Fortran read from fie into arrays - fortran

Question

I need to read from a txt file into three arrays in order to calculate distances between coordinates. I have looked through other answers and pieced together the script bellow. The columns of the input file need to be come my x, y and z arrays respectively. There are 64 rows and 16 decimal places for each entry. Pointers would be greatly appreciated.
the data format:
0.8607595188703266 0.9252035918116585 0.4094258340665792
0.5246038490998378 0.9804633529144733 0.5325820695466118
0.6955271184611949 0.3304908806613460 0.7154502550542654
and my script so far:
PROGRAM readtest
use, intrinsic :: iso_fortran_env
implicit none
integer, parameter :: ArrayLen = 64
real(real64), DIMENSION(ArrayLen) :: x
real(real64), DIMENSION(ArrayLen) :: y
real(real64), DIMENSION(ArrayLen) :: z
integer :: i, ReadCode, num
OPEN(1, FILE='contcar.txt', STATUS='old', ACTION='read')
OPEN(2, FILE='xyz.txt', STATUS='replace', ACTION='write')
num = 0
ReadLoop: do i=1, ArrayLen
read (1, '(A,F18.16)', iostat=ReadCode ) x(i), y(i), z(i)
if ( ReadCode /= 0 ) then
if ( ReadCode == iostat_end ) then
exit ReadLoop
else
write ( *, '( / "Error on read: ", I0 )' ) ReadCode
stop
end if
end if
num = num + 1
end do ReadLoop
WRITE(3, 100) x, y, z
100 format (A,F18.16)
END PROGRAM readtest
The xyz.txt is appearing blank and I am not getting any errors at this stage, what is wrong here that is stopping the array filling and writing to the file?
Sorry, if this is too much of mess to approach, any help would be appreciated.
Thanks

Answer 1

You have two problems with you approach:
Your format specifications are wrong
Your write won't do what you want
First off, the format A,F18.16 reads two items, a character and a floating point number. What you want is to read 3 floating point numbers. With the file provided, there are two spaces before each number so you could use
read (1, '(3(2X,F18.16))', iostat=ReadCode ) x(i), y(i), z(i)
but this is not very flexible if your input format changes and it is easier to just do list-directed input:
read (1, *, iostat=ReadCode ) x(i), y(i), z(i)
which will do what you want and is not sensitive to the exact positioning of the numbers within the file and how many intervening spaces exist.
This will load your data into the arrays. Now for output you want the same thing. You want to duplicate the output so we can use the first format about to stipulate that output (3(2X,F18.16)). This will output 3 numbers per line with 2 spaces before each number. The next problem is that you are attempting
WRITE(3, 100) x, y, z
which will transpose your array. It will write all of the x, then all of the y and lastly all of the z. If you want the same output, put it in a loop. Putting the above together, use:
do i=1, ArrayLen
WRITE(2, 100) x(i), y(i), z(i)
end do
100 format (3(2X,F18.16))
As a note, don't use single digit unit numbers, particularly the first few which conflict with system defined standard input/output/error units on most compilers.