Collections of values appear everywhere in programs, and Julia has the most important built-in collection types. In Chapter 2, Variables, Types, and Operations, we introduced two important types of collection: arrays and tuples. In this chapter, we will look more deeply into multidimensional arrays (or matrices), and into the tuple type as well. A dictionary type, where you can look up a value through a key, is indispensable in a modern language, and Julia has this too. Finally, we will explore the set type. Like arrays, all these types are parameterized; the type of their elements can be specified at the time of object construction.
Collections are also iterable types, the types over which we can loop with for or an iterator producing each element of the collection successively. The iterable types include string, range, array, tuple, dictionary, and set...