unexpected division by zero error

Question

I get a division by zero error when I run this code, can anyone see why? The montecarlo and Vd functions both work perfectly fine until I put them in the for loop at the end and I can't see why, there shouldn't be a zero anywhere to cause the error in the first place.

import numpy as np
import matplotlib.pyplot as plt
import random

d=2

def randaxis(d,N):
    axes={}
    for i in range(0,d):
        axes[i]=[]
        for j in range(0,N):
            axes[i].append(random.uniform(-1,1))
    return axes

def montecarlo(d,N):
    coords=randaxis(d,N)
    inshape=[]
    for i in range(len(list(coords.values())[0])):
        sumsqs=0
        for v in coords.values():
            sumsqs+=v[i]**2
        if sumsqs<=1:
            inshape.append(1)
        else:
            inshape.append(0)

    sumis=0
    for i in inshape:
        sumis+=i

    nratio = sumis/len(inshape)
    return nratio

def Vd(nratio):
    vd = nratio*(2**d)
    return vd

Rep=10 
k=0   
Repeats=np.zeros(Rep)
N=50
for N in range(1000):
    for k in range(Rep):
        Repeats[k]=Vd(montecarlo(2,N))
    print(Repeats)
    N+=50

I think -- and just a wild guess here -- you're probably dividing by zero in some place. What did your debugger say? — Davy M, Feb 06 '18 at 01:38
Please read [How do I ask a good question?](http://stackoverflow.com/help/how-to-ask) before attempting to ask more questions. — , Feb 06 '18 at 01:40
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The error message ends with `nratio = sumis/len(inshape)` before `ZeroDivisionError: division by zero`. Thus, `len(inshape)` is zero. Why is `inshape` empty? What should be in it? All of this should be investigated and reasoned through before calling community to help. — Amadan, Feb 06 '18 at 01:42

score 2 · Answer 1 · answered Feb 06 '18 at 01:43

When you call montecarlo with the second argument of 0, it's going to pass it to randaxis, which uses it to make a dict with a bunch of empty lists as values.

Then you use the length of the first such empty list to determine the number of iterations of the first loop in montecarlo, the one that populates inshape, but of course, since it's empty, you're iterating over range(0), which doesn't iterate at all, and nothing is added to inshape.

Eventually you get here:

nratio = sumis/len(inshape)

wherein you try to divide by the length of an empty list, which is 0, and everything explodes. This will happen anytime you pass 0 as the second argument to montecarlo.

unexpected division by zero error

1 Answers1