CODEWITHSUNDEEP
javaarrays
Simon Baars
Simon Baars

Reputation: 2289

Java - Iterate over a 2-dimensional array starting with the middle

I have this totally normal loop over a 2 dimensional array.

for(int i = 0; i<array.length; i++){
    for(int j = 0; i<array.length; j++){
        array[i][j].doSomething();
    }
}

I want to go through this 2 dimensional array, starting in the middle. For example, if the array has a length of 100 for both dimensions, I want it to go over it like this:

array[50][50] //middle of array
array[49][50] //x-1
array[50][49] //y-1
array[51][50] //x+1
array[50][51] //y+1
array[48][50] //x-2
array[49][49] //x-1 and y-1
array[50][48] //y-2
array[51][49] //x+1 and y-1
array[52][50] //x+2
array[51][51] //x+1 and y+1
array[50][52] //y+2
array[49][51] //x-1 and y+1
etc.

I've been spending hours on finding an efficient way and finding a solution on the internet, but I haven't found a great answer yet. Anyone who knows how to do this?

Upvotes: 4

Views: 1588

Answers (2)

luk2302
luk2302

Reputation: 57124

This will not yet give you any java code but rather an idea on how to approach the problem.

Draw the grid of your array of a far smaller size on a pice of paper and draw the numbers in which the cell should get reached in each cell.

[ ][ ][ ][ ][ ]
[ ][ ][ ][ ][ ]
[ ][ ][1][ ][ ]
[ ][ ][ ][ ][ ]
[ ][ ][ ][ ][ ]

[ ][ ][ ][ ][ ]
[ ][ ][3][ ][ ]
[ ][2][1][4][ ]
[ ][ ][5][ ][ ]
[ ][ ][ ][ ][ ]

[ ][ ][8][ ][ ]
[ ][7][3][9][ ]
[6][2][1][4][A]
[ ][D][5][B][ ]
[ ][ ][C][ ][ ]

[ ][F][8][G][ ]
[E][7][3][9][H]
[6][2][1][4][A]
[L][D][5][B][I]
[ ][K][C][J][ ]

[M][F][8][G][N]
[E][7][3][9][H]
[6][2][1][4][A]
[L][D][5][B][I]
[P][K][C][J][O]

Clearly visible is the "snail" pattern. You always fill the outer neighbor cells of the previously filled cells.

Ignoring the outer bounds of the field the first iteration fills

1 cell - then
4 cells
8 cells
12 cells
... +4 cells

In terms of loops you should not loop using (i,j) which both reflect indices in the array. But rather loop over the rounds and then print the respective cells in that particular round. In round X you start in field arrayLength / 2 - X.

Upvotes: 6

gpasch
gpasch

Reputation: 2682

The basic looop is as follows -

K=1;
while(true) {
    k=K-1;
    for(n=0; n<=k; n++)
      x=-k+n; y=-n;       
    for(n=1; n<=k; n++)
      x=n; y=-k+n;        
    for(n=1; n<=k; n++)
      x=k-n; y=n;         
    for(n=1; n<=k-1; n++)
      x=-n; y=k-n;
    if x out of bounds or y out of bounds continue;
    K++;
    if(K>N/2) break;
}

where N is the size of the array (width or height) and K is the cycle where K=1 at the center.

To find the x y coordinates you go in four loops upper left upper right .... - and since the coordinate system is located at N/2, N/2 you have to add that to x y. There are K+2(K-1)+(K-2) elements in each cycle

Upvotes: 1

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