Fortran: Calling other functions in a function

Question

I wrote the GNU Fortran code in two separate files on Code::Blocks: main.f95, example.f95. main.f95 content:

program testing

   use example

   implicit none
   integer :: a, b

   write(*,"(a)", advance="no") "Enter first number: "
   read(*,*) a

   write(*,"(a)", advance="no") "Enter second number: "
   read(*,*) b

   write(*,*) factorial(a)
   write(*,*) permutation(a, b)
   write(*,*) combination(a, b)

end program testing

example.f95 content:

module example


contains


  integer function factorial(x)

     implicit none
     integer, intent(in) :: x
     integer :: product_ = 1, i

     if (x < 1) then

        factorial = -1

     else if (x == 0 .or. x == 1) then

        factorial = 1

     else

        do i = 2, x
           product_ = product_ * i
        end do

        factorial = product_

     end if

  end function factorial


  real function permutation(x, y)

     implicit none
     integer, intent(in) :: x, y
     permutation = factorial(x) / factorial(x - y)

  end function permutation


  real function combination(x, y)

     implicit none
     integer, intent(in) :: x, y

     combination = permutation(x, y) / factorial(y)

  end function combination


end module example

When I run this code, the output is:

Enter first number: 5
Enter second number: 3
     120
   0.00000000    
   0.00000000    

The permutation and combination functions don't work properly. Thanks for answers.

Answer 1

I think you've fallen foul of one of Fortran's well-known (to those who know it) gotchas. But before revealing that I have to ask how much testing you did ? I ran your code, got the odd result and thought for a minute ...

then I tested the factorial function for a few small values of x which produced

 factorial            1  =            1
 factorial            2  =            2
 factorial            3  =           12
 factorial            4  =          288
 factorial            5  =        34560
 factorial            6  =     24883200
 factorial            7  =    857276416
 factorial            8  =   -511705088
 factorial            9  =   1073741824
 factorial           10  =            0

which is obviously wrong. So it seems that you didn't test your code properly, if at all, before asking for help. (I didn't test your combination and permutation functions.)

O tempora, o mores

You've initialised the variable product_ in the line

 integer :: product_ = 1, i

and this automatically means that product_ acquires the attribute save so its value is stored from invocation to invocation (gotcha !). At the start of each call (other than the first) product_ has the value it had at the end of the previous call.

The remedy is simple, don't initialise product_ . Change

 integer :: product_ = 1, i

to

 integer :: product_ , i
 ...
 product_ = 1

Simpler still would be to not write your own factorial function but to use the intrinsic product function but that's another story.