Aguy Aguy - 1 year ago 84
Python Question

Is there special significance to 16331239353195370.0?

Sorry for the unusual title... :-)


import numpy as np
I've noticed that


gives the number in the title and not


I'm curious about this number. Is it related to some system machine precision parameter? Could I have calculated it from something? (I'm thinking along the lines of something similar to

EDIT: The same result is indeed reproducible in other environments such as Java, octace, matlab... The suggested dupe does not explain why, though.

Answer Source

pi isn't exactly representable as Python float (same as the platform C's double type). The closest representable approximation is used.

Here's the exact approximation in use on my box (probably the same as on your box):

>>> import math
>>> (math.pi / 2).as_integer_ratio()
(884279719003555, 562949953421312)

To find the tangent of that ratio, I'm going to switch to wxMaxima now:

(%i1) fpprec: 32;
(%o1) 32
(%i2) tan(bfloat(884279719003555) / 562949953421312);
(%o2) 1.6331239353195369755967737041529b16

So essentially identical to what you got. The binary approximation to pi/2 used is a little bit less than the mathematical ("infinite precision") value of pi/2. So you get a very large tangent instead of infinity. The computed tan() is appropriate for the actual input!

For exactly the same kinds of reasons, e.g.,

>>> math.sin(math.pi)

doesn't return 0. The approximation math.pi is a little bit less than pi, and the displayed result is correct given that truth.

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