How to add different type values to an array in Ada?

Question

My goal is to receive from standard input an equation, store that in an array for later use/re-printing, then output a line printing the whole equation AND the answer after just like so:

Input: 2+3=

Output: 2 + 3 = 5

I am very confused on how to go about doing this due to the inability of Ada to have dynamic strings and such.

This is a rough idea that I have in pseudo-code..

 Until_loop:                 
       loop 

         get(INT_VAR);
         --store the int in the array?
         get(OPERATOR_VAR);
         --store the operator in the following index of that array? and
         --repeat until we hit the equal sign, signaling end of the equation

         get(CHECK_FOR_EQUALSIGN);
     exit Until_loop when CHECK_FOR_EQUALSIGN = "=";

     end loop Until_loop;

 --now that the array is filled up with the equation, go through it and do the math
 --AND print out the equation itself with the answer

I am guessing the array should look like:

[2][+][5][=][7]

I am also a beginner with Ada, so it's even more difficult to get a grasp of things, I am very good with Java, but I can't get used to the strongly typed syntax. Please ask if you need more info.

Answer 1

Ada can use dynamic fixed strings, without resorting to Unbounded_String or Containers, or allocation and pointers, although these are options.

The insight making this possible is that a string can get its size from its initialisation expression when it is declared - but that declaration can be inside a loop, so that it is executed anew each time round the loop. You can't always structure a program so that this makes sense, though it's possible surprisingly often, with a little thought.

Another feature is that later on, these "declare" blocks make great candidates for very easy refactoring into procedures.

with Ada.Text_IO; use Ada.Text_IO;

procedure Calculator is
begin
   loop
      Put("Enter expression: ");
      declare
         Expression : String := Get_Line;
      begin
         exit when Expression = "Done";
         -- here you can parse the string character by character
         for i in Expression'range loop
            put(Expression(i));
         end loop;
         New_Line;
      end;
   end Loop;
end Calculator;

You should get

brian@Gannet:~/Ada/Play$ gnatmake calculator.adb
gcc-4.9 -c calculator.adb
gnatbind -x calculator.ali
gnatlink calculator.ali
brian@Gannet:~/Ada/Play$ ./calculator
Enter expression: hello
hello
Enter expression: 2 + 2 =
2 + 2 =
Enter expression: Done
brian@Gannet:~/Ada/Play$ 

You still have to write the calculator...