Check if a Lua table member exists at any level

Question

I need to check if a member exists in a table that isn't at the next level, but along a path of members.

foo = {}
if foo.bar.joe then
  print(foo.bar.joe)
end

this will cast an attempt to index field 'bar' (a nil value) because bar isn't defined.

My usual solution is to test the chain, piece-by-piece.

foo = {}
if foo.bar and foo.bar.joe then
  print(foo.bar.joe)
end

but this can be very tedious when there are many nested tables. Are there a better way to do this test than piece-by-piece?

Redouble

Answer 1

I don't understand what you try to mean by "along a path of members". From the example, I assume you are trying to find a value in a "subtable"?

local function search(master, target) --target is a string
    for k,v in next, master do
        if type(v)=="table" and v[target] then return true end
    end
end

A simple example. If you use such a function, you can pass the foo table and the joe string to see if foo.*.joe exists. Hope this helps.

Cicily

Answer 2

debug.setmetatable(nil, {__index = {}})

foo = {}
print(foo.bar.baz.quux)
print(({}).prd.krt.skrz.drn.zprv.zhlt.hrst.zrn)  -- sorry ))

Anode

Answer 3

I think you're looking for something along these lines:

local function get(Obj, Field, ...)
    if Obj == nil or Field == nil then
        return Obj
    else
        return get(Obj[Field], ...)
    end
end

local foo = {x = {y = 7}}
assert(get() == nil)
assert(get(foo) == foo)
assert(get(foo, "x") == foo.x)
assert(get(foo, "x", "y") == 7)
assert(get(foo, "x", "z") == nil)
assert(get(foo, "bar", "joe") == nil)
assert(get(foo, "x", "y") or 41 == 7)
assert(get(foo, "bar", "joe") or 41 == 41)
local Path = {foo, "x", "y"}
assert(get(table.unpack(Path)) == 7)

get simply traverses the given path until a nil is encountered. Seems to do the job. Feel free to think up a better name than "get" though.

As usual, exercise care when combining with or.

I'm impressed by Egor's clever answer, but in general I think we ought to not rely on such hacks.

See also

• The 'Safe Table Navigation' patch for Lua 5.2 : http://lua-users.org/wiki/LuaPowerPatches

• Lengthy discussion on this matter : http://lua-users.org/lists/lua-l/2010-08/threads.html#00519

• Related technique : http://lua-users.org/wiki/AutomagicTables

• I suspect something relevant has been implemented in MetaLua, but I can't find at the moment.

Dismal

I am surprised this simple and elegant answer did not receive any upvote. – Cavalier

Answer 4

To search for an element that is at any level of a table, I would use a method such as this one:

function exists(tab, element)
    local v
    for _, v in pairs(tab) do
        if v == element then
            return true
        elseif type(v) == "table" then
            return exists(v, element)
        end
    end
    return false
end

testTable = {{"Carrot", {"Mushroom", "Lettuce"}, "Mayonnaise"}, "Cinnamon"}
print(exists(testTable, "Mushroom")) -- true
print(exists(testTable, "Apple")) -- false
print(exists(testTable, "Cinnamon")) -- true

Inebriety

Answer 5

If I understood your problem correctly, here's one possibility:

function isField(s)
  local t
  for key in s:gmatch('[^.]+') do
    if t == nil then
      if _ENV[ key ] == nil then return false end
      t = _ENV[ key ]
    else
      if t[ key ] == nil then return false end
      t = t[ key ]
    end
    --print(key) --for DEBUGGING
  end
  return true
end

-- To test

t = {}
t.a = {}
t.a.b = {}
t.a.b.c = 'Found me'

if isField('t.a.b.c') then print(t.a.b.c) else print 'NOT FOUND' end
if isField('t.a.b.c.d') then print(t.a.b.c.d) else print 'NOT FOUND' end

UPDATE: As per cauterite's suggestion, here's a version that also works with locals but has to take two arguments :(

function isField(t,s)
  if t == nil then return false end
  local t = t
  for key in s:gmatch('[^.]+') do
    if t[ key ] == nil then return false end
    t = t[ key ]
  end
  return true
end

-- To test

local
t = {}
t.a = {}
t.a.b = {}
t.a.b.c = 'Found me'

if isField(t,'a.b.c') then print(t.a.b.c) else print 'NOT FOUND' end
if isField(t,'a.b.c.d') then print(t.a.b.c.d) else print 'NOT FOUND' end

Charcoal

I'd suggest implementing it as isField(t, s) — trying to obtain the table from the first component in the string only works with globals. For example, local u = {v = 1}; assert(isField('u.v')) fails. – Dismal

Updated accordingly. Thanks. – Charcoal

Answer 6

foo = {}

foo.boo = {}

foo.boo.jeo = {}

foo.boo.joe is foo['boo']['joe'] and so

i make next function

function exist(t)

    local words = {}
    local command

    for i,v in string.gmatch(t, '%w+') do words[#words+1] = i end

    command = string.format('a = %s', words[1])

    loadstring(command)()

    if a == nil then return false end

    for count=2, #words do
        a = a[words[count]]
        if a == nil then return false end
    end

    a = nil
    return true
end

foo = {}
foo.boo = {}
foo.boo.joe = {}

print(exist('foo.boo.joe.b.a'))

using loadstring to make temp variable. my lua ver is 5.1

remove loadstring at 5.2 5.3, instead using load

Swor

foo.bar.joe is equivalent to _G['foo'].bar.joe (if there's no local variable foo in scope) not _G['foo.bar.joe']! You can't access the latter using the normal variable syntax because it contains characters invalid for variable names. – Equidistant

Now it's raising an error (like in the question and for the same reason). – Equidistant