Lua: set every new element in table to a default value

Question

I want to get the most frequent k-sized substring in a string. To achieve this, I'm using a table to store the number of occurrences for each substring. Here's the code:

function frequentWords(seq, k)
  local subs = ""
  local counter = {}
  for i = 1,(seq:len()-k+1) do
    subs = seq:sub(i, i+k-1)
    counter[subs] = (counter[subs] and counter[subs] + 1 or 1)
    --print(subs .. ": " .. counter[subs])
  end
end

The line counter[subs] = (counter[subs] and counter[subs] + 1 or 1) has the same mean of counter[subs] = (counter[subs] ? counter[subs]+1 : 1). This line would be only counter[subs] = counter[subs] + 1 if we could set every new counter element with 0. Is this possible in Lua? If not, what's the best way of doing something similar?

For instance, in Ruby, this is done by declaring a Hash like this:

counter = Hash.new(0)

Answer 1

You can set an __index metamethod in counter to return 0:

setmetatable(counter,{__index=function () return 0 end})

but this is easier and clearer:

counter[subs] = (counter[subs] or 0) + 1