To avoid accumulation error, try dp (double precision) here.

--- In Fortran@yahoogroups.com, "Eric" <yahoofortran@...> wrote:
>
> I am very new to FORTRAN and need to learn it quick. I wrote the
> following code to find the definite integral of x^2 between 0 and 1
> which should come out to 0.33333 etc. This same code with slight
> syntax changes works in visual basic. When I run it in FORTRAN I
> get .015625
>
> The strange thing is if I try to make it less accurate by setting
> dxval to 1000 and running the DO loop 1000 times I get a much more
> accurate answer of .332828, running the same code in VB 1000 times
> gives me .332833
>
> I am guessing this has to do with a lack of understanding on my part
> of how numbers are stored and calculated in FORTRAN variables but I
> just can't find the mistake. I also tried converting dxval in the DO
> loop to an integer with int(dxval) and still got the same result.
>
> Here's the code. I am using a compiler called Plato3 and running it
> under Windows XP.
>
> PROGRAM ERIC
> IMPLICIT NONE
>
> real,parameter :: dxval=100000000.0,lower=0.0,upper=1.0
> real :: dx,x,y
> integer :: i
>
> y=0.0
> dx=(upper-lower)/dxval
> x=0.0
>
> DO i = 1,dxval, 1
> y = y + ((x**2)*dx)
> x = x + dx
>
> END DO
>
> write(*,*) y
>
>
> END PROGRAM
>