default associative shape for default precedence

//copyright: Nedal Ahmed Husein Aburuqaya, all rights reserved.
#include<time.h>
#include<cstdlib>
#include<chrono>
using namespace std::chrono;
#include <iostream>
using namespace std;
const unsigned int n = 20000;
void fillArray(int a[n]){
    srand(time(0));
    for (int i = 0; i < n; i++)
        a[i] = rand()+1;
}
bool compare(int a[n], int b[n]) {
    for (int i = 0; i < n; i++)
        if (a[i] != b[i])
            return false;
    return true;
}
int main()
{
    int a1[n], a2[n], a3[n], a4[n], a5[n], a6[n], a7[n], a8[n], a[n], b[n];
    fillArray(a1); fillArray(a2); fillArray(a3); fillArray(a4); fillArray(a5); fillArray(a6); fillArray(a7); fillArray(a8);
    auto begin1 = high_resolution_clock::now();
    for (int i = 0; i < n; i++) {
        a[i] = a1[i] * a2[i] + a3[i] / a4[i] + a5[i] * a6[i] - a7[i] / a8[i];
    }
    auto end1 = high_resolution_clock::now();
    auto duration1 = duration_cast<microseconds>(end1 - begin1);
    cout << "duration for default precedence "<< duration1.count() << endl;

    auto begin2 = high_resolution_clock::now();
    for (int i = 0; i < n; i++) {
        b[i] = (((a1[i] * a2[i]) +( a3[i] / a4[i])) + (a5[i] * a6[i]) )- (a7[i] / a8[i]);
    }
    auto end2 = high_resolution_clock::now();
    auto duration2 = duration_cast<microseconds>(end2 - begin2);
    cout << "duration for default precedence associate shape  " << duration2.count() << endl;
    if (compare(a, b))
        cout << "they are the same default precedence and associative shape";
    else cout << "possible errors associative shape redo it";
    return 0;
}