Example
Iterators utilize a form of the for loop known as the generic for loop.
The generic form of the for loop uses three parameters:
- An iterator function that gets called when the next value is needed. It receives both the invariant state and control variable as parameters. Returning
nilsignals termination. - The invariant state is a value that doesn't change during the iteration. It is typically the subject of the iterator, such as a table, string, or userdata.
- The control variable represents an initial value for iteration.
We can write a for loop to iterate all key-value pairs in a table using the next function.
local t = {a=1, b=2, c=3, d=4, e=5}
-- next is the iterator function
-- t is the invariant state
-- nil is the control variable (calling next with a nil gets the first key)
for key, value in next, t, nil do
-- key is the new value for the control variable
print(key, value)
-- Lua calls: next(t, key)
end
