I'm not sure why my variance function returns 0.0000 0.0000

Question

My program runs, but my output should print the variance and the average. I was told as my input array sizes get larger and larger my average should get closer to 5000 and my variance should approach 8333333... Yet I am getting 0 for both. The only thing I can think of is that I'm filling my mydata array/memoryblock incorrectly, but I don't know another way to do it if not the way I have written.

#include <stdio.h> //printf
#include <stdlib.h> //malloc, calloc, realloc, free
#include <math.h> //pow

double average_variance(int data[], int numbers, double* varp){
    
    double sum = 0;
    //sum of array
    for(int i=0; i < numbers; i++){
        sum += data[i];
    }
    double average = sum/numbers;

    double squaresum = 0;
    //sum of squares
    for(int i=0; i < numbers; i++){
        squaresum += pow(data[i], 2);
    }

    //"hack" to return two values by using a pointer; otherwise you can't return two values in a function
    double variance = ((squaresum/numbers) - pow(average, 2));
    *varp = variance;
    
    return average;
}

int main(int argc, char **argv)
{
    int* mydata = NULL;
    int arraysize;
    
    while (1) 
    {
        printf("Enter array size (0 to end): "); scanf("%d", &arraysize);
        
        if (arraysize == 0)
        break;
        
        // 1. Add code to allocate memory and assign to the mydata // pointer. If mydata is already allocated, use realloc ...
        
if(arraysize != 0)
        {   
            //used calloc because in the next if statement I needed to see if values were stored in it or not 
            mydata = calloc(arraysize, sizeof(int));
            if (mydata == NULL) {return -1;}
            
            //if mydata array is already allocated use realloc to make a new memory block 
            if(mydata != 0)
            {
                mydata = realloc(mydata, arraysize * sizeof(int));
                if (mydata == NULL) {return -1;}
            }

        }
    
        // 2. Add code to put random numbers in the array
        for(; mydata < &mydata[-1]; mydata++)
        {
            int randnum = random() % 10000;
            mydata = &randnum;
        }
        
        
        //included
        double variance;
        double avg = average_variance(mydata, arraysize, &variance);
        printf("average = %f variance = %f\n", avg, variance);
    }
    free(mydata);

Answer 1

The array initialization code is broken:

        for(; mydata < &mydata[-1]; mydata++)
        {
            int randnum = random() % 10000;
            mydata = &randnum;
        }

Errors:

• mydata[-1] is not the last element of mydata (a'la Python) but it is an element just before the array. This element does not exist, Undefined Behavior is invoked.
• the mydata is moved during the initiazation to a local variable in the loop. There is no hope for this construct to work.

Just do:

    for (int i = 0; i < arraysize; ++i)
            mydata[i] = random() % 10000;

Moreover, the allocation code is needlessly convoluted. Just replace:

if(arraysize != 0)
        {   
            //used calloc because in the next if statement I needed to see if values were stored in it or not 
            mydata = calloc(arraysize, sizeof(int));
            if (mydata == NULL) {return -1;}
            
            //if mydata array is already allocated use realloc to make a new memory block 
            if(mydata != 0)
            {
                mydata = realloc(mydata, arraysize * sizeof(int));
                if (mydata == NULL) {return -1;}
            }

        }

with:

    mydata = realloc(mydata, arraysize * sizeof *mydata);
    if (!mydata) return -1;