c - Is this a proper for-loop optimization -

for homework assignment need optimize loop run in under 7.5 seconds. think may have done because code runs in 4 seconds. however, worried not doing correctly because instructor told far under 7.5 seconds wrong. worried might not doing things correctly. here original code:

#include <stdio.h> #include <stdlib.h>  #define n_times     600000 #define array_size   10000  int main (void) {     double  *array = calloc(array_size, sizeof(double));     double  sum = 0;     int     i;      (i = 0; < n_times; i++) {          int     j;          (j = 0; j < array_size; j++) {             sum += array[j];             }         }      return 0; } 

here optimization:

   (i = 0; < n_times; i++) {          int     j;          (j = 0; j < array_size/2; j += 20) {             sum += array[j] + array[j+1] + array[j+2] + array[j+3] + array[j+4] + array[j+5] + array[j+6] + array[j+7] + array[j+8] + array[j+9];             sum1 += array[j+10] + array[j+11] + array[j+12] + array[j+13] + array[j+14] + array[j+15] + array[j+16] + array[j+17] + array[j+18] + array[j+19];             }          }     sum += sum1; 

are these doing same number of arithmetic operations? did change code somehow or optimizing well?

your optimizations not correct:

for (j = 0; j < array_size/2; j += 20) { 

you loop half many times in inner loop should.