Check the input data type and whether it is empty or null

Question

I am asking user to give a value at run time to do some calculations.

I want to test if the user entered value is a real/integer number, and if not then give a warning that the program is expecting a real/integer number here.
In addition, I would as well like to know how do we check if a particular variable at the moment is null or empty. i.e. I have declared a variable but what if at the time of calculation its value is null or empty or not yet set, in that case, the program shouldn't crash instead give a warning to provide a correct value.

Both these operations are much more easier in C++ and C#, but I couldn't find a way to do that in Fortran.

No wonder it is easier in C++ for you as you are asking for things that probably have no meaning in Fortran. Could you specify what you mean by null or empty declared variable? — Vladimir F Героям слава
– Vladimir F Героям слава, Commented Aug 26, 2013 at 16:05
I believe the value of an unitialised variable is undefined in C++ as well. — agentp
– agentp, Commented Aug 26, 2013 at 19:56

Community · Accepted Answer · 2017-05-23 12:15:44Z

I guess that "null or empty" you mean whether a variable has been initialized: "not yet set". "null" has a particular meaning for Fortran pointer variables, but I suppose that this is not your question. Fortran doesn't automatically give variables a special value before they are intentionally initialized so there is no easy way to check whether a variable has been initialized. One approach is to initialize the variable its declaration with a special value. That means that you need to know a special value that it will never obtain in the operation of the program. One possibility is to use the huge intrinsic:

program TestVar

   real :: AVar = huge (1.0)

   if ( AVar < huge (1.0) ) then
      write (*, *) "Test 1: Good"
   else
      write (*, *) "Test 1: Bad"
   end if

   AVar = 2.2

   if ( AVar < huge (1.0) ) then
      write (*, *) "Test 2: Good"
   else
      write (*, *) "Test 2: Bad"
   end if

end program TestVar

As warned by @arbautjc, this only works once, even in a subroutine. In a procedure, the initialization with declaration is only done with a first call. Also, if you change the variable type from this example, be sure to understand how huge works (e.g., Long ints in Fortran).

tstev · Accepted Answer · 2023-12-13 07:40:11Z

I ran into the same problem where my program requires the user gives input for a missing value. The way I solved it was by having a logical variable (i.e. missing_value_set) initialize to .false.. Then when parsing the command arguments set the logical variable to .true. when the required argument was indeed passed by the user. After parsing the arguments I have a check on that variable.

program main
  logical :: missing_value_set = .false.
  real :: MISS_VALUE = huge(1.0)

  ! Parse arguments
  ...

  if (.not.missing_value_set) then
    write(*, *) "ERROR: --missing argument is required"
    stop
  end if

  ! Rest of program
  ...

end program

No idea if that makes sense - so far it seems to work for me.

Collectives™ on Stack Overflow

Check the input data type and whether it is empty or null

2 Answers 2

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related