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Task/Sorting-algorithms-Quicksort/Fortran/sorting-algorithms-quicksort.f

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+MODULE Qsort_Module
+
+IMPLICIT NONE
+
+CONTAINS
+
+RECURSIVE SUBROUTINE Qsort(a)
+
+  INTEGER, INTENT(IN OUT) :: a(:)
+  INTEGER :: split
+
+  IF(size(a) > 1) THEN
+     CALL Partition(a, split)
+     CALL Qsort(a(:split-1))
+     CALL Qsort(a(split:))
+  END IF
+
+END SUBROUTINE Qsort
+
+SUBROUTINE Partition(a, marker)
+
+  INTEGER, INTENT(IN OUT) :: a(:)
+  INTEGER, INTENT(OUT) :: marker
+  INTEGER :: left, right, pivot, temp
+
+  pivot = (a(1) + a(size(a))) / 2  ! Average of first and last elements to prevent quadratic
+  left = 0                         ! behavior with sorted or reverse sorted data
+  right = size(a) + 1
+
+  DO WHILE (left < right)
+     right = right - 1
+     DO WHILE (a(right) > pivot)
+        right = right-1
+     END DO
+     left = left + 1
+     DO WHILE (a(left) < pivot)
+        left = left + 1
+     END DO
+     IF (left < right) THEN
+        temp = a(left)
+        a(left) = a(right)
+        a(right) = temp
+     END IF
+  END DO
+
+  IF (left == right) THEN
+     marker = left + 1
+  ELSE
+     marker = left
+  END IF
+
+END SUBROUTINE Partition
+
+END MODULE Qsort_Module
+
+PROGRAM Quicksort
+
+  USE Qsort_Module
+
+  IMPLICIT NONE
+  INTEGER, PARAMETER :: n = 100
+  INTEGER :: array(n)
+  INTEGER :: i
+  REAL :: x
+    CALL RANDOM_SEED
+  DO i = 1, n
+     CALL RANDOM_NUMBER(x)
+     array(i) = INT(x * 10000)
+  END DO
+
+  WRITE (*, "(A)") "array is :-"
+  WRITE (*, "(10I5)") array
+  CALL Qsort(array)
+  WRITE (*,*)
+  WRITE (*, "(A)") "sorted array is :-"
+  WRITE (*,"(10I5)") array
+
+END PROGRAM Quicksort