How to call fortran subroution?

["web" <jiali3(at)21cn.com>]

Hi, I havenot find a perfect example on how to call fortran subroutine from
idl.

For example, a(0) and a(1) are computed in an IDL program, I want a fortran
subroutine to compute c=a(0)+a(1). After c has been returned,  d=a(0)+a(1)+c
is computed in IDL. How to do that? I think we will know how to call fortran
subroutine if we can do above.

1.IDL PROGRAM: test.pro

pro test
a=fltarr(2)
a(0)=100 & a(1)=200
call_external sum_fortran(a,c)
d=a(0)+a(1)+c
print,a,c,d
end

2.FORTRAN SUBROUTINE: sum_fortran.f

subroutine sum_fortran(a,c)
dimension a(2)
c=a(1)+a(2)
return
end

Would you please help me to modify the above and tell me how to run?

Can fortran call idl program?

Best regards
Jiali