Re: Ada Queue

[Ted Dennison <dennison@telepath.com>]

In article <38ed0123@news.hamilton.edu>,
  "Joseph T." <thisthat7@hotmail.com> wrote:
>
> Let me ask you this...how come then, did the author of this page ( a
> CS professor) choose the same method shown here:
>
> http://www.cm.cf.ac.uk/Dave/C/node11.html
>
> I think that the Ada replicates the method in C used by this
> professor. Does his code suffer from the same things you mentioned?

For those who don't want to bother surfing there, the relevent source
is:

listelement * AddItem (listelement * listpointer, int data) {

    listelement * lp = listpointer;

    if (listpointer != NULL) {
        while (listpointer -> link != NULL)
            listpointer = listpointer -> link;
        listpointer -> link = (struct listelement  *) malloc (sizeof
(listelement));
        listpointer = listpointer -> link;
        listpointer -> link = NULL;
        listpointer -> dataitem = data;
        return lp;
    }
    else {
        listpointer = (struct listelement  *) malloc (sizeof
(listelement));
        listpointer -> link = NULL;
        listpointer -> dataitem = data;
        return listpointer;
    }
}
(please forgive the funky formatting Deja may force on this)

To answer your question: No this is not the same. The equivalent of his
interface in (your) Ada would be:

function Enqueue (Q : in Queue; E : in Element_Type) return Queue is


He *does* still suffer from the extraneious assignment problem, but it
happens on the outside of the Enqueue call as an artifact of his
decision to use a function return for the new Queue value. Yours put it
on the inside, where its arguably just as wasteful, but a lot more
obvious.

The reasons I think he probably did it this way are:

1, C has no "in out" parameters like Ada. In order to emulate them, he'd
have to have used something like "listelement ** listpointer", a
construction that is both ugly and error-prone.

2. C automaticly gives you a function return, which subtly encourages
developers to find some use for it.

In other words, I think his interface has more to do with the failings
of C than anything else. This is a very good example of why its not
generally best to do wrote translations of C code into Ada.

--
T.E.D.

http://www.telepath.com/~dennison/Ted/TED.html


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