Composite C Data Types
C supports simple arrays. The initial values in an array are undefined, and the number of elements is a constant set at compile time.
//Creates an array of ten ints, arrayOne to arrayOne
//The initial values are undefined, and the number of elements is a constant
Use the struct keyword to create a composite data type made of fundamental data types. structs are rather like the ancestors of Objective-C’s objects. They’re simpler, they don’t support methods, and they don’t use Objective-C’s automatic memory management.
A union is like a struct and is defined in a similar way, but it can hold only a single value. In the following example, the union can hold a character array, an int called xpos, or an int called ypos. If you set one element in a union, the other elements become undefined. (This exotic behavior saves memory, but it’s largely unnecessary now.)