This first edition was written for Lua 5.0. While still largely relevant for later versions, there are some differences.
The fourth edition targets Lua 5.3 and is available at Amazon and other bookstores.
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7 – Iterators and the Generic for

In this chapter, we cover how to write iterators for the generic for. We start with simple iterators, then we learn how to use all the power of the generic for to write more efficient iterators.