Mastering Algorithms with C

Aggregates and Pointer Arithmetic

One of the most common uses of pointers in C is referencing aggregate data. Aggregate data is data composed of multiple elements grouped together because they are somehow related. C supports two classes of aggregate data: structures and arrays. (Unions, although similar to structures, are considered formally to be in a class by themselves.)

Structures

Structures are sequences of usually heterogeneous elements grouped so that they can be treated together as a single coherent datatype. Pointers to structures are an important part of building data structures. Whereas structures allow us to group data into convenient bundles, pointers let us link these bundles to one another in memory. By linking structures together, we can organize them in meaningful ways to help solve real problems.

As an example, consider chaining a number of elements together in memory to form a linked list (see Chapter 5). To do this, we might use a structure like ListElmt in the following code. Using a ListElmt structure for each element in the list, to link a sequence of list elements together, we set the next member of each element to point to the element that comes after it. We set the next member of the last element to NULL to mark the end of the list. We set the data member of each element to point to the data the element contains. Once we have a list containing elements linked in this way, we can traverse the list by following one next pointer after another.

typedef struct ListElmt_ ...