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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: String filtering
Newsgroups: comp.lang.ada
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References: <m31x3b6n5v.fsf@rr.trudgett> <m2irwnez9b.fsf@hugin.crs4.it>
 <m3mzly6fgy.fsf@rr.trudgett>
Date: Tue, 27 Sep 2005 11:49:28 +0200
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Date: 2005-09-27T11:49:31+02:00
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On Tue, 27 Sep 2005 19:13:17 +1000, David Trudgett wrote:

>     with Ada.Strings.Maps, Ada.Strings.Unbounded;
>     use  Ada.Strings.Maps, Ada.Strings.Unbounded;
> 
>     Lower_Chars : constant Character_Range := ('a', 'z');
>     Upper_Chars : constant Character_Range := ('A', 'Z');
>     Numer_Chars : constant Character_Range := ('0', '9');
>     Alphanumeric : constant Character_Ranges
>       := (Lower_Chars, Upper_Chars, Numer_Chars);
>     Alphanumeric_Set : constant Character_Set := To_Set(Alphanumeric);

with Strings.Maps.Constants;
use  Strings.Maps.Constants;

-- use defined there Alphanumeric_Set

>     function Strip_Non_Alphanumeric
>       (Str : in Unbounded_String) return Unbounded_String
>     is
>        New_Str : Unbounded_String
>          := To_Unbounded_String(Count(Str, Alphanumeric_Set));

If you do this, then use String (1..Count (...));

>     begin
>        New_Str := To_Unbounded_String("");

No need for that, it is initially an empty string.

>        for Char in 1 .. Length(Str) loop
>           if Is_In(Element(Str, Char), Alphanumeric_Set) then
>              Append(New_Str, Element(Str, Char));
>           end if;
>        end loop;
>        return New_Str;
>     end Strip_Non_Alphanumeric;
> 
> Is something like that what y'all do in situations like this?

I don't.

Firstly it is not clear why characters need to be filtered out. Or better
to say, how did it happen, that you get garbage in a string? Either, you
need a character *stream* filtering, long before you get a string token out
of it, or, more realistically an error message (exception), should have
happened, for example if you take some text from a GUI widget.

Secondly, unbounded strings are rarely needed. Especially in text parsing
etc. It is quite uncommon to change a string content there. In your example
you don't do it either. You create a new string. Also both the source and
the result strings have *known* length. So you don't need unbounded strings
here. Usually, after making some trivial analysis like that you'll find out
that only 2% or so really need to be unbounded.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de