Re: Trouble with loop in Ada

[Brian Drummond <brian@shapes.demon.co.uk>]

On Fri, 04 Jan 2013 04:32:28 +0000, Derek Wyss wrote:

> Hello,
> 
> I'm new to Ada and I'm trying to write a toy unix command shell just to
> explore the programming language.  So far I have a very small program,
> but I am confused about its behavior.
 
> Can anyone offer an explanation as to why I need to press enter?  I
> suspect it has to do with my use of End_Of_File and the way it "looks
> ahead" but I'm not really sure.

The confusing thing to me is not that you have to press enter, but that 
you have to press it twice the first time round.

I started it and pressed ijklmn THEN return...
ijklmn
Enter a character> you entered: i
Enter a character> you entered: j
Enter a character> you entered: k
...
which explains it. 
(1) First, End_Of_File must block until there is input. (It has to wait 
to see if you are going to type Ctrl-C) And there isn't input until there 
is at least one character followed by a return.
(2) Then Get returns one character at a time from the line buffer, until 
it runs out, when it blocks again until there is more input.

So the behaviour comes from the library functions behind Get and 
End_Of_File rather than the loop. The question is whether you can get 
what you want from these, or need to find alternatives.

Now the good news : loops are more flexible in Ada; there are other ways 
of using them. So your immediate problem can be solved by:

  loop 
    Put("Enter a character> ");
    Get(input_char);
    Put("you entered: " & input_char);
    New_line;
    exit when End_Of_File;
  end loop;

Oh and very nicely done on the toy example - that's the way to get help!

- Brian