[Numpy-discussion] what goes wrong with cos(), sin()

Robert Kern robert.kern@gmail....
Wed Feb 21 12:26:39 CST 2007


WolfgangZillig wrote:
> Hi,
> 
> I'm quite new to numpy/scipy so please excuse if my problem is too obvious.
> 
> example code:
> 
> import numpy as n
> print n.sin(n.pi)
> print n.cos(n.pi/2.0)
> 
> results in:
> 1.22460635382e-016
> 6.12303176911e-017
> 
> I've expected something around 0. Can anybody explain what I am doing 
> wrong here?

Nothing. You are getting something around 0 to the limit of double-precision
floating point. numpy.pi cannot be exactly π, but within about 1e-16 of it. That
imprecision carries through the computations.

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
 that is made terrible by our own mad attempt to interpret it as though it had
 an underlying truth."
  -- Umberto Eco


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