Constant declaration in Ada

Question

Being new to Ada, I'm exploring its syntax and rules and I would like to draw attention on the code given next. Here I'm trying to set a variable Actual_Stiffness to hold a constant value. Its value is given by the product:

Actual_Stiffness := Stiffness_Ratio * Stiffness_Total

where Stiffness_Total has been defined to be a constant Long_Float in the specification file Material_Data.ads and Stiffness_Total has a value already set in the ads file.

WITH Ada.Text_IO;
WITH Ada.Long_Float_Text_IO;
WITH Material_Data;
USE Material_Data;

PROCEDURE sample IS

   Stiffness_Ratio  : Long_Float;
   Actual_Stiffness : CONSTANT Long_Float :=  Stiffness_Ratio * Stiffness_Total;

BEGIN -- main program
   Ada.Text_IO.Put("Enter stiffness ratio: ");
   Ada.Long_Float_Text_IO.Get(Item => Stiffness_Ratio);
   Ada.Long_Float_Text_IO.Put(Item => Stiffness_Ratio);

   --Ada.Text_IO.New_Line;
   --Ada.Long_Float_Text_IO.Put(Item => Actual_Stiffness);
   --Ada.Text_IO.New_Line;
   --Ada.Long_Float_Text_IO.Put(Item => Stiffness_Total);
END sample;

On compiling I get the warning message

warning: "Stiffness_Ratio" may be referenced before it has a value

and on running the program, Actual_Stiffness doesn't get the right value. I could defined Actual_Stiffness to be just a Long_Float (without adding CONSTANT) and then get its value from the product Actual_Stiffness := Stiffness_Ratio * Stiffness_Total after BEGIN in my program when Stiffness_Ratio would have already gotten a value. This would be the right thing to do.

My question is:

I have defined Stiffness_Total as a constant Long_Float with a prescribed value. How to define Actual_Stiffness to be constant also (as it won't be changing in the program) while keeping the ability for a user to interactively being able to enter a Stiffness_Ratio at the terminal? Is that even possible to do?

Thanks a lot..

Answer 1

Since the 'Stiffness_Ratio' is not determined until run-time, the compiler cannot calculate the value of 'Actual_Stiffness' at compile time as you have asked it to. You will have to make 'Actual_Stiffness' into a non-constant variable and initialize it with the computation after 'Stiffness_Ratio' has a value. (You may even be able to keep it as a CONSTANT within the function as long as the stiffness ratio has a value at the time the actual stiffness is computed.)

This is standard in most programming languages.

So, to answer your question:

• No, you cannot define Actual_Stiffness as a constant as desired.

• A moderately close approximation is that you determine the Stiffness_Ratio value outside this procedure, and pass that value into the procedure as a parameter. But then the Actual_Stiffness is only constant for the duration of the procedure, not for all time.

On the other hand, this is likely to be more useful; someone can run the program with multiple values of the stiffness ratio at different times, thus doing many simulations in a single run.