Using a deferred-length character string to read user input

Question

I would like to use deferred-length character strings in a \"simple\" manner to read user input. The reason that I want to do this is that I do not want to have to declare t

Answer 1

Deferred length character is a Fortran 2003 feature. Note that many of the complicated methods linked to are written against earlier language versions.

With Fortran 2003 support, reading a complete record into a character variable is relatively straight forward. A simple example with very minimal error handling below. Such a procedure only needs to be written once, and can be customized to suit a user's particular requirements.

PROGRAM main
  USE, INTRINSIC :: ISO_FORTRAN_ENV, ONLY: INPUT_UNIT
  IMPLICIT NONE
  CHARACTER(:), ALLOCATABLE :: my_string

  CALL read_line(input_unit, my_string)
  WRITE (*, "(A)") my_string
  PRINT *, ALLOCATED(my_string), LEN(my_string)

CONTAINS
  SUBROUTINE read_line(unit, line)      
    ! The unit, connected for formatted input, to read the record from.
    INTEGER, INTENT(IN) :: unit
    ! The contents of the record.
    CHARACTER(:), INTENT(OUT), ALLOCATABLE :: line

    INTEGER :: stat           ! IO statement IOSTAT result.
    CHARACTER(256) :: buffer  ! Buffer to read a piece of the record.
    INTEGER :: size           ! Number of characters read from the file.
    !***
    line = ''
    DO
      READ (unit, "(A)", ADVANCE='NO', IOSTAT=stat, SIZE=size) buffer
      IF (stat > 0) STOP 'Error reading file.'
      line = line // buffer(:size)
      ! An end of record condition or end of file condition stops the loop.
      IF (stat < 0) RETURN
    END DO
  END SUBROUTINE read_line
END PROGRAM main