f90: Estimate the factorial of full/half-integer positive and half-integer negative values. *.exe and *.out are the Windows and Linux executables.

factorial.exe

factorial.f90

PROGRAM factorial

IMPLICIT NONE

! Declarations

REAL(8):: c, d, h, pi=3.1416
INTEGER(8):: f, f2, g, fact
 CHARACTER(1):: exiit ! f=n! and f2=(2n)!

! Opening prints

PRINT*,
PRINT*,
PRINT*, 'This program is written by Mohammad A. Nilforooshan.'
PRINT*, 'All rights reserved.'
PRINT*, 'http://sites.google.com/site/mannprofile/'

! Ask for the input value

PRINT*,
PRINT*,
PRINT*, 'Insert the input value.'
READ*, c

! Do the half-integer checkup

d=c-AINT(c)

IF (d/=-0.5 .AND. d/=0 .AND. d/=0.5) THEN
    PRINT*,
    PRINT*,
    PRINT*, 'The input value is not full or half integer.'
    STOP
ELSE IF (c<0 .AND. d==0) THEN
    PRINT*,
    PRINT*,
    PRINT*, 'No negative and full integer input'
    STOP
ELSE IF (c==0) THEN
    PRINT*,
    PRINT*,
    PRINT*, '0 ! = 1'
    STOP
ELSE IF (c>0 .AND. d==0) THEN
    g=INT(c)
    f=fact(g)
    h=f
    GO TO 66
ELSE IF (c>0) THEN
    g=INT(CEILING(c))
    f=fact(g)
    f2=fact(2*g)
    h=f2*SQRT(pi)/REAL((4**g)*f)
    GO TO 66
ELSE
    g=-INT(CEILING(c))
    f=fact(g)
    f2=fact(2*g)
    h=(-4**g)*f*SQRT(pi)/REAL(f2)
    GO TO 66
END IF

! Print the output on the screen

66 PRINT*,
PRINT*,
PRINT*, g
PRINT*, f
PRINT*, f2
WRITE(*,*) c,'! =',h

PRINT*,
PRINT*,
PRINT*, 'Press any key to exit.'
READ*, exiit

IF (exiit=='a') THEN
GO TO 82
END IF

82 END PROGRAM factorial

! Introducing the factorial functionn

INTEGER(8) FUNCTION fact(n)
INTEGER(8):: a, b, i, n

fact=1
a=n

DO i=1,n/2
    b=a*(a-1)
    fact=fact*b
    a=a-2
END DO

RETURN
END

factorial.out