Fortran Factorial fortran 90

Tags:
1. May 29, 2015

Md. Abde Mannaf

Code (Text):

program factorial
implicit none
integer::fact,i,n
print*,'enter the value of 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

2. May 29, 2015

Staff: Mentor

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

1
2
3
4
5
6

etc.

3. May 29, 2015

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.

4. Jun 2, 2015

Zondrina

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.

5. Jun 2, 2015

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.

Code (Text):

program torial
implicit none

integer, parameter :: IK = selected_int_kind(18)
integer(IK) :: i

write(*,'(A,\$)') 'Number? : '
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

6. Jun 5, 2015

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!).