Passing an array from python to C using ctypes, then using that array in Python

Question

I am trying to make a histogram of Poisson random generated variables using Python and C. I wanted to use Python for plotting and C for generating. This resulted in the following to codes

Python:

import ctypes
import numpy as np
import matplotlib.pyplot as plt
import time

lam = 5.0
n = 1000000

def generate_poisson(lam, n):
    array = np.zeros(n, dtype= np.int)
    f = ctypes.CDLL('./generate_poisson.so').gen_poisson
    f(ctypes.c_double(lam), ctypes.c_int(n), ctypes.c_void_p(array.ctypes.data))
    return array

start_time = time.time()
array = generate_poisson(lam,n)
print(time.time() - start_time)

plt.hist(array, bins = [0,1,2,3,4,5,6,7,8,9,10,11,12], density = True)
plt.savefig('fig.png')
print(array)

C:

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <math.h>

double get_random() { return ((double)rand() / (double)RAND_MAX); }

int poisson_random(double lam){
    int X;
    double prod, U, explam;

    explam = exp(-lam);
    X = 0;
    prod = 1.0;
    while (1){
        U = get_random();
        prod *= U;
        if (prod > explam){
            X+=1;
        }
        else {
            return X;
        }
    }
}

void gen_poisson(double lam, int n, void * arrayv)
{
    int * array = (int *) arrayv;
    int index = 0;
    srand(time(NULL));

    for (int i =0; i<n; i++, index++){
        //printf("before %d\n", array[i]);
        array[index++] = poisson_random(lam);
        //printf("after %d\n", array[i]);
    }
}

The problem understanding why this works, or at least it seems to work correctly, occurs inside the for loop in gen_poisson(). Somehow using array[index++] instead of array[index] results in a correct histogram. But I dont really understand why this has to be the case. The code also seems to work when the for loop is changed to

for (int i =0; i<2*n; i++){
        //printf("before %d\n", array[i]);
        array[i++] = poisson_random(lam);
        //printf("after %d\n", array[i]);
}

Can someone explain why in this case the loop has to be incremented twice? I just started programming in C while I have some experience in Python. So assume the culprit is my lack of understanding about C. Thank you in advance

Answer 1

Changing the gen_poisson into:

void gen_poisson(double lam, int n, void * arrayv)
{
    long * array = (long *) arrayv;
    srand(time(NULL));
    for (int i =0; i<n; i++){
        array[i] = poisson_random(lam);
    }
}

solves the problem. The problem was as pointed out with declaring the array as int * instead of long *.