Line Filters in Fortran

Here’s the translation of the Go line filter program to Fortran, along with explanations in Markdown format suitable for Hugo:

A line filter is a common type of program that reads input on stdin, processes it, and then prints some derived result to stdout. grep and sed are common line filters.

Here’s an example line filter in Fortran that writes a capitalized version of all input text. You can use this pattern to write your own Fortran line filters.

program line_filter
  use, intrinsic :: iso_fortran_env, only: stdin => input_unit, &
                                           stdout => output_unit, &
                                           stderr => error_unit
  implicit none
  
  character(len=1024) :: line
  integer :: io_status
  
  do
    ! Read a line from standard input
    read(stdin, '(A)', iostat=io_status) line
    if (io_status < 0) exit  ! End of file
    if (io_status > 0) then
      write(stderr, '(A)') 'Error reading input'
      stop 1
    end if
    
    ! Convert the line to uppercase
    call to_upper(line)
    
    ! Write the uppercased line to standard output
    write(stdout, '(A)') trim(line)
  end do

contains

  subroutine to_upper(str)
    character(len=*), intent(inout) :: str
    integer :: i
    
    do i = 1, len_trim(str)
      select case(str(i:i))
        case('a':'z')
          str(i:i) = achar(iachar(str(i:i)) - 32)
      end case
    end do
  end subroutine to_upper

end program line_filter

This Fortran program reads lines from standard input, converts them to uppercase, and writes them to standard output. Here’s a breakdown of how it works:

1. We use the iso_fortran_env module to get standard input, output, and error units.

2. We use a do loop to continuously read lines from standard input.

3. The read statement reads a line into the line variable. The iostat parameter allows us to check for end-of-file or errors.

4. If we’ve reached the end of the file (io_status < 0), we exit the loop.

5. If there’s an error reading (io_status > 0), we print an error message to stderr and stop the program.

6. We call the to_upper subroutine to convert the line to uppercase.

7. We write the uppercased line to standard output.

8. The to_upper subroutine uses a do loop and select case to convert each lowercase letter to uppercase.

To try out our line filter, first make a file with a few lowercase lines.

$ echo 'hello'   > /tmp/lines
$ echo 'filter' >> /tmp/lines

Then compile and use the line filter to get uppercase lines.

$ gfortran line_filter.f90 -o line_filter
$ cat /tmp/lines | ./line_filter
HELLO
FILTER

This Fortran version achieves the same functionality as the original program, reading lines from standard input, converting them to uppercase, and writing them to standard output.