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

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

Answer 2

For your case, lhf's solution is sufficient. Just for completeness, I would like to mention there is a more complex way, that enables some slightly more complex functionality. Specifically, it acts "as you would expect" when you use it with mutable values, such as table: it both creates the item, and assigns it to the key when created.
function defaultTable(constructor)
local new = {}
local function initIndex(key)
local value = constructor()
new[key] = value
return value
end
setmetatable(new, {__index=initIndex})
return new
end