4 - File I/O Operations

Reading input data from file

Scientific codes will often make use of an input file which contains the problem specification. These codes must therefore be able to open an input file and read through it, extracting the relevant data such that it can be used to solve the problem. This can be seen in the code snippet below taken from the Problem module, where an input file has been opened, and the boundary conditions read in.

!!Open input file containing problem specification
open(InputFile, File='input.in', Status='Old')

!!Read in the boundary conditions of the problem
String_Read = ''
do while (String_Read .ne. 'Boundary_Conditions:')
  read(InputFile,*) String_Read
end do
do ii = 1, 2
  read(InputFile,*) String_Read
  if (String_Read == 'Zero') then
    this%Boundary_Conditions(ii) = 0
  else if (String_Read == 'Reflective') then
    this%Boundary_Conditions(ii) = 1
  else
    write(*,*) "ERROR: Unrecognised Boundary Condition"
  end if
end do

When reading an input file, Fortran needs an Integer ID, Filename and Status. The ID allows the file to be referred to easily in future, and is good practice to set through a parameter for readability. The Filename simply matches the name of the file, and the status describes the state of the file, in this case it is an 'Old' file which already exists in the directory.

integer, parameter :: InputFile = 101
open(InputFile, file='input.in', status='Old')

We then wish to read through our file until we reach the name of the boundary conditions. To do so, we loop through our file until we reach the 'Boundary_Conditions:' string. We now know that the next line will contain the name of our boundary condition, and hence this can be read in.

String_Read = ''
String_Read = ''
do while (String_Read /= 'Boundary_Conditions:')
  read(InputFile,*) String_Read
end do

We finally need to check what type of boundary has been specified and store this information. Here we have an If statement which will loop over the known boundary condition names.

Read(InputFile,*) String_Read
if (String_Read == 'Zero') then
  this%Boundary_Conditions(ii) = 0
else if (String_Read == 'Reflective') then
  this%Boundary_Conditions(ii) = 1
else
  write(*,*) "ERROR: Unrecognised Boundary Condition"
end if

Finally, we should close this file, achieved through the close command and the associated ID.

close(InputFile)

Generating output files

Most scientific codes will want to generate outputs that can be read by external software such as GNUPlot. To do so, data generated by the code must be written to an output file in some given format. This is done in a way similar to that of reading the input, with the read statements replaced with write statements.

First we tell the code to open the output file, specifying 'Replace' to tell the code that we wish to overwrite the file if it is already present.

!!Generate textfile
open(textfile,File='OutputFile.txt',Status='Replace')

We then loop over our solutions, writing the values of the position and fluxes to the text file, with each row corresponding to a node. In code snippet below we write down the data for the first node, then loop over the rest of the nodes in each of the regions.

Position = 0._dp
!!First node
NodeID = 1
write(textfile,'(2E14.6)') Position, Flux(NodeID)
do ii = 1, N_Regions
    do jj = 1, RegionNodes(ii)-1
        NodeID = NodeID + 1
        Position = Position + (Boundary_Pos(ii+1)-Boundary_Pos(ii))/Real(RegionNodes(ii)-1,dp)
        write(textfile,'(2E14.6)') Position, Flux(NodeID)
    end do
end do

Finally, we close the file. We now have a complete set of output data stored in text format that can be plotted or used for later analysis.

close(textfile)