Fortran 90 Program Factorial: Fixing Incorrect Output

[Md. Abde Mannaf]

program factorial
  implicit none
  integer::fact,i,n
  print*,'enter the value of n'
  read*,n
  fact=1
  do i=1,n
  fact=fact*i
  end do

  print*,'factorial is ',fact
end program

when input n largest number then answer is incorrect. how to solve

 

[berkeman]

program factorial
  implicit none
  integer::fact,i,n
  print*,'enter the value of n'
  read*,n
  fact=1
  do i=1,n
  fact=fact*i
  end do

  print*,'factorial is ',fact
end program

when input n largest number then answer is incorrect. how to solve

Show us what you get for the first few possibilities for n:

1
2
3
4
5
6

etc.

 

[FactChecker]

Your code looks good to me. When you say the answer is incorrect for "the largest number", don't forget that fact will be larger than the largest allowable integer in that case. The program will either abort or give an incorrect answer. You can only input values of n that will not make n! too large for the integer fact.

 

[STEMucator]

There are implementations of the factorial function that allow the computation of factorials beyond 20!.

The implementations are much more difficult to write, but if you want to get around the storage limits, you have to be a bit more clever.

 

[gsal]

You need to start using Fortran90 intrinsic functions that allow for requesting more storage in a platform compatible manner, I am talking about functions like "selected_int_kind"; in my laptop, I was able to go as high as 18. Needless to say, the factorial function is a nice textbook exercise to show another Fortran90 feature: recursive attribute.

program torial
    implicit none
   
    integer, parameter :: IK = selected_int_kind(18)
    integer(IK) :: i
     
    write(*,'(A,$)') 'Number? : '
    read(*,*) i
    write(*,*) 'Factorial is ',factorial(i)

contains

    recursive function factorial( n ) result (f)
        integer(IK) :: f, n
       
        if( n == 1 ) then
          f = 1
        else
          f = n * factorial( n - 1_IK )
        end if
       
    end function factorial
   
end program torial

> a.exe
Number? : 20
Factorial is 2432902008176640000

 

[theodoros.mihos]

For large numbers may you don't need exact an integer. Try the same code with result as double or extended real (16 byte float number gives answer up to 4930!).