Arrays can be sparse, meaning an array can have a hole in it. Elements can be assigned to index 1 and 2 of an array, leaving 3 and 4 blank and then assigning elements 5 and 6. Any hole in the array will have a default value of nil, as the following code segment demonstrates:

arr = { }

arr[1] = "x"
arr[2] = "y"
-- arr[3] is nil by default
-- arr[4] is nil by default
arr[5] = "z"
arr[6] = "w"

for i=1,6 do
    print (tostring(arr[i]))
end

This implies that setting the value of an existing index to nil will introduce a hole in the array. These holes can sometimes cause unexpected issues.