Re: String filtering

[David Trudgett <wpower@zeta.org.au.nospamplease>]

"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:

> On Wed, 28 Sep 2005 10:06:44 +1000, David Trudgett wrote:
>
>> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:
>> 
>>> On Tue, 27 Sep 2005 14:42:02 +0100, Martin Dowie wrote:
>>>> 2nd sentence of ARM 95 A.4.5 (76) reads:
>>>> 
>>>>   "The function To_Unbounded_String(Length : in Natural)
>>>>     returns an Unbounded_String that represents an uninitialized
>>>>     String whose length is Length."
>>>
>>> Ah, now I see what you meant!
>> 
>> Yep, that's what I meant, too.
>
> No. What Martin meant is that:
>
>    X : Unbounded_String := To_Unbounded_String (Count);
>
> is filled with rubbish

Yes, that's what I meant, too. There was obviously a language problem
happening there, so let's go on to something else, hey? :-)


>
> What I meant is that:
>
>    X : Unbounded_String;
>
> is an empty string, being formally uninitialized. 

Looking back at your original message, it is possible to see how you
meant your words to be taken. However, the way you said it was far
from clear, and at least two people (myself included) understood your
statement differently. You could have better said something like: "And
this line would then become unnecessary."


> You can imagine it as Unbounded_Strings having a default constructor
> setting them empty.

Thanks for pointing that out, by the way, because I wasn't sure
initially. I'm loathe, however, to depend upon default initialisation,
even when it's specified in a standard.


>>    function Strip_Non_Alphanumeric
>>      (Str : in Unbounded_String) return Unbounded_String
>>    is
>>       Dest_Size : Natural := Count(Str, Alpha_Num_Space_Set);
>>       New_Str : Unbounded_String := Null_Unbounded_String;
>
> You don't need initialization here. 

I don't need it, that's true, but I prefer to be explicit about it. I
might change my mind about that when I'm more experienced in Ada.


> Or you can do with Dest_Size. The
> parameter of To_Unbounded_String is a Natural.
>
>>       Dest_Char : Natural := 0;
>>    begin
>>       if Dest_Size > 0 then
>
> You don't need this if. Ada's loops are safe for zero-run.

Its purpose is not to avoid a zero run, though. The test allows an
entire repeat scan of the source string to be avoided in the case that
all of the source characters are not alphanumeric.

The initialisation of New_Str to a null string means that it is ready
to return immediately without code execution passing through the
loop. This avoids the situation of returning an unitialised
unbounded_string when all source string characters are
non-alphanumeric.


>>    function Strip_Non_Alphanumeric
>>      (Str : in String) return String
>>    is
>>       New_Str : String(1 .. Count(Str, Alpha_Num_Space_Set));
>
> You also can do instead:
>
>    New_Str : String(1 .. Length (Str));


Yes, an interesting idea, combined with the sliced return.


>
> here you do:
>
>    return New_Str (1..Dest_Char);
>
> Ada strings has slices!

Interesting. I hadn't considered that!

David




-- 

David Trudgett
http://www.zeta.org.au/~wpower/

    
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