Command-Line Arguments in FORTRAN

Note: this assumes the user has at least FORTRAN 90.

In the vast majority of cases, creating a namelist data file will be faster and easier. Still, you might like to make a command-line fortran program for sheer novelty's sake. Fortran treats all arguments as characters.

The main functionality comes from two commands, IARGC() and GETARG().

• nargs=IARGC() : IARGC() is an integer function that returns the number of arguments supplied to the program. A count of 0 means none were supplied.

• GETARG(iarg,argchar) : The subroutine GETARG has two arguments. The first is an integer specifying which command-line argument to return, and the second is a character string to be filled with the argument. Asking for argument 0 will return the program name.

So, we can figure out how many arguments were passed, and we can put them all into strings. What if we want to pass, say, how many iterations to do something? This requires converting the character strings to, say, integers.

Converting Strings to Integers

The way Fortran handles this is wack. From ​GFortran documentation, the way to do it is to READ the string into an integer,

character(LEN=10) :: string
integer :: ival

string='154'

READ(string,'(I)')ival

Note that trying to WRITE to an integer doesn't work. Attempting something like

WRITE(ival,'(I)')string

produces a fatal "forrtl: severe (61): format/variable-type mismatch." Changing the format code,

WRITE(ival,'(A)')string

does not produce an error, but also does nothing to ival.

Note: Presumably this is how you would convert strings to floats, but I haven't tried it.

Converting Integers to Strings

In contrast, the way to convert from integer to character is with a WRITE statement,

WRITE(string,'(I10)')ival