CODEWITHSUNDEEP
cqsort
PatrickSteiner
PatrickSteiner

Reputation: 527

How to sort the rows of a 2D Matrix using qsort?

Many colleagues of mine were asking me if it is possible to sort each row of a 2D-Array, by using the function qsort() from <stdlib.h> to arrange a matrix like:

 5,  8,  7,  6, 
 1,  4,  3,  2, 
11, 12, 10,  9,

into something like:

 5,  6,  7,  8, 
 1,  2,  3,  4, 
 9, 10, 11, 12,

Upvotes: 2

Views: 2279

Answers (2)

Rakshith Murukannappa
Rakshith Murukannappa

Reputation: 589

Patrick,

Imagine the 2d array as a combination of many 1d arrays.

Perform a sort on each of the 1d arrays(rows), using 2 loops. One loop to iterate through the (columns) and one though the 1d arrays. In the second inner loop you can perform sorting.

Upvotes: 0

PatrickSteiner
PatrickSteiner

Reputation: 527

The solution to the problem looks like the following:

#include <stdio.h>   // scanf() printf()
#include <stdlib.h>  // qsort()

int compare (const void *a, const void *b)
{
  int x = *(int *)a;
  int y = *(int *)b;

  if (x<y) return -1; 
  if (x>y) return 1; 
  return 0;
}

int main()
{
  // Syntax of a 2D Array: array[rows][cols]
  int rows = 3, cols = 4;
  int array[3][4] = { {5,8,7,6,}, {1,4,3,2}, {11,12,10,9} };

  // Print the matrix unsorted:
  printf("\nUnsorted rows:\n");
  for (int i = 0; i < rows; i++) {
    for (int j = 0; j < cols; j++) {
      printf("%2d, ", array[i][j]);
    }
    printf("\n");
  }

  // Sort the matrix using qsort:
  for(int j = 0; j < rows; j++)
    qsort(array[j], cols, sizeof(int), compare);

  // Print the matrix sorted:
  printf("\nSorted rows:\n");
  for (int i = 0; i < rows; i++) {
    for (int j = 0; j < cols; j++) {
      printf("%2d, ", array[i][j]);
    }
    printf("\n");
  }

  return 0;
}

Output:

Unsorted rows:
 5,  8,  7,  6, 
 1,  4,  3,  2, 
11, 12, 10,  9, 

Sorted rows:
 5,  6,  7,  8, 
 1,  2,  3,  4, 
 9, 10, 11, 12,

Thanks to flukey for the helpful answer in: Qsorting 2d pointer arrays

Upvotes: 1

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