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

User · Answer

This is a swiss-armyknife function you can use:

function table.find(t, val, recursive, metatables, keys, returnBool)
    if (type(t) ~= "table") then
        return nil
    end

    local checked = {}
    local _findInTable
    local _checkValue
    _checkValue = function(v)
        if (not checked[v]) then
            if (v == val) then
                return v
            end
            if (recursive and type(v) == "table") then
                local r = _findInTable(v)
                if (r ~= nil) then
                    return r
                end
            end
            if (metatables) then
                local r = _checkValue(getmetatable(v))
                if (r ~= nil) then
                    return r
                end
            end
            checked[v] = true
        end
        return nil
    end
    _findInTable = function(t)
        for k,v in pairs(t) do
            local r = _checkValue(t, v)
            if (r ~= nil) then
                return r
            end
            if (keys) then
                r = _checkValue(t, k)
                if (r ~= nil) then
                    return r
                end
            end
        end
        return nil
    end

    local r = _findInTable(t)
    if (returnBool) then
        return r ~= nil
    end
    return r
end

You can use it to check if a value exists:

local myFruit = "apple"
if (table.find({"apple", "pear", "berry"}, myFruit)) then
    print(table.find({"apple", "pear", "berry"}, myFruit)) -- 1

You can use it to find the key:

local fruits = {
    apple = {color="red"},
    pear = {color="green"},
}
local myFruit = fruits.apple
local fruitName = table.find(fruits, myFruit)
print(fruitName) -- "apple"

I hope the recursive parameter speaks for itself.

The metatables parameter allows you to search metatables as well.

The keys parameter makes the function look for keys in the list. Of course that would be useless in Lua (you can just do fruits[key]) but together with recursive and metatables, it becomes handy.

The returnBool parameter is a safe-guard for when you have tables that have false as a key in a table (Yes that's possible: fruits = {false="apple"})

User · Answer

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     break   end end

User · Answer

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

User · Answer

function table find t value      if t and type t    quot table quot  and value then         for    v in ipairs  t  do             if v    value then                 return true              end         end         return false      end     return false  end

User · Answer

Use the following representation instead   local items     apple true  orange true  pear true  banana true   if items apple then         end

User · Answer

you can use this solution  items      a    b    for k v in pairs items  do   if v     a  then    --do something  else    --do something  end end   or items     a    b   for k v in pairs items  do    while v do     if v     a  then        return found     else        break     end   end   end  return nothing

User · Answer

Sort of solution using metatable     local function preparetable t   setmetatable t    newindex function self k v  rawset self v true  end   end  local workingtable    preparetable workingtable  table insert workingtable 123  table insert workingtable 456   if workingtable 456  then     end

User · Answer

function valid data  array   local valid       for i   1   array do   valid array i     true  end  if valid data  then   return false  else   return true  end end   Here s the function I use for checking if data is in an array

User · Answer

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

User · Answer

Write it however you want  but it s faster to iterate directly over the list  than to generate pairs   or ipairs       usr bin env lua  local items      apple    orange    pear    banana     local function locate  table  value       for i   1   table do         if table i     value then print  value     found    return true end     end     print  value     not found    return false end  locate  items   orange    locate  items   car     orange found car not found

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