Anne Archibald schrieb: > On 21/02/07, WolfgangZillig <[EMAIL PROTECTED]> 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? > > Well, nothing. It is around zero. Try (without numpy) >>>> (1.+1e-16)-1. > 0.0 > That is, for floating-point numbers, 1e-16 is about the fractional > error you can expect from any calculation, and since pi is of order > unity, you should expect its representation to have about that big an > error on it; feed that through sin and you get an error of about the > same size. > > Or, to see more clearly, try taking (on a pocket calculator, say) > sin(3.14) (or even sin(pi)). Roundoff error is a basic fact of life in > numerical computations. > > Anne
Thanks for your answers. I was already aware that it won't be exactly 0 but it wasn't clear to me that the rounding precision is around 1e-16. Thanks for your help! Wolfgang _______________________________________________ Numpy-discussion mailing list Numpy-discussion@scipy.org http://projects.scipy.org/mailman/listinfo/numpy-discussion