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MODULE Sort
CONTAINS
RECURSIVE SUBROUTINE Module_merge_sort (a, ascend)
IMPLICIT NONE
INTEGER, DIMENSION(:), INTENT(INOUT) :: a
LOGICAL, INTENT(IN), OPTIONAL :: ascend
LOGICAL :: up
INTEGER low, high, mid
IF (PRESENT(ascend)) THEN
up = ascend
ELSE
up = .TRUE.
ENDIF
low=LBOUND(a,1)
high=UBOUND(a,1)
IF (low<high) THEN
mid=(low+high)/2
CALL Module_merge_sort(a(low:mid), up)
CALL Module_merge_sort(a(mid+1:high), up)
a(low:high) = Merge(a(low:mid), a(mid+1:high), up)
END IF
END SUBROUTINE Module_merge_sort
FUNCTION Merge (a, b, up)
INTEGER, DIMENSION(:), INTENT(INOUT) :: a, b
INTEGER, DIMENSION(SIZE(a)+SIZE(b)) :: Merge
LOGICAL, INTENT(IN) :: up
INTEGER a_ptr, a_high, a_low
INTEGER b_ptr, b_high, b_low
INTEGER c_ptr
LOGICAL condition
a_low=LBOUND(a,1)
a_high=UBOUND(a,1)
b_low=LBOUND(b,1)
b_high=UBOUND(b,1)
a_ptr=a_low
b_ptr=b_low
c_ptr=1
DO WHILE (a_ptr<=a_high .AND. b_ptr<=b_high)
IF (up) THEN
condition= (a(a_ptr) <= b(b_ptr))
ELSE
condition= (a(a_ptr) >= b(b_ptr))
END IF
IF (condition) THEN
Merge(c_ptr)=a(a_ptr)
a_ptr=a_ptr+1
ELSE
Merge(c_ptr)=b(b_ptr)
b_ptr=b_ptr+1
END IF
c_ptr = c_ptr + 1
END DO
IF (a_ptr>a_high) THEN
Merge(c_ptr:) = b(b_ptr:b_high)
ELSE
Merge(c_ptr:) = a(a_ptr:a_high)
END IF
END FUNCTION Merge
END MODULE Sort
PROGRAM Merge_sort
USE Sort
INTEGER, DIMENSION(:), ALLOCATABLE :: array
INTEGER i, n
REAL r, time
PRINT*, "Enter array size:"
READ(*,*) n
ALLOCATE( array(n) )
DO i=1, n
CALL RANDOM_NUMBER(r)
array(i)=100 * r
END DO
PRINT '(20I3)', array(1:20)
time = Second()
CALL Module_merge_sort(array)
PRINT '("[Sort time = ",F10.3," seconds ]")', Second() - time
PRINT '(20I3)', array(1:20)
time = Second()
CALL Module_merge_sort(array, .FALSE.)
PRINT '("[Sort time = ",F10.3," seconds ]")', Second() - time
PRINT '(20I3)', array(1:20)
CONTAINS
REAL FUNCTION Second()
IMPLICIT NONE
INTEGER i, timer_count_rate, timer_count_max
CALL SYSTEM_CLOCK( i, timer_count_rate, timer_count_max )
Second = REAL(i) / timer_count_rate
END FUNCTION Second
END PROGRAM Merge_sort
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