• Subject: Re: Lua Basics: tables as lists?
• From: Warren Merrifield <wmerrifield@...>
• Date: Tue, 23 Aug 2005 17:48:07 +0100

```I may be getting the wrong end of the stick here, but isn't it as
simple as writing:

Python:
a = "a"
if a in ["a", "b", "c"]:
print "a is in the list"

Lua:
mylist = {"a", "b", "c"};
a = "a"
if mylist[a] ~= nil then
print "a is in the list"
end

Warren

On 23/08/05, Rici Lake <lua@ricilake.net> wrote:
>
> On 22-Aug-05, at 9:52 PM, William Trenker wrote:
>
> > It's easy to set up lua lists:
> > mylist = {"a", "b", "c"}
> > What I'm wondering is what is the design-intended best practice for
> > testing for list membership.  And what is the fastest way to do this?
>
> If you don't actually care about the order of the objects (which is a
> surprisingly common case), then do it like this:
>
> myset = {a = true, b = true, c = true}
>
> -- is an object in the set?
>    if myset[obj] then ...
>
> -- get all the objects out of the set (in undetermined order):
>    for k in pairs(myset) do ...
>
> Note that you cannot add objects to the set during the iteration, which
> is a bit annoying if you're doing a workqueue. However, you can fake it
> by using three sets.
>
> This assumes that we're storing a digraph as a table of sets, such that
> digraph[node_a][node_b] is true if there is a line node_a -> node_b.
> Then the following function computes the set of nodes reachable from a
> given node.
> It may not be the fastest way of doing it, but it seemed like a simple
> example:
>
> function reachable(digraph, node)
>    local empty = {}
>    local done, work, pending = {}, {}, {}
>    local function insert(stateset)
>      if not done[stateset] then
>        done[stateset], pending[stateset] = true, true
>      end
>    end
>
>    insert(node)
>
>    repeat
>      pending, work = {}, pending
>      for node in pairs(work) do
>        for next_node in pairs(digraph[node] or empty) do
>          insert(next_node)
>        end
>      end
>    until not next(pending)
>
>    return done
> end
>
> ("not next(pending)" is true if pending is empty, assuming that false
> is not a possible key. If false were possibly you'd have to say "until
> nil == next(pending)", which would be more readable but slightly
> slower.)
>
>
> > Assuming that for-loops are run-time expensive for large lists, is
> > something like this considered good practice?
>
> For loops are not expensive
>
> > myList = {"a", "b", "cde"}
> > myListString = "|" .. table.concat(myList, "|") .. "|"
> > a = "cde"
> > if string.find(myListString, "|"..a.."|") then
> >     print("found!")
> > else
> > end
>
> That is very expensive
>
>

--
()()       Warren Merrifield
( '.')
(")_(")   wmerrifield@gmail.com

```