Dynamic Memory in C

I can’t understand what doing the row

*(m[i] + sizes[i] - 1) = n;

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

#define MAXSTR 100

int main() 
  {
    int i, j, k, n;
    char str[MAXSTR];
    printf("Enter amount of rows: ");
    fgets(str, MAXSTR, stdin);
    k = atoi(str);
    int* sizes = (int * ) calloc(k, sizeof(int));
    int* sum = (int * ) calloc(k, sizeof(int));
    int** m = (int ** ) calloc(k, sizeof(int * ));
    printf("Enter matrix:\n");
    for (i = 0; i < k; i++) 
        {
          fgets(str, MAXSTR, stdin);
          char* sym = str;
          while (1) 
             {
               m[i] = (int * ) realloc(m[i], (++sizes[i]) * sizeof(int));
               n = strtol(sym, & sym, 10);
               sum[i] += n;
               if (n) 
                  {
                  *(m[i] + sizes[i] - 1) = n;
                  } 
               else 
                  {
                   --sizes[i];
                   break;
                  }
           }
       }

   printf("\nMatrix: \n");
   for (i = 0; i < k; i++) 
      {
        for (j = 0; j < sizes[i]; j++)
        printf("%i ", *(m[i] + j));
        printf("\n");
      }
    printf("\nSum of elements of row:\n");
    for (i = 0; i < k; i++)
      printf("#%i - %i\n", i + 1, sum[i]);

    free(sizes);
    free(sum);
    free(m);

    return 0;

>Solution :

m is the matrix. Or more formally, it appears to be an array of "rows". Where each row is an array of integers.

sizes[i] is the length of the row represented by m[i].

This expression

*(m[i] + sizes[i] - 1) = n;

Appears to assign the value n to the last index of the row identified by m[i]. Essentially, it’s appending to the end of the reallocated array.

This entire block of code is a bit complex:

      while (1) 
         {
           m[i] = (int * ) realloc(m[i], (++sizes[i]) * sizeof(int));
           n = strtol(sym, & sym, 10);
           sum[i] += n;
           if (n) 
              {
              *(m[i] + sizes[i] - 1) = n;
              } 
           else 
              {
               --sizes[i];
               break;
              }
       }

It could be simplified to just:

int rowsize = 0;
while (1)
{
    n = strtol(sym, &sym, 10);   // parse the next number in str
    if (n == 0)                  // the row ends when 0 is read
    {
       break;
    } 
    m[i] = (int *)realloc(m[i], (rowsize+1) * sizeof(int);  // grow the row's size by 1
    m[i][rowsize] = n;
    sum[i] += n;
    rowsize++;
}
sizes[i] = rowsize;