Changing values in a 2D array

Question

Alright so basically, I have a multi-dimensional array, Board[8][8]. I'm trying to take random values within the array and make them a different value. The value that I am changing must already be a certain value. The code I am running keeps yeilding these results:

java.lang.ArrayIndexOutOfBoundsException: 8
       at checkers.init(checkers.java:32)
       at sun.applet.AppletPanel.run(Unknown Source)
       at java.lang.Thread.run(Unknown Source)

This is the code causing the problem. Note that line 8 is a variable declaration:

int BLACK = 1;

    Random generator = new Random();
    int checkersCount_B = 0, checkersCount_W = 0, x, y;

    while(checkersCount_B < 9){
        x = generator.nextInt(9);
        y = generator.nextInt(9);

        if(Board[x][y] == BLACK){
            Board[x][y] = BLACK_CHECKER;
        //  System.out.println(x + " " + y);
            checkersCount_B ++;
        } else{
            //nothing
        }
    }

Line 32 is the if statement.

The code works for a couple runs through the while loop, but never makes past two or three, any suggestions?

Here is a more in depth description about why arrays are 0 indexed: developeronline.blogspot.com/2008/04/… — Jason Sperske
– Jason Sperske, Commented Mar 21, 2013 at 20:45

Jean Waghetti · Accepted Answer · 2013-03-21 20:40:53Z

4

The indexes of your array go from 0 to 7. Iterating while (index<9), will take the 9th element (given by index 8).

answered Mar 21, 2013 at 20:40

Jean Waghetti

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1 Comment

Kosjfn Sdgn Over a year ago

Thanks, it's one of those days... you just saved me hours

rgettman · Accepted Answer · 2013-03-21 20:41:21Z

1

You will run off the end of one of the arrays, because eventually nextInt will return 8, but the indexes of your arrays are 0-7 (length 8).

Use generator.nextInt(8) to return a random number between 0 and 7.

answered Mar 21, 2013 at 20:41

rgettman

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Comments

Mohammed Hossain · Accepted Answer · 2013-03-21 20:41:13Z

0

You are generating numbers from 0 to 8 with your generator.nextInt(9). Since the board's width and height are 8, you should generate indexes that range from 0 to 7. Change the 9 to 8 in your nextInt call.

answered Mar 21, 2013 at 20:41

Mohammed Hossain

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user1697575 · Accepted Answer · 2013-03-21 20:41:14Z

0

In arrays indexes start from 0 (not 1) so for array of 8 elements you will have to use indexes from 0 to 7.

answered Mar 21, 2013 at 20:41

user1697575

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Franklin · Accepted Answer · 2013-03-21 20:42:14Z

0

Indices go from 0 to 7; thus, you must generate a value within that range. The length, however, is 8.

answered Mar 21, 2013 at 20:42

Franklin

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Aaron_H · Accepted Answer · 2013-03-21 21:30:17Z

0

Copy-and-pasteable solution:

Random generator = new Random();
int checkersCount_B = 0, checkersCount_W = 0, x, y;

while(checkersCount_B < 8){
    x = generator.nextInt(8);
    y = generator.nextInt(8);

    if(Board[x][y] == BLACK){
        Board[x][y] = BLACK_CHECKER;
    //  System.out.println(x + " " + y);
        checkersCount_B ++;
    } else{
        //nothing
    }
}

answered Mar 21, 2013 at 21:30

Aaron_H

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