Search for an item in a Lua list

Question

If I have a list of items like this:

local items = { "apple", "orange", "pear", "banana" }

how do I check if "orange" is in this list?

In Python I could do:

if "orange" in items:
    # do something

Is there an equivalent in Lua?

Answer 1

You could use something like a set from Programming in Lua:

function Set (list)
  local set = {}
  for _, l in ipairs(list) do set[l] = true end
  return set
end

Then you could put your list in the Set and test for membership:

local items = Set { "apple", "orange", "pear", "banana" }

if items["orange"] then
  -- do something
end

Or you could iterate over the list directly:

local items = { "apple", "orange", "pear", "banana" }

for _,v in pairs(items) do
  if v == "orange" then
    -- do something
    return
  end
end

Answer 2

Lua tables are more closely analogs of Python dictionaries rather than lists. The table you have create is essentially a 1-based indexed array of strings. Use any standard search algorithm to find out if a value is in the array. Another approach would be to store the values as table keys instead as shown in the set implementation of Jon Ericson's post.

Answer 3

Use the following representation instead:

local items = { apple=true, orange=true, pear=true, banana=true }
if items.apple then
    ...
end

Answer 4

You're seeing firsthand one of the cons of Lua having only one data structure---you have to roll your own. If you stick with Lua you will gradually accumulate a library of functions that manipulate tables in the way you like to do things. My library includes a list-to-set conversion and a higher-order list-searching function:

function table.set(t) -- set of list
  local u = { }
  for _, v in ipairs(t) do u[v] = true end
  return u
end

function table.find(f, l) -- find element v of l satisfying f(v)
  for _, v in ipairs(l) do
    if f(v) then
      return v
    end
  end
  return nil
end