Access a 2d array index without coordinates in Java

Question

Lets say we have a 8 x 8 2d integer array named grid and trying to select the element 0 at [5][5]

int[][] grid = new int{{1,1,1,1,1,1,1,1},
                       {1,1,1,1,1,1,1,1},
                       {1,1,1,1,1,1,1,1},
                       {1,1,1,1,1,1,1,1},
                       {1,1,1,1,1,1,1,1},
                       {1,1,1,1,1,0,1,1},
                       {1,1,1,1,1,1,1,1},
                       {1,1,1,1,1,1,1,1}};

Now the question is, is it possible to access the index of element 0 without using coordinates and just use a single number?

example: access the element 0 with the number 45, like a 1d array

I tried approaching this problem with a for loop but it is giving me out of range errors.

int x = 0;
int y = 0;

for (int i = 0;i<45;i++) {

   x += 1;

   if (x > grid[y].length) {

     x = 0;
     y += 1;

}

The code above is supposed to add x and y until it reaches the target element.

It should probably be if (x == grid[y].length)

Eran
– Eran

2018-03-11 22:26:35 +00:00
Commented Mar 11, 2018 at 22:26 — Eran
– Eran, Commented Mar 11, 2018 at 22:26

Jose da Silva Gomes · Accepted Answer · 2018-03-11 22:42:31Z

1

Eventually you would have to use the two indexes.

You could calculate the x and y given just a single number.

public static int getAt(int[][] matrix, int position) {
    int columns = matrix[0].length; // column size, matrix has to be at least 1x1
    return matrix[position / columns][position % columns];
}

public static void setAt(int[][] matrix, int position, int value) {
    int columns = matrix[0].length; // column size, matrix has to be at least 1x1
    matrix[position / columns][position % columns] = value;
}

Also in your example:

1) You don't need to use a for loop (and again eventually to access or modify the matrix you would have to use both indexes).

2) When y is greater or equal than the row size (8 in this case) you will receive an out of bounds exception because you only have 8 columns.

Finally the only way to access it with one index is if you transform the matrix to a 1d array.

Here you can see how: how to convert 2d array into 1d?

edited Mar 11, 2018 at 22:42

answered Mar 11, 2018 at 22:36

Jose da Silva Gomes

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5 Comments

void00 Over a year ago

could you transform the 1d array back into a 2d one again

Jose da Silva Gomes Over a year ago

Yeah sure, but that wouldn't make much sense. Is it a requirement?. Here you can see how: stackoverflow.com/questions/2706529/…

void00 Over a year ago

I will give each solution a try and see which one works best

void00 Over a year ago

Calculating the position using your modulus method was the most efficient solution for me

Jose da Silva Gomes Over a year ago

Great, if using just one index is not an absolute requirement, using mod div is easier.

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