How to sort a 2 dimensional array with all elements in ascending order in java

Question

Is there any efficient way to sort a 2 dimensional array in java like using Arrays.sort().For ex:-

a={{3 ,1},{0,2})
result={{0,1},{2,3}}

This was my approach:

  for (int x = 0; x < n; x++) {
      for (int y = 0; y < n; y++) {
            for (int i = 0; i < n; i++) {
                for (int j = 0; j < n; j++) {
                    if (grid[i][j] > grid[x][y]) {
                        int t = grid[x][y];
                        grid[x][y] = grid[i][j];
                        grid[i][j] = t;
                    }
                }
            }
        }
    }

Given that your array loses a zero and gains a four, I don't think it counts as sorting. — Joe C, Jun 18 '17 at 09:52
@Aominè that post deals with sorting column wise..I do'nt want to sort only the column..but the entire array — ani, Jun 18 '17 at 09:54
It seems you just want a 1D array, and then group elements by pairs. — JB Nizet, Jun 18 '17 at 09:54
@JBNizet I have to input a 2d array and output a sorted 2d array — ani, Jun 18 '17 at 09:55
The answer to your question is "yes". Now, have you tried something? Are you having problems with your attempts? If so, what are they? — Joe C, Jun 18 '17 at 09:56
@ani what if one of the 2d array has just 1 element? Should it get sorted independently? — Darshan Mehta, Jun 18 '17 at 09:56
What do you mean by 'efficient'? Space-efficient or time-efficient? — Sam, Jun 18 '17 at 09:59
Then make a 1D array containing all the elements of your 2D array, sort it, then create a 2D array from the sorted 1D array. — JB Nizet, Jun 18 '17 at 10:02

Youcef LAIDANI · Answer 1 · 2017-06-18T10:17:09.443

You can solve your problem using :

public static void main(String[] args) {
    Integer[][] a = {{3, 1}, {0, 2}};
    List<Integer> list = new ArrayList<>();
    for (Integer[] i : a) {//<--convert the 2d to list-------------------------(1)
        list.addAll(Arrays.asList(i));
    }
    Collections.sort(list);//<--sort this list---------------------------------(2)
    Integer[][] result = new Integer[a.length][];//create new 2d array---------(3)
    int k = 0;
    for (int i = 0; i < a.length; i++) {//loop throw the original array--------(4) 
        //creae temp array with the same size of each 1d array-----------------(5)
        Integer[] temp = new Integer[a[i].length];
        //loop and fill temp array with elements of the list-------------------(6)
        for (int j = 0; j < a[i].length; j++) {
            temp[j] = list.get(k);
            k++;
        }
        result[i] = temp;//add the temp to the new array-----------------------(7)
    }

    System.out.println(Arrays.deepToString(result));//print the new array
}

example

input                                           output
{{3, 1}, {0, 2}}                                [[0, 1], [2, 3]]
{{3, 1, 5}, {0, 2}}                             [[0, 1, 2], [3, 5]]     
{{3, 1, 5}, {0, 5}, {3, 7, 5}, {10, 9, 11}}     [[0, 1, 3], [3, 5], [5, 5, 7], [9, 10, 11]]

Note this solution will assure the same length of each node of the original array.

Might be worth looking into this Q&A for a cleaner way to flatten out the array: https://stackoverflow.com/questions/2569279/how-to-flatten-2d-array-to-1d-array — christopher, Jun 18 '17 at 10:10

score 1 · Accepted Answer · answered Jun 18 '17 at 10:09

You can sort all the elements first and then generate pair or n elements, e.g.;

int[][] a={{3 ,1},{0,2}};
int count = a[0].length;

//Sort the elements
List<Integer> sortedElements = Arrays.stream(a)
    .flatMapToInt(e -> Arrays.stream(e))
    .boxed()
    .sorted()
    .collect(Collectors.toList());

//Now, generate 2D arrays
int[][] arrays = new int[a.length][];
int[] temp = new int[count];
int index = 0;
for(int i = 0; i < sortedElements.size() ; i++){
    if(i % count == 0 && i != 0){
        arrays[index++] = temp;
        temp = new int[count];
    }
    temp[i % count] = sortedElements.get(i);
}
arrays[index++] = temp;

for(int[] e : arrays){
    System.out.println(Arrays.toString(e));
}

@ani this: https://stackoverflow.com/questions/31301471/big-o-complexity-of-java-util-stream-streamt-sorted — Darshan Mehta, Jun 18 '17 at 10:22

How to sort a 2 dimensional array with all elements in ascending order in java

2 Answers2