Computing Factorial

Problem Statement

The factorial of a non-negative integer n, written as n!, is defined as follows:

Write a program that reads in an integer and computes its factorial. This program should detect if the input is negative and display an error message.

Solution

! ----------------------------------------------------------
! Given a non-negative integer N, this program computes
! the factorial of N.  The factorial of N, N!, is defined as
!         N! = 1 x 2 x 3 x .... x (N-1) x N
! and 0! = 1.
! ----------------------------------------------------------

PROGRAM  Factorial
   IMPLICIT  NONE

   INTEGER :: N, i, Answer

   WRITE(*,*)  'This program computes the factorial of'
   WRITE(*,*)  'a non-negative integer'
   WRITE(*,*)
   WRITE(*,*)  'What is N in N! --> '
   READ(*,*)   N
   WRITE(*,*)

   IF (N < 0) THEN                 ! input error if N < 0
      WRITE(*,*)  'ERROR: N must be non-negative'
      WRITE(*,*)  'Your input N = ', N
   ELSE IF (N == 0) THEN           ! 0! = 1
      WRITE(*,*)  '0! = 1'
   ELSE                            ! N > 0 here
      Answer = 1                   ! initially N! = 1
      DO i = 1, N                  ! for each i = 1, 2, ..., N
         Answer = Answer * i       ! multiply i to Answer
      END DO
      WRITE(*,*)  N, '! = ', Answer
   END IF

END PROGRAM  Factorial

Program Input and Output

• If the input is -5, a negative number, the program generates the following output indicating the input is wrong.

  This program computes the factorial of
  a non-negative integer
  
  What is N in N! -->
  -5
  
  ERROR: N must be non-negative
  Your input N = -5
  

• If the input is a zero, the output is 0! = 1.

  This program computes the factorial of
  a non-negative integer
  
  What is N in N! -->
  0
  
  0! = 1
  

• If the input is 5, the factorial of 5 is 5!=1*2*3*4*5=120.

  This program computes the factorial of
  a non-negative integer
  
  What is N in N! -->
  5
  
  5! = 120
  

• If the input is 15, the factorial of 15 is 15! = 1*2*3*...*13*14*15=2004310016

  This program computes the factorial of
  a non-negative integer
  
  What is N in N! -->
  15
  
  15! = 2004310016
  

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