Original matrix:
2 4 5 6 
3 4 6 2
6 7 7 2
1 1 1 1
So after swapping diagonals we get:
6 4 5 2
3 6 4 2
6 7 7 2
1 1 1 1

let temp = mxt[x][x]
mxt[x][x] = mxt[x][size-x-1]
mxt[x][size-x-1] = temp

import Foundation
import Glibc

// Size of the array 
var size = 3

// Function to interchange the diagonals of the matrix
func diagonalInterchange(M:[[Int]]){
   var mxt : [[Int]] = M

   // Interchanging diagonals by
   // swapping the elements
   for x in 0..<size{
      if (x != size/2){
         let temp = mxt[x][x]
         mxt[x][x] = mxt[x][size-x-1]
         mxt[x][size-x-1] = temp
      }
   }

   // Displaying matrix
   print("Matrix after diagonals interchange:")
   for m in 0..<size{
      for n in 0..<size{
         print(mxt[m][n], terminator: " ")
      }
      print("\n")
   }
}

// Creating 3x3 matrix of integer type
var matrix : [[Int]] = [[2, 3, 4], 
   [5, 6, 7], 
   [8, 3, 2]]

print("Original Matrix:")
for x in 0..<size{
   for y in 0..<size{
      print(matrix[x][y], terminator:" ")
   }
   print("\n")
}

// Calling the function 
diagonalInterchange(M:matrix)

Original Matrix:
2 3 4 

5 6 7 

8 3 2 

Matrix after diagonals interchange:
4 3 2 

5 6 7 

2 3 8