I don't understand the logic behind this code please explain it.

Question

class TwoDAgain {
public static void main(String args[]) {
int twoD[][] = new int[4][];
twoD[0] = new int[1];
twoD[1] = new int[2];
twoD[2] = new int[3];
twoD[3] = new int[4];

int i, j, k = 0;

for(i=0; i<4; i++)
for(j=0; j<i+1; j++) {
twoD[i][j] = k;
k++;
}

for(i=0; i<4; i++) {
for(j=0; j<i+1; j++)
System.out.print(twoD[i][j] + " ");
System.out.println();
}
}
}

Answer 1

// Create a 2 dimension array with 4 rows
// each row has different number of column
int twoD[][] = new int[4][];
twoD[0] = new int[1];
// row 0 - 1 column
twoD[1] = new int[2];
// row 1 - 2 column
twoD[2] = new int[3];
// row 2 - 3 column
twoD[3] = new int[4];
// row 3 - 4 column

int i, j, k = 0;
/*
* <i> and <j> are loop iterators
* <k> is the number to print
* Outer loop goes from 0 to 3 (rows)
* Inner loop goes from 0 to <i> (columns)

<i> = 0, <j> ranges from 0 to 0
i j k
twoD[0][0] = 0

<i> = 1, <j> ranges from 0 to 1
i j k
twoD[1][0] = 1
twoD[1][1] = 2

<i> = 2, <j> ranges from 0 to 2
i j k
twoD[2][0] = 3
twoD[2][1] = 4
twoD[2][2] = 5

<i> = 3, <j> ranges from 0 to 3
i j k
twoD[3][0] = 6
twoD[3][1] = 7
twoD[3][2] = 8
twoD[3][3] = 9
*/
for(i=0; i<4; i++)
for(j=0; j<i+1; j++) {
twoD[i][j] = k;
k++;
}

// Next loops uses similar logic, but for showing values.