Not working as intended

So this is returning a crazy number. It's not working as intended because the number it returns... well, it's impossible to be right because (99*99*99*99*99) = 9509900499, and the returned number is greater, therefore something is broken.

It's supposed to return the greatest product of 5 adjacent numbers in the 20x20 array.

Code:

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#include <iostream>

using namespace std;

int grid[20][20] = { { 8, 2, 22, 97, 38, 15, 0, 40, 0, 75, 4, 5, 7, 78, 52, 12, 50, 77, 91, 8 },
                                                        { 49, 49, 99, 40, 17, 81, 18, 57, 60, 87, 17, 40, 98, 43, 69, 48, 4, 56, 62, 0 },
                                                        { 81, 49, 31, 73, 55, 79, 14, 29, 93, 71, 40, 67, 53, 88, 30, 3, 49, 13, 36, 65 },
                                                        { 52, 70, 95, 23, 4, 60, 11, 42, 69, 24, 68, 56, 1, 32, 56, 71, 37, 2, 36, 91 },
                                                        { 22, 31, 16, 71, 51, 67, 63, 89, 41, 92, 36, 54, 22, 40, 40, 28, 66, 33, 13, 80 },
                                                        { 24, 47, 32, 60, 99, 3, 45, 2, 44, 75, 33, 53, 78, 36, 84, 20, 35, 17, 12, 50 },
                                                        { 32, 98, 81, 28, 64, 23, 67, 10, 26, 38, 40, 67, 59, 54, 70, 66, 18, 38, 64, 70 },
                                                        { 67, 26, 20, 68, 2, 62, 12, 20, 95, 63, 94, 39, 63, 8, 40, 91, 66, 49, 94, 21 },
                                                        { 24, 55, 58, 5, 66, 73, 99, 26, 97, 17, 78, 78, 96, 83, 14, 88, 34, 89, 63, 72 },
                                                        { 21, 36, 23, 9, 75, 0, 76, 44, 20, 45, 35, 14, 0, 61, 33, 97, 34, 31, 33, 95 },
                                                        { 78, 17, 53, 28, 22, 75, 31, 67, 15, 94, 3, 80, 04, 62, 16, 14, 9, 53, 56, 92 },
                                                        { 16, 39, 5, 42, 96, 35, 31, 47, 55, 58, 88, 24, 0, 17, 54, 24, 36, 29, 85, 57 },
                                                        { 86, 56, 0, 48, 35, 71, 89, 7, 5, 44, 44, 37, 44, 60, 21, 58, 51, 54, 17, 58 },
                                                        { 19, 80, 81, 68, 5, 94, 47, 69, 28, 73, 92, 13, 86, 52, 17, 77, 4, 89, 55, 40 },
                                                        { 4, 52, 8, 83, 97, 35, 99, 16, 7, 97, 57, 32, 16, 26, 26, 79, 33, 27, 98, 66 },
                                                        { 88, 36, 68, 87, 57, 62, 20, 72, 3, 46, 33, 67, 46, 55, 12, 32, 63, 93, 53, 69 },
                                                        { 4, 42, 16, 73, 38, 25, 39, 11, 24, 94, 72, 18, 8, 46, 29, 32, 40, 62, 76, 36 },
                                                        { 20, 69, 36, 41, 72, 30, 23, 88, 34, 62, 99, 69, 82, 67, 59, 85, 74, 4, 36, 16 },
                                                        { 20, 73, 35, 29, 78, 31, 90, 1, 74, 31, 49, 71, 48, 86, 81, 16, 23, 57, 5, 54 },
                                                        { 1, 70, 54, 71, 83, 51, 54, 69, 16, 92, 33, 48, 61, 43, 52, 1, 89, 19, 67, 48 } };

int main() {
        int i = 0;
        int m0 = 0;
        int m1 = 1;
        int m2 = 2;
        int m3 = 3;
        int m4 = 4;
        unsigned long long int ans = 0;

        while (i < 20)        {
                        while (m4 < 20) {
                                unsigned long long int temp = (grid[i][m0] * grid[i][m1] * grid[i][m2] * grid[i][m3] * grid[i][m4]);
                                if (temp > ans)
                                        ans = temp;
                                m0++; m1++; m2++; m3++; m4++;
                        }
                m0 = 0, m1 = 1, m2 = 2, m3 = 3, m4 = 4;
                i++;
        }
        cout << ans;
}

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edit: the number I get is: 18446744071741414188

The problem may be with the unsigned long long type. I tested your program on the MinGW port of g++ and got the same wrong answer. Changing to unsigned long gave a more reasonable answer. In any case, unsigned long long is not one of the built-in types mentioned in the C++ Standard, and there might be a bug in this MinGW port of the g++ compiler extension.

Hmm you're right, that does appear to be the case, I just tried with 'long' and got a more reasonable answer aswell. Hopefully that's all it was, thanks for the help. If I find out it was something else I'll post again ;)

The error appears to be due to integer overflow. You multiply 5 int's and the largest results overflow, only then do you convert the result to unsigned long long which is long enough. Since you are essentially casting a negative value, the upper bits of the result will be all 1, whereas the lower 32 bits contain the correct result - and altogether looking like a complete mystery. You could cast one of the multipliers to unsigned long long to start with if you want to use that type.

(What a relief that it didn't really turn out a compiler bug! :))

Quote:

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Originally Posted by anon

The error appears to be due to integer overflow. You multiply 5 int's and the largest results overflow, only then do you convert the result to unsigned long long which is long enough.

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Yes, that seems to be the case. Changing to unsigned long happened to work due to implementation defined behaviour.

Quote:

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Originally Posted by anon

What a relief that it didn't really turn out a compiler bug!

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I wish I had carefully documented my problems when I attempted to use long long with MinGW in the past; perhaps I similiarly encountered such an overflow without realising it.