/0.3) 1 But pulling in the whole of Numeric just to have that one handy function is often overkill. So I was wondering if module math (and perhaps by symmetry module cmath, too) shouldn't grow a function 'areclose' (calling it just 'close' seems likely to engender confusion, since 'close' is more often used as a verb than as an adjective; maybe some other name would work better, e.g. 'almostequal') taking two float arguments and optional tolerances and using roughly the same specs as Numeric, e.g.: def areclose(x,y,rtol=1.e-5,atol=1.e-8): return abs(x-y)<atol+rtol*abs(y) What do y'all think...?

atol sounds suspicious to me, but otherwise fine.

Georg

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