CODEWITHSUNDEEP
luaconditional-statementsnulllua-table
vitto
vitto

Reputation: 19476

How to check if a table element is nil

I have a table called frameActions which could not contain some property in some case:

action = 'UP'
frameActions = {}
frameActions['UP'] = { a = 1, b = 2 }

How do I check if the table have a specific property name?

if frameActions[action].c ~= nil then
    -- do something
end

this condition throws the error: attempt to index a nil value

Upvotes: 4

Views: 10592

Answers (2)

wpedrak
wpedrak

Reputation: 687

You can use some Lua magic and rewrite Etan Reisner's comment as

local E = {}
local my_val = ((frameActions or E).action or E).c
if my_val ~= nil then
    --your code here
end

This code checks if frameAction is nil aswell.

Explanation:

This is how lua will evaluate second line (consider that frameActions = {foo='bar'}):

(frameActions or E) --> {}, because frameAction is not nil or false and then will be take as result
(frameAction.action or E) --> E, because there is no 'action' key in frameAction table, so second 'or' argument is taken
E.c --> nil, because there is no 'c' key in empty table

Those 'check chains' could be even longer. For example:

local E = {}
local my_val = ((((foo or E).bar or E).baz or E).xyz or E).abc
if my_val ~= nil then
    --code if foo['bar']['baz']['xyz']['abc'] is not nil
end

Upvotes: 3

A.G.
A.G.

Reputation: 1319

You can use metamethods to check if your code is trying to access an undefined index. It is well documented on lua wiki.

With the following code the function check_main_index(t,k) will be called when action is defined to an index that is not defined. The function check_sub_index(t,k) is called when accessing a property that is not defined.

But the code you wrote works fine if action is defined to 'UP' and throws the error attempt to index a nil value only if action is defined to something else. (Tested with Lua 5.2).

action = 'UP'

local function check_main_index(t,k)
  print ( "main index : " .. k .. " does not exist" )
  return nil
end
local function check_sub_index(t,k)
  print ( "sub index : " .. k .. " does not exist" )
  return nil
end

frameActions = setmetatable({}, {__index = check_main_index})
frameActions['UP'] = setmetatable({ a = 1, b = 2 }, {__index = check_sub_index})

if frameActions[action].c ~= nil then
    print( "defined" )
else
    print( "not defined" )
end

Upvotes: 3

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