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Lesson 4: A compound data type

Purpose: Introduce the struct data type.
Contents: The struct data type, use of typedef.
References: The tutorialspoint about

There is also relevant material in wikibooks: C Programming.

Composite data type data type

There is a compound data type in C called struct. In an array, all the elements are of the same type and accessed using indexing. In a struct the elements are labeled and can be of different type.
Examples:
struct Point { int x, y; }; struct Person { char *name; int age; };
It is then possible to create variables of these types:
struct Point a, b, c; struct Point d = {0, 0}; struct Person p1; struct Person p2 = {"Charlie", 12};
As seen, the fields can be initialized in the declaration.

The fields can be accessed using a "dot notation".
Examples:

a.x = 3; b.y = a.x*a.x; printf("Name: %s of age %d\n", p2.name, p2.age);
Variable of struct type can be assigned as ordinary variables: Examples:
c = a; p1 = p2;
and also returned as function values. It becomes quite cumbersome to repeat  struct Person  in every declaration which leads us to introduce typedef

Type aliases

It is possible to create aliases for types in C. Examples.
typedef float real; typedef char string[100]; typedef int * intPointer; real x, y, z; string name, address; intPointer ip, jp;
It is particular useful for structs:
struct Point { int x, y; }; typedef struct Point Point; Point a, b, c;
The struct and typedef can be combined:
typedef struct Point { int x, y; } Point; Point a, b, c;

Exercises

  1. Write a function that asks the user for a person's name and and age.
    Try three different ways of returning the result:
    1. using a parameter of type *Person,
    2. as a Person function value and
    3. as a pointer to Person.
  2. Write three main functions showing how the three versions above should be called.
  3. The following struct is used to store a, from the beginning, unknown number of integers:
    typedef struct Container { int *buffer; // Pointer to the storage int size; // Allocated size int used; // Current number of used places } Container;
    The integers are stored from the beginning so the first used is the index for the first free position. The container is full when  used == size.

    A function for creating containers is given:

    Container *makeContainer(int initialSize) { Container *c = (Container *)malloc(sizeof(container)); (*c).size = initialSize; (*c).used = 0; (*c).buffer = (int *)malloc(initialSize*sizeof(int)); return c: }
    Make sure that you understand the code!

    Write a function int store(int data, Container *c) that stores data in the first free position and returns the index of that position. If the container is full, it should be expanded.

Go to next lesson.

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