Fortran function for geometric series

Question

I am implementing this equation with a Fortran function.

Write call in function (g) consistently returns 6, but when I call function in program (z) output depends, e.g., -2123950080 -529463296 929961984.

Why g and z are not the same? What should I change in function to accomplish geometric series calculation?

function  geomSeries(n)
implicit none
integer :: i, n, g = 0, x = 1, geomSeries

g = 1 + x
do i = 2, n
    g = g + x**i
end do
write (*,*) g
return
end function geomSeries

program geomFunc
implicit none
integer :: n = 5, geomSeries, z

z =  geomSeries(n)
write (*,*) z
end program geomFunc

P.S. Ideally I would like it to be pure function, as I don't see how it produces side effects, but haven't managed to compile it that way (why?).

Your variables g and x are saved and are initialized *only once*. We have duplicates. — Vladimir F Героям слава, Sep 26 '19 at 10:09
Rather more crucially, you don't assign to the function result `geomseries`. `g` is not the same thing. — francescalus, Sep 26 '19 at 10:12
Ups, yes this is actually the immediate problem, the linked one will only appear after a second call. — Vladimir F Героям слава, Sep 26 '19 at 10:15
Thanks, assigning geomSeries = g did the trick! I'm new to Fortran and I didn't know function returns only what's assigned to function name, because I'm used to passing different variables to return in Python or R. — tRash, Sep 26 '19 at 10:49
@tRash a function statement could be `function geomSeries(n) result(g)`. More information at [wikibooks](https://en.wikibooks.org/wiki/Fortran/Fortran_procedures_and_functions) — PTRK, Sep 26 '19 at 12:17
Or, indeed, in the link above https://stackoverflow.com/questions/31061507/what-is-the-purpose-of-result-variables-in-fortran — Vladimir F Героям слава, Sep 26 '19 at 15:12

Fortran function for geometric series

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