[boost] Re: [string algo formal review]

From: David Abrahams (dave_at_[hidden])
Date: 2003-10-27 14:21:04

Rob Stewart <stewart_at_[hidden]> writes:

> From: Pavol Droba <droba_at_[hidden]>
>> On Mon, Oct 27, 2003 at 02:07:47AM +1100, Thorsten Ottosen wrote:
>> > "Pavol Droba" <droba_at_[hidden]> wrote in message
>> > news:20031026141956.GD17011_at_lenin.felcer.sk...
>> 
>> > > Then you would also need a type which holds values in range 1..n. Arrays
>> > > and
>> > > containers are 0-indexed also because unsigned types are denoting exactly
>> > > this
>> > > domain. And actualy what conteptual difference between "first element" and
>> > > "first match of substring"?
>> > 
>> > well, you almost said it The first element of an array is index 0. The first
>> > substring of a string
>> > is a index X. Also, array objects(chars). are not substrings unless we're
>> > talking about substrings of lenght 1.
>> > You also called your functions firnd_first, not find_0th, and we have
>> > 
>> > find_first( s, XX ) == find_nth( s, XX, 0 )
>> > 
>> > which doesn't seem natural. It's like having an integer variable and making
>> > zero mean you have got one
>> > of something.
>
> A substring is not a structural element of the container.  A
> container is, by definition, composed of elements.  A substring
> is a semantic notion temporarily associated with a portion of a
> container.  Thus, the former should be denoted using the usual
> C/C++ indexing notation.  The latter, OTOH, should be denoted
> using the vernacular: we use ordinals to refer to things like
> substrings.  That means numbering from one for substrings.
>
>> What would then denote find_nth( s, XX, 0 )? Should it throw an exception?
>
> Sure.

I don't get it.  The first element of a sequence is s[0].  Why
shouldn't the first matching substring be find_nth(s, XX, 0)?

You can think of find_nth as indexing into a view onto the matching
substrings of s if you want.

I imagine most C++ programmers will have the same trouble
understanding why the first should be 1 in this case.

Finally, interfaces which allow values to be passed at compile-time
which are *always* illegal at runtime are a bad idea.  That's
especially true for an unsigned zero, and especially in this case
which is clearly confusable.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com