Re: String indexing again

[<jgiors@...>]

> ------------------------------
> Date: Mon, 20 Dec 2010 15:58:49 +0200
> From: Dirk Laurie <dpl@sun.ac.za>
> Subject: Re: String indexing again
> To: Lua mailing list <lua-l@lists.lua.org>
> Message-ID: <20101220135849.GA7839@rondloper>
> Content-Type: text/plain; charset=us-ascii
> 
> On Mon, Dec 20, 2010 at 02:35:09PM +0200, Randy Kramer wrote:
> > For those of us (maybe just me) just starting to watch from the peanut 
> > gallery, without the long lecture, can you tell me what s[3] does 
> > indicate--I mean what does the s function (is it a function?) do?
> > 
> > On Sunday 19 December 2010 11:57:19 pm Dirk Laurie wrote:
> > > Except for the long lecture you need to explain why you get this:
> > > > s='hello'; print(s[3])
> > >
> > > nil
> 
> Indexing for strings was not defined at all in Lua 5.0:
> --
> Lua 5.0.3  Copyright (C) 1994-2006 Tecgraf, PUC-Rio
> > s='hello'; print(s[3])
> stdin:1: attempt to index global `s' (a string value)
> stack traceback:
>     stdin:1: in main chunk
>     [C]: ?
> --
> It is still not defined in Lua 5.1 and 5.2.  But in the meantime, 
> strings have acquired a metatable, in order to allow you to write
> s:sub(3,3) instead of string.sub(s,3,3).  Since indexing for
> strings is not defined, s[3] falls through to the metatable,
> which does not have an entry with key value 3.  Therefore, as 
> with all table references, nil is returned.

This is a good point. The behavior could be modified by changing __index
to a function which generates an error when the index is a number (and
otherwise falls back to the current behavior), but I'm not certain it
would be worthwhile.

> In my opinion, this sort of non-intuitive behaviour, which needs
> very careful reading of the reference manual to understand, is 
> a much greater evil than the alleged illogic and Cobol-likeness
> of making s[3] mean the third character of the string s.  But 
> I promised to stop ranting about this.
> 
> Dirk

I think an argument against indexing of characters in a string is that
it implies indexing is acceptable on the left-hand side of an
assignment:

s = "ABxDEF"
s[3] = "C"    -- Oh-oh...

If I am not mistaken, this cannot work in standard Lua (i.e. there is no
way to make s contain "ABCDEF" after the assignment). The failure of
assigning to a character in a string would probably be just as confusing
as what you've mentioned above, especially if reading a character from a
string (with an array index) is allowed.

John Giors
Independent Programmer
Three Eyes Software
jgiors@ThreeEyesSoftware.com
http://ThreeEyesSoftware.com