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Row-wise vs column-wise traversal of matrix

Two common ways of traversing a matrix are row-major-order and column-major-order.
Row Major Order : When matrix is accessed row by row.
Column Major Order : When matrix is accessed column by column.

Examples:

Input : mat[][] = {{1, 2, 3}, 
                  {4, 5, 6}, 
                  {7, 8, 9}}
Output : Row-wise: 1 2 3 4 5 6 7 8 9
         Col-wise : 1 4 7 2 5 8 3 6 9

Difference: If we see according to time complexity, both lead to O(n2), but when it comes to cache level one of the orders access will be faster as compare to other one. It depends on the language we are using. Like in C, store matrix in row major form so while accessing the i+1th element after ith, most probably it will lead to a hit, which will further reduce the time of program.
Following is C code showing the time difference in row major and column major access.

// C program showing time difference
// in row major and column major access
#include <stdio.h>
#include <time.h>
  
// taking MAX 10000 so that time difference
// can be shown
#define MAX 10000
  
int arr[MAX][MAX] = {0};
  
void rowMajor() {
  
  int i, j;
  
  // accessing element row wise
  for (i = 0; i < MAX; i++) {
    for (j = 0; j < MAX; j++) {
      arr[i][j]++;
    }
  }
}
  
void colMajor() {
  
  int i, j;
  
  // accessing element column wise
  for (i = 0; i < MAX; i++) {
    for (j = 0; j < MAX; j++) {
      arr[j][i]++;
    }
  }
}
  
// driver code
int main() {
  int i, j;
  
  // Time taken by row major order
  clock_t t = clock();
  rowMajor();
  t = clock() - t;
  printf("Row major access time :%f s "
                t / (float)CLOCKS_PER_SEC);
  
  // Time taken by column major order
  t = clock();
  colMajor();
  t = clock() - t;
  printf("Column major access time :%f s "
               t / (float)CLOCKS_PER_SEC);
  return 0;
}

Output:

Row major access time :0.492000 s
Column major access time :1.621000 s


This article is attributed to GeeksforGeeks.org

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