Why does loop order affect too much to execution time ?

Revision en2, by ExpectoPatronum, 2021-09-26 18:56:10
ll res = -oo;
    for(int k=1; k<=n; k++)
        for(int i=1; i<=n-k+1; i++)
            for(int j=1; j<=n-k+1; j++)
            {
                f[i][j][k] = f[i][j][k &mdash; 1] + a[i + k &mdash; 1] * b[j + k &mdash; 1];
                res = max(res, f[i][j][k]);
            }
    cout << res;
ll res = -oo;
    for(int i=1; i<=n; i++)
        for(int j=1; j<=n; j++)
            for(int k=1; k<=n; k++)
            {
                if(k + i > n || k + j > n) break;
                f[i][j][k] = f[i][j][k &mdash; 1] + a[i + k &mdash; 1] * b[j + k &mdash; 1];
                res = max(res, f[i][j][k]);
            }
    cout << res;

These code seem not to be differ too much but execution time have much differ. Example in case n = 500, code1 run in 3103ms but code 2 run only 789ms. What is the reason ? Pls explain to me. Thanks.

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  Rev. Lang. By When Δ Comment
en3 English ExpectoPatronum 2021-09-26 18:57:28 48
en2 English ExpectoPatronum 2021-09-26 18:56:10 101 Tiny change: ' << res;\n\n\n\n\n~~' -> ' << res;\n~~~~~\n\n\n\n~~'
en1 English ExpectoPatronum 2021-09-26 09:27:41 997 Initial revision (published)