Algorithm to rotate a matrix using transpose

This is a horrible question but there is a neat trick.

To rotate 90 degrees just take transpose then reverse the rows. To rotate -90 (270) degrees take the transpose then reverse the columns.

val matrix = Array(Array(1,2), Array(3,4), Array(5,6))
print(matrix)

1 2
3 4
5 6

Transpose

def transpose(matrix: Array[Array[Int]]): Array[Array[Int]] = {
  matrix.head.indices.map(i => matrix.map(_(i))).toArray
}

print(transpose(matrix))

1 3 5
2 4 6

Rotate 90 degrees clockwise 

Transpose and reverse rows:

def rotate90(matrix: Array[Array[Int]]): Array[Array[Int]] = {
  transpose(matrix).map(_.reverse)
}

print(rotate90(matrix))

5 3 1
6 4 2

Voila.

Rotate 90 degrees anticlockwise

Transpose and reverse columns:

def rotateMinus90(matrix: Array[Array[Int]]): Array[Array[Int]] = {
  val t = transpose(matrix)
  t.head.indices.foreach { i =>
    (0 until (t.length/2)).foreach { j =>
      val temp = t(j)(i)
      t(j)(i) = t(t.length - j - 1)(i)
      t(t.length - j - 1)(i) = temp
    }
  }
  t
}

print(rotateMinus90(matrix))

2 4 6
1 3 5

Full code:

	object Rotation extends App {
	val matrix = Array(Array(1,2), Array(3,4), Array(5,6))
	print(matrix)
	print(transpose(matrix))
	print(rotate90(matrix))
	print(rotateMinus90(matrix))
	def transpose(matrix: Array[Array[Int]]): Array[Array[Int]] = {
	matrix.head.indices.map(i => matrix.map(_(i))).toArray
	}
	def rotate90(matrix: Array[Array[Int]]): Array[Array[Int]] = {
	transpose(matrix).map(_.reverse)
	}
	def rotateMinus90(matrix: Array[Array[Int]]): Array[Array[Int]] = {
	val t = transpose(matrix)
	t.head.indices.foreach { i =>
	(0 until (t.length/2)).foreach { j =>
	val temp = t(j)(i)
	t(j)(i) = t(t.length - j - 1)(i)
	t(t.length - j - 1)(i) = temp
	}
	}
	t
	}
	def print(matrix: Array[Array[Int]]): Unit = {
	matrix.foreach(r => println(r.mkString(" ")))
	println()
	}
	}

view raw Rotation.scala hosted with ❤ by GitHub