Re: Interesting Experiment With String/Sequence Slicing
On 21 Aug 2001, at 14:06, Derek Parnell wrote:
>
> > From: slie at theage.fairfax.com.au
> > To: EUforum <EUforum at topica.com>
> > Reply-To: EUforum at topica.com
> > Subject: Re: Interesting Experiment With
> > String/Sequence Slicing
> > Date: 21/08/2001 12:58:45 PM
> >
> >
> > i am just wild guessing here, but could it
> > be true that a function can be
> > specifically written to deal with string
> > type sequences and so speeding up
> > the slicing function?
> >
>
> Hi Sam,
> we have discovered that the fastest method of inserting, deleting, and
> replacing text in sequences is to minimize the number and size of
> concatenations.
>
> Consider the general case where you have a text of <aaa><bbb><ccc> and a
> replacement text of <ddd> which is going to replace <bbb>.
>
> If the length of <ddd> is longer than <bbb> then create a new sequence (
> using the repeat() function) with the length of <aaa> + <ddd> + <ccc> then
> copy these sub-strings into the new sequence and return the new sequence.
>
> Where <ddd> is shorter than <bbb>, you can avoid the overhead of creating a
> new
> sequence and reuse the space in the exiting string. The trick here is to move
> either <aaa> closer to <ccc> or <ccc> closer to <aaa>, depending on which is
> smaller, and leaving enough room for <ddd> to slide in between them. Then you
> just return the truncated sequence. If you didn't move <aaa> then that is S[1
> ..
> length(<aaa>)+length(<ddd>)+length(<ccc>)], but if you did move <aaa> it is
> S[length(S) - (length(<aaa>)+length(<ddd>)+length(<ccc>)) .. length(S)].
>
> There are further optimisations if <aaa>, <ccc>, or <ddd> are empty
> sequences.
>
> I know this looks complex but it is almost always faster than creating
> temporary sequences and concatenating them, depending on the length of the
> sequences involved.
This is something the interpreter should be doing.
Kat
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