Creating a 2d matrix from an array (java)

Question

I'm supposed to write a method that creates a 2d matrix from an array, for instance: ({1, 2, 3, 4}, 3) should return the matrix {{1, 2, 3}, {4}}

public class Matrix {
  public static int[][]toM(int[] array, int a) {
    int[][]matrix = new int [(array.length + a- 1)/ a][a];
    for (int i = 0; i < array.length; i++){
      int value = array[i];
      value = value++;
      for (int row = 0; row < (array.length + a- 1)/a; row++) {
        for (int col = 0; col < a; col++) {
          matrix[row][col]= value++;
        }
      } 
    }
    return matrix;
  }
}

a is the number of elements in each row. how am i supposed to get [[1, 2, 3], [4]] if my input is int[] array = {1,2,3,4} and int n =3? I'm getting [[4, 5, 6], [7, 8, 9]]?

Answer 1

I think the following is closer to what you have in mind:

public static int[][] toM2(int[] array, int a) {
    int[][] matrix = new int [(array.length + a- 1)/ a][a];
    for (int i = 0; i < array.length; i++) matrix[i/a][i%a] = array[i];
    return matrix;
}

The line 'int value = array[i]' followed by value++ indicate that you are thinking about this problem like a C programmer. The array[i] does not give you a pointer to a value, it just gives you a value.

So, the key is to take the index and transform it into row and column references:

int row = i/a;
int col = i%a;

There is still the problem of the rows all being the same length. With java you don't have to allocate all the rows at the same time. In fact, you can a new array for each row. The following is complicated, but it works:

public static int[][] toM3(int[] array, int a) {
    int[][] matrix = new int[(array.length + a - 1) / a][];
    int rowStart = 0;
    for (int i = 0; i < array.length; i++) {
        int row = i/a;
        if (matrix[ row ] == null) {
            matrix[ row ] = new int[ Math.min( a, array.length-rowStart) ];
            rowStart += a;
        }
        matrix[ row ][i % a] = array[i];
    }
    return matrix;
}

Answer 2

I think something like this is a lot more readable:

static int[][] transform(int[] arr, int N) {
    int M = (arr.length + N - 1) / N;
    int[][] mat = new int[M][];
    int start = 0;
    for (int r = 0; r < M; r++) {
        int L = Math.min(N, arr.length - start);
        mat[r] = java.util.Arrays.copyOfRange(arr, start, start + L);
        start += L;
    }
    return mat;
}

Your resulting matrix will be MxN, with the last row possibly having less. It uses Arrays.copyOfRange instead of manually allocating and copying rows, and some math to figure out M (how many rows will this matrix have?), and L (how many elements will be on this row?)

    System.out.println(Arrays.deepToString(
        transform(new int[] {1,2,3,4,5,6}, 4)
    )); // prints "[[1, 2, 3, 4], [5, 6]]"