Fortran read file into array - transposed dimensions

Question

I'm trying to read a file into memory in a Fortran program. The file has N rows with two values in each row. This is what I currently do (it compiles and runs, but gives me incorrect output):

program readfromfile
  implicit none

    integer :: N, i, lines_in_file
    real*8, allocatable :: cs(:,:)

    N = lines_in_file('datafile.txt') ! a function I wrote, which works correctly

    allocate(cs(N,2))

    open(15, 'datafile.txt', status='old')
    read(15,*) cs

    do i=1,N
        print *, cs(i,1), cs(i,2)
    enddo

end

What I hoped to get was the data loaded into the variable cs, with lines as first index and columns as second, but when the above code runs, it first gives prints a line with two "left column" values, then a line with two "right column" values, then a line with the next two "left column values" and so on.

Here's a more visual description of the situation:

In my data file:       Desired output:        Actual output:
A1   B1                A1   B1                A1   A2
A2   B2                A2   B2                B1   B2
A3   B3                A3   B3                A3   A4
A4   B4                A4   B4                B3   B4

I've tried switching the indices when allocating cs, but with the same results (or segfault, depending on wether I also switch indices at the print statement). I've also tried reading the values row-by-row, but because of the irregular format of the data file (comma-delimited, not column-aligned) I couldn't get this working at all.

How do I read the data into memory the best way to achieve the results I want?

Solution

I do not see any comma in your data file. It should not make any difference with the list-directed input anyway. Just try to read it like you write it.

do i=1,N
    read (*,*) cs(i,1), cs(i,2)
enddo

Otherwise if you read whole array in one command, it reads it in column-major order, i.e., cs(1,1), cs(2, 1), ....cs(N,1), cs(1, 2), cs(2,2), ... This is the order in which the array is stored in memory.

Comments

[Tomas Aschan]

I'm amazed at why I couldn't get this working before, but now it does. Thanks! Regarding the commas in my input file, they are there in the actual file, but in the example I just wanted to visualize the ordering of the elements.

[Tomas Aschan]

And an upvote for the explanation of why it did what it did when it wasn't working. More rep to you, sir! =)

[Tomas Aschan]

For future readers of this post: I wanted to generalize the read to a subroutine that just takes the file name and the number of rows and cols to read, and ended up with the following implicit do construct: read(f, *) ((cs(row,col),col=1,Ncols),row=1,Nrows). Note that the columns have to be in the inner loop, for reasons mentioned by @Vladimir F in the answer above.