Simple Ada program with wrong output

Question

Complete beginner to Ada here. I am trying to compile and run a simple Ada program, from here: http://www.dwheeler.com/lovelace/s1sf.htm

Here is the code:

-- Demonstrate a trivial procedure, with another nested inside.
with Ada.Text_IO, Ada.Integer_Text_IO;
use Ada.Text_IO, Ada.Integer_Text_IO;

procedure Compute is

 procedure Double(Item : in out Integer) is
 begin -- procedure Double.
   Item := Item * 2;
 end Double;

 X : Integer := 1;   -- Local variable X of type Integer.

begin -- procedure Compute
 loop
  Put(X);
  New_Line;
  Double(X);
 end loop;
end Compute;

I'm on Linux, using gnat so I do:

gnatmake -c compute.adb
gnatmake compute

Which gives me the executable. Running the executable gives a list of zeros, as it seems to initialize X to 0, even though it says to initialize it to 1, so I should get a list 1,2,4,...

Can anyone explain either where my code or my thinking is wrong? Oh and using gnat is there a way to compile and create the executable in a single command?

user_1818839 user_1818839 · Accepted Answer · 2013-03-05T10:38:49

I can only guess that when you added "-gnato", gnatmake simply replied gnatmake: "compute" up to date. leaving you with the same executable.

brian@Gannet:~/Ada/Play$ gnatmake -gnato compute.adb
brian@Gannet:~/Ada/Play$ ./compute
          1
          2
...
  536870912
 1073741824
raised CONSTRAINT_ERROR : compute.adb:9 overflow check failed

Then without -gnato (I had to touch the source or I got the "up to date" message)

brian@Gannet:~/Ada/Play$ gnatmake compute.adb
brian@Gannet:~/Ada/Play$ ./compute
          1
          2
...
  536870912
 1073741824
-2147483648
          0
          0
          0
          0

which subsequently appears as a string of zeroes. Adding your extra "if" statement touched the file, forcing recompilation : the "if" itself is not strictly necessary (though testing and preventing constraint error is a Good Thing!)

The moral : without -gnato, or rather, without at least the flags -gnataoE -fstack_check, Gnat is not an Ada compiler.

Simple Ada program with wrong output

2 Answers