Activity: Structures & Unions

Brief Intro — Structures & Unions

In C, structures allow you to group different types of data — like a person's name, age, and salary — under a single name, making it easier to manage complex records. Unions take this further by letting different data types share the exact same memory space.

In this activity, you'll learn how to define and use structs and unions to organize your program's data efficiently and manage computer memory effectively.

Task 1: Why Structs?

In C, a struct (structure) is a collection of variables (can be of different types) under a single name. To store information about one person you can use separate variables name, citNo and salary — but for many people you would need name1, citNo1, salary1, name2, citNo2, salary2, ... A better approach is a single struct Person used for every person.

Task 2: Defining a Struct

To define a structure, the struct keyword is used:

struct structureName {
    dataType member1;
    dataType member2;
    ...
};

Example:

struct Person {
    char name[50];
    int citNo;
    float salary;
};

Task 3: Creating Struct Variables

When a struct type is declared, no storage or memory is allocated. To allocate memory and work with it, we need to create variables. Two equivalent ways:

int main() {
    struct Person person1, person2, p[20];
    return 0;
}

or, naming variables right after the closing brace:

struct Person { ... } person1, person2, p[20];

Both create two variables and an array of 20 elements.

Task 4: Accessing Members

Two operators access structure members:

• The dot (.) operator, also called the member operator, e.g. person2.salary.
• The pointer ( -> ) operator, also called the structure pointer operator, used to access through pointer variables.

Task 5: The Distance Program Trace

The notes' program reads two distances (feet + inches), adds the feet and inches, then converts excess inches with:

while (sum.inch >= 12) {
    ++sum.feet;
    sum.inch = sum.inch - 12;
}

Output: Sum of distances = 15'-5.7". Try the Distance Adder above with 12ft 7.9in and 2ft 9.8in.

Task 6: The typedef Keyword

typedef creates an alias name for data types. It is commonly used with structures to simplify declaring variables. struct Distance { ... } with struct Distance d1, d2; is equivalent to typedef struct Distance { ... } distances; with distances d1, d2;.

Task 7: Nested Structures

You can create structures within a structure:

struct complex {
    int imag;
    float real;
};

struct number {
    struct complex comp;
    int integers;
} num1, num2;

Task 8: Nested Member Access

To set the imag of num2 to 11, chain the dots:

num2.comp.imag = 11;

Each dot moves one level deeper into the nested structure.

Task 9: Defining a Union

A union is a user-defined type similar to structs except for one key difference: structures allocate space to store all their members, whereas unions allocate space to store only the largest member. The union keyword defines one:

union car {
    char name[50];
    int price;
};

Union variables are created the same ways as struct variables (union car car1, car2, *car3;).

Task 10: Accessing Union Members

Use the dot (.) operator to access union members through a variable, and the -> operator through a pointer. For example: car1.price; through the pointer car3 either (*car3).price or car3->price.

Task 11: Struct vs Union Memory

With name[32] (32 bytes), salary (4 bytes), workerNo (4 bytes): the struct sJob is 40 bytes (all members side by side), but the union uJob is 32 bytes — the size of a union variable is always the size of its largest element. Output: Size of union = 32, Size of structure = 40.

Task 12: Sharing Memory

With a union, all members share the same memory. After j.salary = 12.3; then j.workerNo = 100;, salary no longer holds 12.3 — the program prints Salary = 0.0 and Number of workers = 100. Use the layout buttons above to watch the overwrite.