File Input/Output

Here we will generate some data and write it to a file:

src/08_file_write.f90

! Program : Write output to a file

PROGRAM file_write
  IMPLICIT NONE

  INTEGER :: ii

  OPEN(unit=10, file='output.dat', status='new')
    DO ii = 1, 10
      WRITE(1, '(i4)') ii*ii
    ENDDO
  CLOSE(unit=10)
END PROGRAM file_write

info

Note that unit=0, 5, and 6 are reserved. Do not use them to identify files in your programs. Some older compilers might other other low unit numbers for internal usage. It is safer and conventional to use unit number 10 and above for user files.

Reading from file: we will read the data from file that is generated in the above example.

src/09_file_read.f90

! Program : Read data from file

PROGRAM file_read
  IMPLICIT NONE

  INTEGER :: ii, data(10)

  OPEN(unit=10, file='output.dat', status='old')
    DO ii = 1, 10
      READ(1, '(i4)') data(ii)
    END DO
  CLOSE(unit=10)

  DO ii = 1, 10
    PRINT '(i3)', data(ii)
  END DO
END PROGRAM file_read

FORTRAN Namelist

FORTRAN Namelist provides an easy way to read and write groups of variables. Namelists simplify reading and writing variables to files without needing to specify variable formats for each I/O operation. The namelist group is defined in the Fortran code, and the corresponding input/output text uses a specific format:

• Begins with &groupname
• Lists variable assignments (comma or space separated)
• Ends with /.

Namelist example

Namelists are used in Quantum ESPRESSO input files.

Here is an example namelist:

src/30_qe.in

&SYSTEM
    ibrav   = 2
    nat     = 2
    ntyp    = 1
    ecutwfc = 30.0
    ecutrho = 240.0
/

We can read and parse various inputs following way:

src/30_namelist.f90

program read_qe_input
  implicit none

  ! Declare variables matching those in the QE SYSTEM namelist
  integer :: ibrav, nat, ntyp
  double precision :: ecutwfc, ecutrho
  character(len=256) :: input_file
  integer :: ios

  ! Namelist definition
  namelist /SYSTEM/ ibrav, nat, ntyp, ecutwfc, ecutrho

  ! Set default values
  ibrav = 0
  nat = 0
  ntyp = 0
  ecutwfc = 0.0d0
  ecutrho = 0.0d0

  ! Set input file name
  input_file = '30_qe.in'

  ! Open and read the SYSTEM namelist
  open(unit=10, file=input_file, status='old', action='read')
  read(10, nml=SYSTEM, iostat=ios)
  if (ios /= 0) then
     print *, "Error reading SYSTEM namelist from file ", trim(input_file)
  else
     print *, "ibrav   =", ibrav
     print *, "nat     =", nat
     print *, "ntyp    =", ntyp
     print *, "ecutwfc =", ecutwfc
     print *, "ecutrho =", ecutrho
  end if
  close(10)
end program read_qe_input