dynamic array size determined at runtime in ada

Question

Is it possible to have an array with size that is determined at runtime like so,

Procedure prog is
   type myArray is array(Integer range <>) of Float;
   arraySize : Integer := 0;
   theArray : myArray(0..arraySize);
Begin
   -- Get Array size from user.
   put_line("How big would you like the array?");
   get(arraySize);

   For I in 0..arraySize Loop
      theArray(I) := 1.2 * I;
   End Loop;
End prog;

Is there a way to achieve this result other than using dynamically Linked Lists or another similar structure? Or is there a simple built in data structure that would be simpler than using dynamically linked lists?

Answer 1

Sure, declare it in a block as follows:

procedure prog is
   arraySize : Integer := 0;
   type myArray is array(Integer range <>) of Float;
begin
   -- Get Array size from user.
   put_line("How big would you like the array?");
   get(arraySize);

   declare
      theArray : myArray(0..arraySize);
   begin
      for I in 0..arraySize Loop
         theArray(I) := 1.2 * I;
      end Loop;
   end;
end prog;

or pass the arraySize as an argument into a subprogram and declare and operate on it in that subprogram:

procedure Process_Array (arraySize : Integer) is

    theArray : myArray(0..arraySize);

begin
   for I in arraySize'Range Loop
      theArray(I) := 1.2 * I;
   end Loop;
end;

This is just illustrative (and not compiled :-), as you need to deal with things like an invalid array size and such.

Answer 2

Yes, you can defer the declaration of a constrained object until you know the size. In this example, the array Candidates can be allocated in a nested block (introduced by the keyword declare) or on the heap (using the keyword new). In this related example, Line has a different size each time through the loop, depending on what Get_Line finds.