CODEWITHSUNDEEP
pythonmathscipynormal-distribution
Leon
Leon

Reputation: 881

Alternative for scipy.stats.norm.pdf?

Does anyone know of an alternative for scipy.stats.norm.pdf()? I'm hosting my python site on Google App Engine and Google doesn't support SciPy.

I've tried this function, but that didn't return the same results as scipy:

def normpdf(x, mu, sigma):
    u = (x-mu)/abs(sigma)
    y = (1/(sqrt(2*pi)*abs(sigma)))*exp(-u*u/2)
    return y

For example:

print scipy.stats.norm.pdf(20, 20, 10)
print normpdf(20, 20, 10)

print scipy.stats.norm.pdf(15, 20, 10)
print normpdf(15, 20, 10)

print scipy.stats.norm.pdf(10, 20, 10)
print normpdf(10, 20, 10)

Returns these values:

0.0398942280401
0.0398942280401

0.0352065326764
0.0146762663174

0.0241970724519
0.0146762663174

Upvotes: 6

Views: 18034

Answers (2)

Chris Taylor
Chris Taylor

Reputation: 47402

The division by 2 inside of the exp is being interpreted as integer division whenever u evaluates to an int. To prevent this you can ensure that u always evaluates to a float by manually casting it:

def normpdf(x, mu=0, sigma=1):
    u = float((x-mu) / abs(sigma))
    y = exp(-u*u/2) / (sqrt(2*pi) * abs(sigma))
    return y

(I also provided default arguments for mu and sigma, you could remove those if you wanted)

Upvotes: 6

David Zwicker
David Zwicker

Reputation: 24328

You got tricked by pythons integer division arithmetics! Here is some working code:

from __future__ import division

import scipy.stats
from numpy import *

def normpdf(x, mu, sigma):
    u = (x-mu)/abs(sigma)
    y = (1/(sqrt(2*pi)*abs(sigma)))*exp(-u*u/2)
    return y


print scipy.stats.norm.pdf(20, 20, 10)
print normpdf(20, 20, 10)

print scipy.stats.norm.pdf(15, 20, 10)
print normpdf(15, 20, 10)

print scipy.stats.norm.pdf(10, 20, 10)
print normpdf(10, 20, 10)

Note the first line! Otherwise, you could convert each input variable to a float, e.g. by multiplying by 1.

Upvotes: 10

Related Questions