Reading and writing binary files (GrADS) in fortran - fortran

Question

I am running a model and writing the model output to a binary file (a GrADS *gra file) such that, for example:
integer,parameter :: nvar =3 ,& !number of variables to be written to file
nx=10,ny=10,& !number of girdboxes in lat & long
nt = 5
integer :: it, & ! loop counter
irec ! Record number
real :: var1(nx,ny), var2(nx,ny),var3(nx,ny)
OPEN(30,file='Outfile.gra',action='write',form='unformatted',access='direct',&
recl=4*nVar*nx*ny,status='replace')
!loop over timesteps
it = 1, nt
irec = irec + 1
WRITE(1,rec=irec) Var1(:,:),Var2(:,:),Var3(:,:)
enddo
The file can be read in GrADS and the *ctl file looks like this
dset /mypath/Outfile.gra
title MyTitle
options little_endian
xdef 10 linear 1 1
ydef 10 linear 1 1
zdef 1 linear 1.0 1.0
tdef 5 linear 00:00Z01jan2012 1hr
vars 3
var1
var2
var3
endvars
What I would like to do, from a separate program, is write all the x&y of 1 variable at 1 timestep to a text file. I tried a number of ways but nothing has worked. My latest attempt is this one:
integer,parameter :: &
t = 3, & !Timestep I want to write to file
field = 2, & !Variable I want to write to file
nvar =3 , & !number of variables to be written to file
nx=10,ny=10, & !number of girdboxes in lat & long
nt = 5 !number of timesteps
inteǵer :: it,ix,iy,& ! loop counters
irec ! Record number
real :: val(nx,ny) ! Data to be written to file
open(1,file='NewFile.txt',status='replace')
open(2,file='Outfile.gra',action='read',form='unformatted',access='direct',&
recl=4*nVar*nx*ny,status='old')
irec = 0
do it = 1,nt
irec=irec + nvar*nx*ny
if(it == t) then
irec = irec + (field-1)*nx*ny
do ix = 1,nx
do iy = 1,ny
irec=irec+1
read(2,rec=irec) val(ix,iy)
enddo
enddo
write(1,*) val(:,:)
This particular example gives me the following error
Fortran runtime error: Non-existing record number
but I tried other variations that did not give me any errors but simply didn't write what I was trying to write to file. Could somebody tell me what I am doing wrong and how to solve this? Thank you.

Answer 1

OK, let me try again. When you write outfile.gra you appear to write nt records to it in the block
!loop over timesteps
it = 1, nt
irec = irec + 1
WRITE(1,rec=irec) Var1(:,:),Var2(:,:),Var3(:,:)
enddo
I guess that irec is initialised to 0 somewhere in the code. nt is set to 5 so, if my guess is correct, your code writes 5 records to outfile.gra.
Later, you read the same file in this block
irec = 0
do it = 1,nt
irec=irec + nvar*nx*ny
if(it == t) then
irec = irec + (field-1)*nx*ny
do ix = 1,nx
do iy = 1,ny
irec=irec+1
read(2,rec=irec) val(ix,iy)
enddo
enddo
It's unclear where the if statement closes but from your question I guess that it closes after the loops over nx and ny, like this:
irec = 0
do it = 1,nt
irec=irec + nvar*nx*ny
if(it == t) then
irec = irec + (field-1)*nx*ny
do ix = 1,nx
do iy = 1,ny
irec=irec+1
read(2,rec=irec) val(ix,iy)
enddo
enddo
end if
Again, if my guess is correct, then irec has the value 401 when the read statement is first executed.
It seems that you have written 5 records to outfile.gra and are trying to read the 401st record from it. It's entirely reasonable for the run-time to report that you are trying to read a record that doesn't exist.

Answer 2

High Performance Mark and hristo-iliev identified the problem with the record number. In order to be able to do what I want, i.e. write a single variable at a single timestep the right code is
irec = 0
do it = 1, t
irec = irec + 1
read(2,rec=irec) val(:,:,:)
enddo
write(1,*) val(:,:,field)
where val(nx,ny,nVar)