Java how to fill down a 2d array

Question

So for example, this is what I'm asking: how can you convert this:

{{0, 1, 2},
 {3, 4, 5},
 {6, 7, 8},
 {9, 0, 1}}

to this:

{{0, 1, 2},
 {0, 1, 2},
 {0, 1, 2},
 {0, 1, 2}}

This is what I have so far:

void fillDown(int[][] grid) {
    int[][] m = {{}};
        int[][] newArray = zero(m);
        for (int i = 0; i < m.length; i++) {
            for (int j = 0; j < m[0].length; j++) {
                if(j== m.length-1){
                    print(newArray[i][j]);
                }else{
                    print(newArray[i][j] + " ");
                }
            }
        }
    }


    int[][] zero(int[][] m) {
        int rows = m.length;
        int columns = m[0].length;
        int[][] tempArray = new int[rows][columns];

        for (int i = 0; i < rows; i++) {
            for (int j = 0; j < columns; j++) {
                tempArray[i][j] = m[i][j];
            }
        }
        return tempArray;
}

But, when I input some values, it doesn't work as expected

For example, when I input something like:

{{0, 1, 2},
 {3, 4, 5},
 {6, 7, 8},
 {9, 0, 1}}

it will still return what I inputed:

{{0, 1, 2},
 {3, 4, 5},
 {6, 7, 8},
 {9, 0, 1}}

How would I do this?

I didn't look at your code, but from what you're stating that you're trying to accomplish, it looks like you're filling each cell with the value of the column number (outer loop). Why wouldn't you just do something like: for(int i = 0; i < array.length; i++){ for(int j = 0; j < array[0].length; j++){array[i][j] = i;}} — robotlos
– robotlos, Commented Apr 6, 2016 at 22:08
What exactly are you trying to accomplish? You never use the grid[][] you pass into the fillDown method. It's unclear what you are trying to do. — Nick Suwyn
– Nick Suwyn, Commented Apr 6, 2016 at 22:18

Yassin Hajaj · Accepted Answer · 2016-04-06 22:18:27Z

1

Use nested loops.

static void fillDown(int[][] grid) {
    for (int i = 1 ; i < grid.length ; i++){
        for (int j = 0 ; j < grid[i].length ; j++) {
            grid[i][j] = grid[0][j];
        }
    }
}

Or make a copy of the original array.

static void fillDown(int[][] grid) {
    for (int i = 1 ; i < grid.length ; i++){
        grid[i] = Arrays.copyOf(grid[0], grid[0].length);
    }
}

I would not use the following method because it just copies the reference and thus every change made on one of the indexes will impact all the other indexes.

for (int i = 1 ; i < grid.length ; i++) {
    grid[i] = grid[0]; // /!\ NOT A GOOD IDEA
}

edited Apr 6, 2016 at 22:18

answered Apr 6, 2016 at 22:13

Yassin Hajaj

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3 Comments

SomeDude Over a year ago

There is no need of nested loop, just replace each row starting from second with first one.

Yassin Hajaj Over a year ago

@svasa This keeps a reference to the original array.

Ceelos Over a year ago

fillDown is not the method with the issue.

robotlos · Accepted Answer · 2016-04-06 22:33:42Z

After staring at your question, it looks like you're talking about your int[][] zero(int[][] m) method (correct me if I'm wrong).

In your int[][] zero(int[][] m) method, replace

    tempArray[i][j] = m[i][j];

with

    tempArray[i][j] = j;

It should look like so:

public static int[][] zero(int[][] m) {
    int rows = m.length;
    int columns = m[0].length;
    int[][] tempArray = new int[rows][columns];

    for (int i = 0; i < rows; i++) {
        for (int j = 0; j < columns; j++) {
            tempArray[i][j] = j;
        }
    }
    return tempArray;
}

Running the following:

int[][] array = new int[][]{{0, 1, 2},
                            {3, 4, 5},
                            {6, 7, 8},
                            {9, 0, 1}};
System.out.println("Before:");
for (int[] i : array) {
    for (int j : i) {
        System.out.print(j + " ");
    }
    System.out.println();
}
array = zero(array);
System.out.println("After:");
for (int[] i : array) {
    for (int j : i) {
        System.out.print(j + " ");
    }
    System.out.println();
}

Resulted in:

run:
Before:
0 1 2 
3 4 5 
6 7 8 
9 0 1 
After:
0 1 2 
0 1 2 
0 1 2 
0 1 2 
BUILD SUCCESSFUL (total time: 0 seconds)

SomeDude · Accepted Answer · 2016-04-06 22:22:31Z

0

Actually just curious why would you want to do this. Anyway, it can be done like :

int[][] a = {{0, 1, 2},
             {3, 4, 5},
             {6, 7, 8},
             {9, 0, 1}};
int[] replacewith = Arrays.copyOf( a[0], a[0].length );
for ( int i = 1; i < a.length; i++ )
{
   a[i] = replacewith;
}

edited Apr 6, 2016 at 22:22

answered Apr 6, 2016 at 22:12

SomeDude

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