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- Subject: Re: digit strings as table indexes
- From: Javier Guerra <javier@...>
- Date: Thu, 25 Jan 2007 13:30:26 -0500
On Thursday 25 January 2007 11:22 am, Jimmie Houchin wrote:
> Okay, I learned that '000' is not a valid name in Lua.
> But for whatever reason it is accepted successfully as a table index.
> I don't if that is something I can depend on or not.
you're mixing two different concepts used at very different parts of the
language: names and values.
a value can be a lot of things: a number, a string, nil, a table, a function,
true, false... (anything else?)
you can use as table keys any value except nil and NaN, as Rici explained.
a Name, OTOH, is a parser concept. its a small part of your source code, you
can use names to identify variables or special words. since the parser can
pick a name from the source code, and any name can be expressed as a string
value, there's a nice notation to use tables as 'records', or 'objects'.
simply put:
table.name ==== table["name"]
this is called "syntactic sugar", because it makes code more attractive, but
don't add any nutritional value. the compiled code is exactly the same on
both cases.
but, since there's no quoting around the name in the left form, you can't
expect it to work with keys that are somewhat 'ugly'
some examples:
table["two words"] -- ok
table.two words -- wrong!!
table["123"] -- ok
table.123 -- wrong!!
table["this-that"] -- ok
table.this-that -- wrong!!
table["you!@#$@#$#%"] -- ok
table.you!@#$@#$#% -- wrong!!
table["_"] -- ok
table._ -- wrong!
also, since the dot notation is sugar for _string_ values, you have to watch
out when the key isn't a string:
table[true] -- key is the boolean true value
table.true -- key is the string "true"
table[123] -- key is a number
table["123"] -- key is a string
table[var] -- key is the value of the variable
table.var -- key is the string "var"
in short: you are really free to use almost anything as a table key, the
limitation is only for the "dot notation" syntactic sugar.
as David noted, you should only use dot notation for constant keys, typically
record fields, object members and such. for anything else (especially when
the key is variable, and user entered!) use the 'real' form: []
--
Javier
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