Question

Hey, I wrote this (fortran) with the aim of finding the minimum spanning tree of a bunch of points (syscount of them). I know for a fact that this approach works, since i wrote it in javascript earlier today. js is slow though, and i wanted to see how much faster fortran would be!!
only problem is it’s not working, i’m getting an annoying error;

prims.f95:72.43:

if((check == 1) .and. (path(nodesin(j))(k) < minpath)) then

                                 1

Error: Expected a right parenthesis in expression at (1)

What the hell is that about?! the 43rd character on the line is the “h” of “path”

nodesin(1) = 1

do i = 1,syscount-1
    pathstart = -1
    pathend = -1
    minpath = 2000

    do j = 1,i
        do k = 1, syscount

            check = 1
            do l = 1, i
                if(nodesin(l) == k) then
                    check = 0
                end if
            end do

            if((check == 1) .and. (path(nodesin(j))(k) < minpath)) then
                minpath = path(nodesin(j))(k)
                pathstart = nodesin(j)
                pathend = k
            end if

        end do
    end do

    nodesin(i+1) = pathend
    minpaths(i)(1) = pathstart
    minpaths(i)(2) = pathend

end do

Also, i’m fairly new to fortran, so i have a few other questions;

can i use && instead of .and. ?

is there a versions of the for(object in list){} loop found in many other languages?

is there a verion of the php function in_array ? i.e. bool in_array(needle,haystack), and if there is, is there a better way of doing it than:

check = false
Asize = size(array)
do i = 1, Asize
    if(array(i) == needle) then
        check = true
    end if
end do

then to using the check variable to see if it’s there?

(I haven’t posted anything on stackoverflow before. please don’t get angry if i’ve broken loads of etiquette things!)

Answer 1

It looks like you have defined path and minpaths as two-dimensional arrays. Multi-dimensional arrays are accessed differently in Fortran when compared to C-like languages. In Fortran you separate the indices by commas within one set of parentheses.

I’m guessing by the use of these variables they are integer arrays. Here is how you access elements of those arrays (since you didn’t share your variable declarations I am making up the shape of these arrays):

integer :: path(n1, n2)
integer :: minpaths(n3, 2)

your if statement should be:

if((check == 1) .and. (path(nodesin(j), k) < minpath)) then

your access to minpaths should be:

minpaths(i, 1) = pathstart
minpaths(i, 2) = pathend

Also, if you are not using IMPLICIT NONE I recommend you consider it. Not using it is dangerous, and you are using variable names that are close to each other (minpath and minpaths). You could save hours of hair pulling debugging by using IMPLICIT NONE.