Fortran program giving a wrong output?

Question

I'm new to Fortran, so forgive me if I'm doing something stupid in the following code:

program test2
    implicit none

    ! Variable declaration
    integer :: i, n
    real    :: s

    ! Initialization
    n = 1e+9
    s = 0.0

    do i=1,n
        s = s + real(i)
        s = s + sqrt(s)
    end do

    print *, s

end program test2

This small program outputs: 1.8014399E+16 and I expect it to give 1.0000000010000024E+18. I use the GNU Fortran compiler on a Windows 10 machine.

I usually run long simulations involving many matrices. Matrix multiplications, concatenations, reshaping, indexing, FFTs, random number generations, MAX, mean, powers, complex numbers, etc are the typical operations I use and I read that Modern Fortran comes with heavy support for matrix operations with super high speeds. — AboAmmar
– AboAmmar, Commented Nov 30, 2016 at 23:01
If you change real ::s to double precision :: s do you get the answer you expect? — francescalus
– francescalus, Commented Nov 30, 2016 at 23:20
Wow, that is right. But I tried many other things like kind = 16, real*16, etc, but all didn't work, do you know why? — AboAmmar
– AboAmmar, Commented Nov 30, 2016 at 23:23
@francescalus -- Please elaborate your comment into an answer. — AboAmmar
– AboAmmar, Commented Nov 30, 2016 at 23:33
WRITE ( * , * ) 'SIZEOF(s)=', SIZEOF(s)... You may need REAL(i) to become DBLE(I) on the s = s + line... — Holmz
– Holmz, Commented Dec 2, 2016 at 13:04

AboAmmar · Accepted Answer · 2016-12-02 18:28:24Z

1

Well, I ended up using the following definition for the double precision variable s. This method has the advantage of being reusable and system independent. Thanks to @francescalus for the comment.

program test2
    implicit none

    ! Variable declaration
    integer, parameter :: dp = kind(0.d0)
    integer  :: i, n
    real(dp) :: s

    ! Initialization
    n = 1e+9
    s = 0.0

    do i=1,n
        s = s + real(i)
        s = s + sqrt(s)
    end do

    print *, s

end program test2

answered Dec 2, 2016 at 18:28

AboAmmar

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