Is there an easily available implementation of erf() for Python?

Question

I can implement the error function, erf, myself, but I'd prefer not to. Is there a python package with no external dependencies that contains an implementation of this function? I have found this but this seems to be part of some much larger package (and it's not even clear which one!).

Answer 1

Since v.2.7. the standard math module contains erf function. This should be the easiest way.

http://docs.python.org/2/library/math.html#math.erf

Answer 2

I recommend SciPy for numerical functions in Python, but if you want something with no dependencies, here is a function with an error error is less than 1.5 * 10^-7 for all inputs.

def erf(x):     # save the sign of x     sign = 1 if x >= 0 else -1     x = abs(x)      # constants     a1 =  0.254829592     a2 = -0.284496736     a3 =  1.421413741     a4 = -1.453152027     a5 =  1.061405429     p  =  0.3275911      # A&S formula 7.1.26     t = 1.0/(1.0 + p*x)     y = 1.0 - (((((a5*t + a4)*t) + a3)*t + a2)*t + a1)*t*math.exp(-x*x)     return sign*y # erf(-x) = -erf(x) 

The algorithm comes from Handbook of Mathematical Functions, formula 7.1.26.