CODEWITHSUNDEEP
javaarraysmultidimensional-arraysub-array
Defqon
Defqon

Reputation: 107

Splitting columns of 2D array into a separate 2D array

I would like to send certain columns of a 2D array to another method as a 2D array. For example, I have the following array:

1 2 6 4
2 5 3 9
2 5 1 3

I would like to send lets say the middle 2 columns to a method as the 2D array:

2 6
5 3
5 1

How would I split this in a way that I could clarify the amount of columns being split and where I want the split to start and end. My understanding is that System.arrayCopy() would be an efficient way of performing this, however, I can only copy a 1D array with it.

Upvotes: 0

Views: 1136

Answers (3)

Joop Eggen
Joop Eggen

Reputation: 109597

There is a lesson in this.

It is easier to pick out a couple of rows of a two dimensional array. So maybe represent columns as rows.

int[][] arr = {
        {1, 2, 2},
        {2, 5, 5},
        {6, 3, 1},
        {4, 9, 3}
};

int[][] sub = new int[] {
        Arrays.copyOf(arr[1], 3),
        Arrays.copyOf(arr[2], 3)
};

int[][] sharing = Arrays.copyOfRange(arr, 1, 3); // 3 exclusive.

The last sharing shares the rows (columns) with arr.

One could make one's matrix class.

There is a story behind this. The Asian soroban is 90° rotated with respect to the Roman abacus. This makes it more readable IMHO, as reading numbers, and the soroban indeed is still a good pedagogical and useful tool.

Utilities: Arrays.copyOfRange​(T[],int,int).

Upvotes: 2

user14940971
user14940971

Reputation:

To get a 2d sub-array from an existing 2d array - you can iterate over the rows of this array, and for each row, iterate over the specified range using the Arrays.stream(int[],int,int) method:

int[][] arr1 = {
        {1, 2, 6, 4},
        {2, 5, 3, 9},
        {2, 5, 1, 3}};

int[][] arr2 = Arrays.stream(arr1)
        // range from '1' inclusive to '3' exclusive
        .map(row -> Arrays.stream(row, 1, 3).toArray())
        .toArray(int[][]::new);

// output
Arrays.stream(arr2).map(Arrays::toString).forEach(System.out::println);

[2, 6]
[5, 3]
[5, 1]

Upvotes: 1

Thiyagu
Thiyagu

Reputation: 17900

Try this.

int[][] arr = new int[][]{
        {1, 2, 6, 4},
        {2, 5, 3, 9},
        {2, 5, 1, 3}
};

int startColumn = 1;
int endColumn = 2;
int[][] result = Arrays.stream(arr)
        .map(row -> IntStream.rangeClosed(startColumn, endColumn)
                .map(j -> row[j]).toArray())
        .toArray(int[][]::new);

System.out.println(Arrays.deepToString(res));

Output: [[2, 6], [5, 3], [5, 1]]

It streams the 2D array and for each row, it builds a 1D array by picking the columns from startColumn to endColumn inclusive and collects every array in a 2D array.

Upvotes: 2

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