Fortran read() reads last line twice? - file-io

Question

So, suppose I'm trying to read in a file the length of which I don't know before hand. We can use iostat and a while loop to break when we need to, but I'm having an issue with this. Namely, the code I've written reads the last line twice. I'm sure there is an obvious solution, but I can't seem to figure it out. I don't really understand how either the read() or iostat functions work entirely (I'm pretty new at fortran), but I can't glean much from documentation so I'm hoping someone here can help.
Here is the (relevant bit of) code I've written:
filename = 'test.txt'
iostat_1 = 0
iostat_2 = 0
open(newunit = lun, file = filename, status = 'old', iostat = iostat_1)
if (iostat_1 == 0) then
do while(iostat_2 == 0)
if(iostat_2 == 0) then
read(lun,*,iostat = iostat_2) dum, real_1,real_2,int_1
print *, dum, real_1,real_2,int_1
endif
enddo
endif
So, supposing my input file is
1 1.0 1.0 1
2 2.0 2.0 1
3 3.0 3.0 1
4 4.0 4.0 4
Then the output to the terminal from the print statement will be
1 1.0 1.0 1
2 2.0 2.0 1
3 3.0 3.0 1
4 4.0 4.0 4
4 4.0 4.0 4
So keep in mind the following: The main purpose here is to be able to read in a file with an arbitrary number of lines. I'm not interested in a solution involving reading the number of lines first.
Thanks for the help!
UPDATE Okay I just solved the problem. That being said, I'm wondering if there is a solution less clumsy than mine. Here is what I did to fix the issue
! Body of ReInsert
filename = 'rpriov3.dat'
iostat_1 = 0
iostat_2 = 0
open(newunit = lun, file = filename, status = 'old', iostat = iostat_1)
if (iostat_1 == 0) then
do while(iostat_2 == 0)
if(iostat_2 == 0) then
read(lun,*,iostat = iostat_2) dum, real_1,real_2,int_1
if(iostat_2 == 0) then !<---- Added this nested if statement
print *, dum, real_1,real_2,int_1
endif
print *, iostat_2
endif
enddo
endif

Answer 1

As you found out, when you set an iostat parameter, the read command doesn't overwrite the variables it asks for.
Your solution is, as you already noticed, somewhat convoluted.
Firstly:
do while (condition)
if (condition) then
...
end if
end do
In this case, the inner if statement is complete surplus. The loop doesn't run unless condition is true, so unless the evaluation of condition itself doesn't change the result 1), the if clause will always be executed.
The second thing I'd look at is: What should happen if the open fails? In most cases, I want to print an error and quit:
open(..., iostat=ios)
if (ios /= 0) then
print*, "Error opening file"
STOP 1
end if
do while (...)
...
end do
Even if you don't want to exit the program in case of an error in open, there are usually ways to make the code more readable than eternal nesting. For example, you could ask the user for a filename again and again (in its own loop) for a file name, until either the file opens, or the user enters some quit message.
ios = 1
do while (ios /= 0)
write(*, *, advance='no') "Enter filename (or 'quit') :"
read(*, *) filename
if ( trim(filename) == "quit" ) STOP
open(newunit=lun, file=filename, ..., iostat=ios)
end do
Finally there's the most inner if block. Since you want to exit the loop anyway when you reach an error, you can use the exit statement inside a loop to exit it immediately without executing the rest of the loop block:
do
read(..., iostat=ios) ...
if (ios /= 0) exit
print *, ....
end do
This is an infinite loop with an explicit exit as soon as it encounters a read error (usually, but not necessarily an EOF). Since the print statement is after the exit, it won't be executed in case of such an error.
1) What I mean by that is something like this C snippet i++ < 10, which both tests i against 10 and increments it.