write_vector_data Subroutine

subroutine write_vector_data ( data_file_name, n, x, y, dx, dy, ierror )
!
!*******************************************************************************
!
!! WRITE_VECTOR_DATA writes vector data to a file, for plotting by GNUPLOT.
!
!
!  Discussion:
!
!    Each vector is described by 4 values, X, Y, dX, dY, indicating that
!    a vector is to be drawn from (X,Y) to (X+dX,Y+dY).
!
!  Modified:
!
!    22 February 2001
!
!  Author:
!
!    John Burkardt
!
!  Parameters:
!
!    Input, character ( len = * ) DATA_FILE_NAME, the name of the data file.
!
!    Input, integer N, the number of vectors.
!
!    Input, real X(N), Y(N), DX(N), DY(N), the vector data.
!
!    Output, integer IERROR, nonzero if an error occurred.
!
  implicit none
!
  integer n
!
  character ( len = * ) data_file_name
  real dx(n)
  real dy(n)
  integer file_unit
  integer i
  integer ierror
  integer ios
  real x(n)
  real y(n)
!
  ierror = 0

  call get_unit ( file_unit )

  if ( file_unit == 0 ) then
    ierror = 1
    write ( *, '(a)' ) ' '
    write ( *, '(a)' ) 'WRITE_VECTOR_DATA - Fatal error!'
    write ( *, '(a)' ) '  Could not get a free FORTRAN unit.'
    return
  end if

  open ( unit = file_unit, file = data_file_name, status = 'replace', &
    iostat = ios )

  if ( ios /= 0 ) then
    ierror = 2
    write ( *, '(a)' ) ' '
    write ( *, '(a)' ) 'WRITE_VECTOR_DATA - Fatal error!'
    write ( *, '(a)' ) '  Could not open the output file.'
    return
  end if

  do i = 1, n
    write ( file_unit, * ) x(i), y(i), dx(i), dy(i)
  end do

  close ( unit = file_unit )

  write ( *, '(a)' ) ' '
  write ( *, '(a)' ) 'WRITE_VECTOR_DATA:'
  write ( *, '(a)' ) '  Wrote the GNUPLOT vector data file "' // &
    trim ( data_file_name ) // '"'

  return
endsubroutine write_vector_data