Check if a Lua table member exists at any level - lua

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?

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.

Answer 2

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

Answer 3

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

Answer 4

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.

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

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