Java: Sorting a 2D Array by Columns without Arrays.sort()

Question

I'm trying to set up a method that will sort a two dimensional array of doubles by column. Based on the provided specifications, this method is also not supposed to take ragged arrays with rows of unequal lengths. I'm testing this using the double[][] mdarray = {{3.0, 4.0, 1.0, 8.0}, {13.0, 2.0, 12.0, 9.0}

Using a print method, this should be displayed as

3.0, 2.0, 1.0, 8.0,

13.0, 4.0, 12.0, 9.0,

When using a separate print method to output the result, the array appears to be unchanged. I think my problem is figuring out how to check every row for a given column before moving onto the next, but I'm not sure how to do that easily without the Arrays.sort() method, which is not allowed for this exercise.

public void sort(boolean byColumn)
{
    if(isRagged() == true)
    {
        System.out.println("Ragged arrays cannot be sorted by column.");
    }
    else
    {
        for(int i = 0; i < mdarray.length; i++)
        {
            for(int j = i + 1; j < mdarray.length - 1; j ++)
            {
                for(int k = 0; k < mdarray[i].length; k++)
                {
                    if(mdarray[i][k] > mdarray[j][k])
                    {
                        double temp = mdarray[i][k];
                        mdarray[i][k] = mdarray[j][k];
                        mdarray[i][k] = temp;
                    }
                }
            }
        }
    }
}

@shmosel you probably meant it's supposed to be mdarray[j][k] = temp — radoh
– radoh, Commented Dec 8, 2016 at 7:58
I switched it to what you suggested. The original array didn't change still, but a separate test array {{1.0, 5.0, 7.0}, {4.0, 1.0, 3.0}, {9.0, 2.0, 8.0} changed to {{1.0, 1.0, 3.0}, {4.0, 5.0, 7.0}, {9.0, 2.0, 8.0}. It's definitely closer. — user7218814
– user7218814, Commented Dec 8, 2016 at 8:04

radoh · Accepted Answer · 2016-12-08 08:22:22Z

1

You've actually switched the bounds of i and j variables as well. Since j iterates from i+1, it should gain values up to mdarray.length - 1, therefore the condition should be j < mdarray.length and similarly i's upper bound should be mdarray.length - 2 therefore the condition should be i < mdarray.length - 1.

Oh and also there's the mistake we mentioned in comments - when switching the elements, assign the temp variable to mdarray[j][k].

answered Dec 8, 2016 at 8:22

radoh

4,8555 gold badges34 silver badges50 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Andrea Grillo · Accepted Answer · 2016-12-08 08:47:36Z

As you know you can consider a 2D array as a group of columns or rows. This program has to sort columns, so we'll consider the array as a group of columns.

The program is done:

loops all columns

choose your favorite algorithm for sorting a 1D array (take a look here)

double[][] mdarray = {{3.0, 4.0, 1.0, 8.0}, {13.0, 2.0, 12.0, 9.0}};

//loop all columns
for(int col = 0; col < mdarray[0].length; col++){

    //loops all rows and use bubble sort algorithm in ascending order
    for(int i = 1; i < mdarray.length; i++){

        for(int j = i; j < mdarray.length; j++){
            if(mdarray[j - 1][col] > mdarray[j][col]){
                //swap
                double temp = mdarray[j][col];
                mdarray[j][col] = mdarray[j-1][col];
                mdarray[j-1][col] = temp;
            }
        }
    }
}

If you print each row of mdarray, the output will be:

mdarray[0]: 3.0  2.0 1.0  8.0
mdarray[1]: 13.0 4.0 12.0 9.0

Collectives™ on Stack Overflow

Java: Sorting a 2D Array by Columns without Arrays.sort()

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related