Lua Table Access with Location Given as a String

I have a table and I'm trying to access a specific location that is passed in as a String. What is the easiest way to use the string to access the correct location?

Example, if the table looks like this:

a.b1 = true
a.b2.c1 = true
a.b2.c2 = false
a.b3 = true


How can I change a.b2.c2 to true given a location 'a.b2.c2' as a string.

a[str_var]

2 Answers
2

If you have just a single level, you can use square-brace indexing:

function setSingle(obj, key, value)
obj[key] = value
end
setSingle(a, "b1", "foo")
print(a.b1) --> foo


If you have multiple, you need to do several iterations of this indexing. You can use a loop to do that:

function setMultiple(obj, keys, value)
for i = 1, #keys - 1 do
obj = obj[keys[i]]
end

-- Merely "obj = value" would affect only this local variable
-- (as above in the loop), rather than modify the table.
-- So the last index has to be done separately from the loop:
obj[keys[#keys]] = value
end
setMultiple(a, "b2", "c1", "foo")
print(a.b2.c1) --> foo


You can use string.gmatch to parse a properly formatted list of keys. [^.]+ will match "words" made of non-period symbols:

string.gmatch

[^.]+

function parseDots(str)
local keys =
for key in str:gmatch "[^.]+" do
table.insert(keys, key)
end
return keys
end


Putting this all together,

setMultiple(a, parseDots("b2.c2"), "foo")
print(a.b2.c2) --> foo


One issue you may run into is that you cannot create new tables with this function; you will have to create the containing table before you can create any keys in it. For example, beforing ading "b4.c3" you would have to add "b4".

"b4.c3"

"b4"

You can use loadstring to build the statement you want to execute as a string.

loadstring

a = b2 =

a.b1 = true
a.b2.c1 = true
a.b2.c2 = false
a.b3 = true

str = "a.b2.c2"
loadstring(str .. " = true")()
print(a.b2.c2)


a

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.