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I am a new programmer, I have had a class but haven't been back to school yet, so I am trying to get ahead on my own when I have time b doing the problems at projecteuler.net. I have searched on this site and on google for the solution but all of their fixes are using the wrong variable in the for loops which I checked multiple times.

The exception:

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 20 at Euler11.main(Euler11.java:37)

My code: 

public class Euler11 {
    /**
     * @param args
     */
    public static void main(String[] args) {
        int[][] grid = new int[][] {
                {8, 02, 22, 97, 38, 15, 00, 40, 00, 75, 04, 05, 07, 78, 2, 12, 50, 77, 91, 8},
                {49, 49, 99, 40, 17, 81, 18, 57, 60, 87, 17, 40, 98, 43, 69, 48, 04, 56, 62, 00},
                {81, 49, 31, 73, 55, 79, 14, 29, 93, 71, 40, 67, 53, 88, 30, 03, 49, 13, 36, 65},
                {52, 70, 95, 23, 04, 60, 11, 42, 69, 24, 68, 56, 01, 32, 56, 71, 37, 02, 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, 03, 45, 02, 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, 02, 62, 12, 20, 95, 63, 94, 39, 63, 8, 40, 91, 66, 49, 94, 21},
                {24, 55, 58, 05, 66, 73, 99, 26, 97, 17, 78, 78, 96, 83, 14, 88, 34, 89, 63, 72},
                {21, 36, 23, 9, 75, 00, 76, 44, 20, 45, 35, 14, 00, 61, 33, 97, 34, 31, 33, 95},
                {78, 17, 53, 28, 22, 75, 31, 67, 15, 94, 03, 80, 04, 62, 16, 14, 9, 53, 56, 92},
                {16, 39, 05, 42, 96, 35, 31, 47, 55, 58, 88, 24, 00, 17, 54, 24, 36, 29, 85, 57},
                {86, 56, 00, 48, 35, 71, 89, 07, 05, 44, 44, 37, 44, 60, 21, 58, 51, 54, 17, 58},
                {19, 80, 81, 68, 05, 94, 47, 69, 28, 73, 92, 13, 86, 52, 17, 77, 04, 89, 55, 40},
                {04, 52, 8, 83, 97, 35, 99, 16, 07, 97, 57, 32, 16, 26, 26, 79, 33, 27, 98, 66},
                {88, 36, 68, 87, 57, 62, 20, 72, 03, 46, 33, 67, 46, 55, 12, 32, 63, 93, 53, 69},
                {04, 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, 04, 36, 16},
                {20, 73, 35, 29, 78, 31, 90, 01, 74, 31, 49, 71, 48, 86, 81, 16, 23, 57, 05, 54},
                {01, 70, 54, 71, 83, 51, 54, 69, 16, 92, 33, 48, 61, 43, 52, 01, 89, 19, 67, 48}
        };  
        long product = 0;
        long hp = 0;
        long vp = 0;
        long d1p = 0;
        long d2p = 0;

        for (int h = 3; h < 23; h++) {
            for (int v = 3; v < 23; v++) {
                hp = grid[h][v] * grid[h][v + 1] * grid[h][v + 2] * grid[h][v + 3];
                vp = grid[h][v] * grid[h + 1][v] * grid[h + 2][v] * grid[h + 3][v];
                d1p = grid[h][v] * grid[h + 1][v + 1] * grid[h + 2][v + 2] * grid[h + 3][v + 3];
                d2p = grid[h][v] * grid[h - 1][v + 1] * grid[h - 2][v + 2] * grid[h - 3][v + 3];

                if (hp > product) {
                    product = hp;
                }

                if (vp > product) {
                    product = vp;
                }

                if (d1p > product) {
                    product = d1p;
                }

                if (d2p > product) {
                    product = d2p;
                }
            }
        }
    }
}

I apologize for any sloppiness in the code and if you have any advice on that I am always willing to accept criticism.

Looking over the documentation for the ArrayIndexOutOfBoundsException, it mentions negative numbers and <= symbols but I do not have any that I have noticed.

I tried setting h = 0 and v = 0 and having them go to 19 instead of 23 and got the exception but it said -1 instead of 20 and was on line 40. The MultiDimensional array was my friends idea and I feel like there is probably an easier way, but I don't know what it could be so I went with his suggestion. If you have a site that you used to find the answer and could link it I would appreciate it.

Thank you ahead of time for any advice you can give me.

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  • 1
    Wow, because the the grid is [20][20] and you want to access the 23 index (h variable) Commented Jul 1, 2013 at 21:10

3 Answers 3

Reset to default
6

I don't want to come up with the answer for you, since this is for your learning, and that wouldn't help out much. But I will give a hint for you:

ArrayOutOfBoundsException means that you tried to access an element of an array that doesn't exist. You've got a 20x20 array, so you can use integers between 0 and 19 to access the elements in the array (Remember that arrays are 0-indexed, meaning they start counting from 0 rather than 1). Think on this: Is your code ever trying to access the array with numbers outside of that range?

These array bounds issues are quite common when starting programming, and they are a little confusing at first. Once you understand them, however, they're trivial mistakes forever after.

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2 Comments

Thank you for your response, I got so use yo using counts for the for loops that I set it up for 20 loops with a counter. I enjoy how you phrased it by not giving me the answer and still explained my issue for me. Thank you very much.
No problem, I remember having these same bugs when I was learning to program! Keep up the hard work, learning to program well is all about practice, practice, and more practice.
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Your array is 20x20 elements. Inside code you access indexes with [h+3] (maximum) and [h-3] (minimum). Use the for loops like this:

for (int h = 3; h < 17; h++) {
     for (int v = 3; v < 17; v++) {
....

Hope this helps.

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1

You are attempting to access a part of the array that is greater than the size of the array (which is 20 by 20; i.e. its indexes go between 0 and 19), i.e, a part of the array that doesn't exist.

Your loops go between 3 and 23, so on this line:

hp = grid[h][v] * grid[h][v + 1] * grid[h][v + 2] * grid[h][v + 3];

When h=0 and v=20 you attempt to access grid[0][20] and the exception is raised. If the exception wasn't raised you'd go on to access grid[0][21] and grid[0][22] on the next time round the loop; even more wrong; even further outside the array.

When you go between 0 and 19 you get the reverse problem d2p = grid[h][v] * grid[h - 1][v + 1] * grid[h - 2][v + 2] * grid[h - 3][v + 3];

when h=0 and v=0 grid[h - 1][v + 1] attempts to access grid[0 - 1][1], i.e. grid[-1][1], this also does not exist

Without knowing exactly what your goal is (and not wanting to spoil your learning) I can't advise on what to do instead but you must not access parts of the array that do not exist. But it seems like you are manipulating the "inner region" of the array, possibly you don't want to go the whole way from 0-19, maybe you only want 3-16 (for example, see if that makes sense for the problem you're actually solving)

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