Re: Multiple indexing (was: 'in' keyword today)
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- Subject: Re: Multiple indexing (was: 'in' keyword today)
- From: Sean Conner <sean@...>
- Date: 2014年4月11日 01:59:42 -0400
It was thus said that the Great Thomas Jericke once stated:
> On 04/10/2014 10:04 PM, Sean Conner wrote:
> >It was thus said that the Great Thomas Jericke once stated:
> >>
> >>-----Original Message-----
> >>>From: "Sean Conner" <sean@conman.org>
> >>> How about:
> >>>
> >>> foo = { a = 1 , b = 2 , c = 3 }
> >>> bar = { x = 'one' , y = 'two' , z = 'three' }
> >>> baz = { one = 'alpha' , two = 'beta' , three = 'gamma' }
> >>>
> >>> a,b,c in foo = x,y,z in bar in baz
> >>Depends on the order of the in operarator, IMO it should be right to left:
> >>
> >>foo["a", "b", "x"] = baz["bar"]["x", "y", "z"]
> >>
> >>As baz["bar"] is nil, you will get:
> >>
> >>attempt to index field bar (a nil value)I think what you wanted to write
> >>is:
> >>
> >>a,b,c in foo = baz[x,y,z in bar]
> > Interesting. Because in my mind, I was parsing it as
> >
> > a,b,c in foo = (x,y,z in bar) in baz
> >
> > (left to right) That is:
> >
> > foo.a = baz[bar.x]
> > foo.b = baz[bar.y]
> > foo.c = baz[bar.z]
> >
> > I'm not saying I'm right or you are wrong, just how I would interpet it.
>
> You have a mistake in you inner interpreter right there
That's why I'm asking these questions, to clarify the concept.
> I wrote:
> a,b,c in table -> table["a", "b", "c"]
>
> If you want to expand left to right you get
>
> a,b,c in foo = (x,y,z in bar) in baz
>
> foo["a", "b", "c"] = (bar["x, y, z"]) in bat -- syntax error: name
> expected near "(bar["x, y, z"])"
>
> The statements in front of the in must be names not expressions, if you
> start to mix them the code will get ambiguous. Example:
>
> local t = {}
> local table = { t = "lala" }
> table[t] = "dada"
>
> local test = t in table
> print (test) -- lala or dada ?
No ambiguity here.
local t = {} -- table: 0x896b2b8
local table = { t = "lala" }
table[t] = "dada" -- LINE 3
You now have
table[ table: 0x896b2b8 ] = "dada"
table[ 't' ] = "lala"
because at line 3, you used the table t as the index, not the string "t",
and in lua, tables are valid keys. As I understand you,
x in y
resolves to
y["x"]
that is, x is used as a string, "x".
What you failed to mention (or I did not pick up on) is that the syntax
x in y
is NOT an expression, even though it looks like one, because you can do:
test = t in table -- how is this not an expression?
which is
test = table['t']
which is "dada". So, given what you said, I would assume the following is a
syntax error:
function blah()
return "one","two","three"
end
foo = { "one" = 1 , "two" = 2 , "three" = 3 }
a,b,c = blah() in foo
because of the same reason that
a = foo[blah()]
works but
a = foo.blah() -- or
a = foo.(blah())
fails.
-spc (And remember, you have to explain all this to someone learning Lua)