Re: ada loops

[Stephen Leake <Stephe.Leake@nasa.gov>]

rm@gunlab.com.ru (Roman V. Isaev) writes:

> I've got bitten by this (code was translated from pascal by p2ada, so 
> please don't bash using goto):
> 
>       for Ijk in 1 .. 40 loop
>          Sss:=Fnmis-Si(Ijk);
>          if  Sss<0.0 then
>             goto Label_1;
>          end if;
>       end loop; --  IJK
>       <<Label_1>>    null;
> 
> Before the loop Ijk is 0, after the loop Ijk is still 0. Huh? Why? 

You left out the first declaration of Ijk. You must actually have
something like:

procedure Foo is
    Ijk : Integer;
begin
    for Ijk in 1 .. 40 loop
        Sss:=Fnmis-Si(Ijk);
        if  Sss<0.0 then
           goto Label_1;
        end if;
    end loop; --  IJK
    <<Label_1>>    null;
end Foo;

There are _two_ variables named Ijk; one in the Foo procedure, and one
in the loop. The loop index variable is _not_ visible outside the
loop. Inside the loop, it hides the outer declaration.

I don't remember how Pascal did this; I suspect the loop variable was
not a new declaration. In standard C++, the loop variable is a new
declaration (as in Ada), but in early versions of C++, it wasn't.

> I replaced Ijk with temp variable and put Ijk := temp before goto
> and it solved the problem, but why we can't have final value when we
> break out of the loop? What's the logic behind this language
> behaviour?

I'm not sure of all of the reasons (see the Ada Rationale for more).
It may have to do with optimizations.

> Also when I try to debug this in GVD I can see Ijk variable, but not
> Sss (it's a global variable described in outer procedure). I see
> following message in the bottom window:
> 
> (gdb) print Sss
> Cannot access memory at address 0x24803ec

That means you have a bug in your code, which is clobbering the stack
or something. Or you have a version of gdb that is not Ada-aware. What
OS and compiler are you using?

-- 
-- Stephe