Java, transpose a 2d array with dynamic rows and columns

Question

I am atempting to transpose a matrix with wach row having a different number of columns. For example what I am trying is this

public Object[][] transpose (Object[][] data) {
    Object temp = new Object[data[0].length][];
    for (int i = 0; i < data.length; i++) {
         temp[i] = new Object[data[i].length];
         for ( int j = 0; j < data[i].length; j++) {
              temp[j][i] = data[i][j]
         }
    }
    return temp;
}

It is appearing that the fourth line is not working correctly which I am not quite sure how to work around it.

That code doesn't compile. It should be Object[][] temp, and you're missing a semi-colon after data[i][j]. — Andreas
– Andreas, Commented Jan 10, 2016 at 21:55

Andreas · Accepted Answer · 2016-01-10 22:02:44Z

If your input looks like this:

[ [ 1, 2       ],
  [ 3, 4, 5, 6 ],
  [ 7          ] ]

The transposed result could be one of these:

        SIMPLE                 COMPACT
[ [    1, 3,    7 ],      [ [    1, 3, 7 ],
  [    2, 4, null ],        [    2, 4    ],
  [ null, 5, null ],        [ null, 5    ],
  [ null, 6, null ] ]       [ null, 6    ] ]

In either case, the result should have as many columns as the longest row, not the first row.

public static Object[][] transposeSimple(Object[][] data) {
    int maxLen = 0;
    for (Object[] row : data)
        if (row.length > maxLen)
            maxLen = row.length;
    Object[][] temp = new Object[maxLen][data.length];
    for (int i = 0; i < data.length; i++)
        for (int j = 0; j < data[i].length; j++)
            temp[j][i] = data[i][j];
    return temp;
}

public static Object[][] transposeCompact(Object[][] data) {
    int maxLen = 0;
    for (Object[] row : data)
        if (row.length > maxLen)
            maxLen = row.length;
    Object[][] temp = new Object[maxLen][];
    for (int i = 0; i < temp.length; i++) {
        for (maxLen = data.length; maxLen > 0; maxLen--)
            if (data[maxLen - 1].length > i)
                break;
        temp[i] = new Object[maxLen];
        for (int j = 0; j < maxLen; j++)
            if (i < data[j].length)
                temp[i][j] = data[j][i];
    }
    return temp;
}

Test

System.out.println(Arrays.deepToString(transposeSimple(new Object[][] {
        { 1, 2       },
        { 3, 4, 5, 6 },
        { 7          },
})));
System.out.println(Arrays.deepToString(transposeCompact(new Object[][] {
        { 1, 2       },
        { 3, 4, 5, 6 },
        { 7          },
})));

Output

[[1, 3, 7], [2, 4, null], [null, 5, null], [null, 6, null]]
[[1, 3, 7], [2, 4], [null, 5], [null, 6]]

Collectives™ on Stack Overflow

Java, transpose a 2d array with dynamic rows and columns

1 Answer 1

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related