# Recursive function

I am trying to write a recursive function in fortran that sums numberes 1 to n? I just need help with the function, I understand how to call from the main program...Thanks!
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Principal Software EngineerCommented:
Many (I would venture to say, most) versions of FORTRAN do not support recursion.  Recursion was first supported in FORTRAN 90, and in the real world not in all F90 implementations.  First check the reference manual for the particular FORTRAN compiler to see if recursion is supported.

If recursion is supported, then:

sum(n) = n + sum(n-1)
sum(0) = 0
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Author Commented:
For the version that I am using, the recursion function seems to work.
So when I try to write the recursive function, the Sum(n) is the function or is that the result?
I know that the recursive function calls on itself, but is the Sum(n) the function?

I was going to go about it this way:

....
recursive function Sumofnumbers(n) result(res)

implicit none

integer :: res, n, i

if (n .eq. 0) then
res = 0
else
do i = 1, n
res = Sumofnumbers(n) + n
end do
endif

end function Sumofnumbers
....

But I don't seem to get the right sum...
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Principal Software EngineerCommented:
Consider this pseudo-code example instead, which more clearly demonstrates recursion:

integer function sum(input)
if (input ?= 0) then
sum = 0
else
sum = input + sum(input-1)
end if

Note that if this example is fed a negative integer, overflows will occur.
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Author Commented:
Thank you Dr. Klahn! I am going to close this question, but I appreciate the help and I got my recursive code to finally work! Best, Cynthia.
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