Lua 5.3.5 unexpected table.insert behavior

When using table.insert(...) like this:

table.insert(...)

test =
table.insert(test, 1)
print(test[1]) -- 1

table.insert(test, 2)
print(test[1]) -- 1


The expected print result is

1

1

Now when you do this:

local A = b =
local B = c = x=0

function add(t, X)
local temp = B
temp.c = x=X
table.insert(t.b, temp)
end

local a = A
add(a, 1)
print(a.b[1].c.x) -- 1

add(a, 2)
print(a.b[1].c.x) -- 2


The result is

1

2

But shouldn't it be 1 and 1, too? This is my expected behavior, since I'm accessing the first element of the inner table b both times. What am I missing here?

b

2 Answers
2

I don't quite understand what you are trying to do with your code, but you get 2 instead of 1 that you expect because first and second elements point to the same table, so the modifications you think are applied only to the second element are actually applied to both (because of your local temp = B assignment, which makes B used in each of the inserted elements).

local temp = B

B

Add print(a.b[1] == a.b[2]) to the end of your script to confirm.

print(a.b[1] == a.b[2])

add(..)

B

temp

A.b

B

local temp = c = x=0

local temp = B

The problem is that Lua does not copy tables by values. You would need a deep copy-mechanism to copy really everything from a table. This code here works like expected while providing the requested 'defaults' of A and B.

A

B

function init_A(B)
local t =
if not (B== nil) then
t.b = B
else
t.b =
end
return t
end

function init_B(C)
local t =
if not (C== nil) then
t.c = C
else
t.c = x=0
end
return t
end

function add(t, X)
local temp = init_B(x=X)
table.insert(t.b, temp)
end

local a = init_A()
add(a, 1)
print(a.b[1].c.x) -- 1

add(a, 2)
print(a.b[1].c.x) -- 1
print(a.b[2].c.x) -- 2


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