How to use command line arguments in Fortran?

Question

GCC version 4.6

The Problem: To find a way to feed in parameters to the executable, say a.out, from the command line - more specifically feed in an array of double precision numbers.

Attempt: Using the READ(*,*) command, which is older in the standard: Program test.f -

PROGRAM MAIN  
     REAL(8)    :: A,B  
     READ(*,*) A,B
     PRINT*, A+B, COMMAND_ARGUMENT_COUNT()
END PROGRAM MAIN

The execution -

$ gfortran test.f
$ ./a.out 3.D0 1.D0

This did not work. On a bit of soul-searching, found that

$./a.out
3.d0,1.d0
   4.0000000000000000                0

does work, but the second line is an input prompt, and the objective of getting this done in one-line is not achieved. Also the COMMAND_ARGUMENT_COUNT() shows that the numbers fed into the input prompt don't really count as 'command line arguments', unlike PERL.

Answer 1

If you want to get the arguments fed to your program on the command line, use the (since Fortran 2003) standard intrinsic subroutine GET_COMMAND_ARGUMENT. Something like this might work

PROGRAM MAIN  
     REAL(8)    :: A,B
     integer :: num_args, ix
     character(len=12), dimension(:), allocatable :: args

     num_args = command_argument_count()
     allocate(args(num_args))  ! I've omitted checking the return status of the allocation 

     do ix = 1, num_args
         call get_command_argument(ix,args(ix))
         ! now parse the argument as you wish
     end do

     PRINT*, A+B, COMMAND_ARGUMENT_COUNT()
END PROGRAM MAIN

Note:

• The second argument to the subroutine get_command_argument is a character variable which you'll have to parse to turn into a real (or whatever). Note also that I've allowed only 12 characters in each element of the args array, you may want to fiddle around with that.
• As you've already figured out read isn't used for reading command line arguments in Fortran programs.

Since you want to read an array of real numbers, you might be better off using the approach you've already figured out, that is reading them from the terminal after the program has started, it's up to you.