Need to be able to traverse diverse data structures in a common abstract way.
The main principle is to take the responsibility for access and traversal out of the data structure and put that into an Iterator object.
- Memento (Iterators can store the current position as a Memento)
- Factory (to instantiate appropriate subclasses if uncommon types are used)
Iterators are fundamental to "generic programming", which seeks to boost productivity by reducing configuration costs.
A real-world example is the seek button on a radio. Though you may know the frequency of several stations, you may want to simply get to the next one. The seek button iterates to the next available station.
Generally, iterators keep track of where it is in the structure. In Lua, the generic for loop keeps track of that. Though the iterator is often tricky to write, the use of that iterator is very easy.
function allwords () local line = io.read() -- current line local pos = 1 -- current position in the line return function () -- iterator function while line do -- repeat while there are lines local s, e = string.find(line, "%w+", pos) if s then -- found a word? pos = e + 1 -- next position is after this word return string.sub(line, s, e) -- return the word else line = io.read() -- word not found; try next line pos = 1 -- restart from first position end end return nil -- no more lines: end of traversal end end for word in allwords() do print(word) end