Insert numbers to array

Question

I need to insert for every number sum of his digits in array using pointer arithmetic. Auxiliary arrays are not allowed.

Example:

123, 456, 789

Output:

123, 6, 456, 15, 789, 24

Code:

#include <stdio.h>
int sum_of_digits(int n){
    int sum=0;
    while(n!=0){
        sum=sum+n%10;
        n/=10;
    }
    return sum;
}

void add(int *arr, int n)
{
    int *p=arr, *y=arr+n*2-2,i;
    while(p++<arr+n);
    p-=2;
    while(p>arr)
    {
        *y=*p;
        *p=0;
        p--;
        y-=2;
    }
    p=arr;
    while(p<arr+n*2)
    {
        *(p+1)=sum_of_digits(*p);
        p+=2;
    }
    for(i=0;i<n;i++)
    printf("%d ", arr[i]);
}

void main() {
    int arr[20]={123, 456, 789},n=3;
    add(arr,n*2);
}

This works correct, but I don't understand the part of code with pointers. Could you explain me how this works?

Answer 1

I have formatted function add and added some comments to explain it, I hope they clarify what's happening. Note that arr[i] == *(arr + i) and *arr == arr[0]

void add(int *arr, int n)
{
    // p points to last number in arr
    // Equivalent to &arr[n-1]
    int *p = arr + n - 1;
    
    // y points to where last number will be after spreading
    // Equivalent to &arr[2*n - 2]
    int *y = arr + n * 2 - 2;
    
    // for n = 4 arr looks like this
    // [A, B, C, D, _, _, _, _]
    //           ^           ^
    //           p           y
    
    // Spread numbers of arr
    // [A, B, C, D, _, _, _, _] -> [A, _, B, _, C, _, D, _]
    while (p > arr)
    {
        // Copy D to last position
        *y = *p;
        // Zero original D location, unnecessary
        *p = 0;
        // Move p backwards by one
        p--;
        // Move y backwards by two
        y -= 2;
        
        // arr now looks like this
        // [A, B, C, _, _, _, D, _]
        //        ^     ^   
        //        p     y
        
        // After next iteration
        // [A, B, _, _, C, _, D, _]
        //     ^  ^         
        //     p  y      
        
        // After next iteration
        // [A, _, B, _, C, _, D, _]
    }
    
    // Iterate over every other value 
    // and assign arr[i+1] to sum_of_digits[i]
    p = arr;
    while (p < arr + n * 2)
    {
        *(p + 1) = sum_of_digits(*p);
        p += 2;
    }
    // Equivalent to
    // for (int i = 0; i < n * 2; i += 2) {
    //     arr[i + 1] = sum_of_digits(arr[i]);
    // }
    
    // [A, sum_of_digits(A), B, sum_of_digits(B), C, sum_of_digits(C), 
    //  D, sum_of_digits(D)]

    // Print array
    for (int i = 0; i < n; i++)
        printf("%d ", arr[i]);
}