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 func

Answer 1

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.