Infinite loop in a nested while and for loop

Question

The code is for generating bead positions of a 3d polymer. It is getting into an infinite loop. I could not figure out the mistake.

import numpy as np
import math as m

atoms = 249
pos = [[]]
sigma = 1.0
pos[0] = [0, 0, 0]

for p in range(atoms):
    md = 0.0
    c = 0
    n = p

    while md < sigma:
        rnd_vector = (2 * (np.random.rand(3))) - [1, 1, 1]
        rnd_vector = np.round(rnd_vector, 6)

        d = m.sqrt(sum(map(lambda x: x * x, rnd_vector)))

        rnd_vector = [x / d for x in rnd_vector]

        next_pos = [pos[p][x] + rnd_vector[x] for x in range(3)]

        c += 1

        for i in range(n):
            temp = [rnd_vector[x] - pos[i][x] for x in range(3)]
            td = m.sqrt(sum(map(lambda x: x * x, temp)))

            if i == 0:
                md = td

            if td < md:
                md = td

    pos.append(next_pos)

tobias_k · Accepted Answer · 2020-01-25 10:03:42Z

3

Difficult to tell without knowing what exactly the program is supposed to do, however, the reason for the infinite loop is that in the very first iteration, when n=p=0, the inner-most loop is never executed and thus md is never updated.

If you change the inner loop to for i in range(n+1):, then the program eventually terminates, though I can not tell if that's still doing what it is supposed to do.

answered Jan 25, 2020 at 10:03

tobias_k

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