Simple swapping code not working in swift - ios

Question

I am trying to take transpose of a 2d array in swift. But I don't know why the swapping is not happening.
The array remains the same after taking transpose. I am working with the following code:
var array_4x4 = [[Int]](count: 4, repeatedValue: [Int](count: 4, repeatedValue: 4))
for i in 0..<4
{
for j in 0..<4
{
let temp = Int(arc4random_uniform((100 + 1)) - 1) + 1
array_4x4[i][j] = temp
}
}
for i in 0..<4
{
for j in 0..<4 // code in this loop is not working
{
let temp = array_4x4[i][j]
array_4x4[i][j] = array_4x4[j][i]
array_4x4[j][i] = temp
}
}

Answer 1

Your nested loop runs over all possible array indices (i, j), which means that
each array element is swapped with the transposed element twice.
For example, when i=1 and j=2, the (1, 2) and the (2, 1) array elements are swapped.
Later, when i=2 and j=1, these elements are swapped back.
As a consequence, the matrix is identical to the original matrix in the end.
The solution is to iterate only over (i, j) pairs with i < j,
i.e. swap only the elements above the diagonal with their
counterpart below the diagonal:
for i in 0..<4 {
for j in (i + 1)..<4 {
let temp = array_4x4[i][j]
array_4x4[i][j] = array_4x4[j][i]
array_4x4[j][i] = temp
}
}
Note that the Swift standard library already has a function to
exchange two values:
for i in 0..<4 {
for j in (i + 1)..<4 {
swap(&array_4x4[i][j], &array_4x4[j][i])
}
}
And just for the sake of completeness:
an alternative solution would be to compute the transposed matrix as
a value, and assign it to the same (or a different) variable:
array_4x4 = (0..<4).map { i in (0..<4).map { j in array_4x4[j][i] } }