Lua can't unpack a table starts with two nil assigned elementwise

Question

I tried the following code in Lua. The first four cases work well but the last two fail. I get nil as results in those two. What is the problem?

v = {nil, 10, nil}
a, b, c = unpack(v)
-- Output: a, b, c =  nil 10  nil
print('a, b, c = ', a, b, c)

v = {nil, nil, 10}
a, b, c = unpack(v)
-- Output: a, b, c =  nil nil 10
print('a, b, c = ', a, b, c)

v = {}
v[2] = 10
a, b, c = unpack(v)
-- Output: a, b, c =  nil 10  nil 
print('a, b, c = ', a, b, c)

v = {}
v[1] = nil
v[2] = 10
v[3] = nil
a, b, c = unpack(v)
-- Output: a, b, c =  nil 10  nil
print('a, b, c = ', a, b, c)

v = {}
v[3] = 10
a, b, c = unpack(v)
-- Output: a, b, c =  nil nil nil
print('a, b, c = ', a, b, c)

v = {}
v[1] = nil
v[2] = nil
v[3] = 10
a, b, c = unpack(v)
-- Output: a, b, c =  nil nil nil
print('a, b, c = ', a, b, c)

Answer 1

When using a table as an array, all elements are required to have values different than nil.

Setting the value for a key to nil effectively removes that key from the table. But in an array all integer keys from 1 to the length of that array must be set. “Holes” are not allowed. So the behaviour in all your cases is unspecified.

You can verify by printing all key/value pairs in the table:

t = {0, nil, 2, 3}

print("pairs:")
for k, v in pairs(t) do
    print("["..k.."]", v)
end

And note how ipairs breaks, since it stops at the first nil element.

print("ipairs:")
for k, v in ipairs(t) do
    print("["..k.."]", v)
end

Workaround in your case is described in an answer here: Lua unpack bug?