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

Alex Martelli aleaxit at gmail.com
Sun Feb 5 18:31:48 CET 2006

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.
'almost_equal') 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...?

Alex

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