Explain why unpack() returns different results in Lua

Question

The following script finds prime numbers in a range from 1 to 13. When I explicitly iterate over the table that contains the results I can see that the script works as expected. However, if I use unpack() function on the table only the first 3 numbers get printed out.

From docs: unpack is "a special function with multiple returns. It receives an array and returns as results all elements from the array, starting from index 1".

Why is it not working in the script below?

t = {}
for i=1, 13 do t[i] = i end

primes = {}
for idx, n in ipairs(t) do
  local isprime = true
  for i=2, n-1 do
    if n%i == 0 then
      isprime = false
      break
    end
  end
  if isprime then
    primes[idx] = n
  end
end
print('loop printing:')
for i in pairs(primes) do
  print(i)
end
print('unpack:')
print(unpack(primes))

Running

$ lua5.3 primes.lua
loop printing:
1
2
3
5
7
13
11
unpack:
1   2   3

Answer 1

Change

primes[idx] = n

to

primes[#primes+1] = n

The reason is that idx is not sequential as not every number is a prime.