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Structure and Union (C)

Learning Objectives

• Define and initialize structures and unions.
• Access members correctly.
• Explain memory difference between structure and union.

Concept

• struct: each member gets separate memory.
• union: all members share the same memory block.

C Example

#include <stdio.h>
#include <string.h>

typedef struct {
    int id;
    char name[20];
    float marks;
} Student;

typedef union {
    int i;
    float f;
    char ch;
} Data;

int main(void) {
    Student s1 = {101, "Riya", 89.5f};
    printf("Student -> id=%d name=%s marks=%.1f\n", s1.id, s1.name, s1.marks);

    Data d;
    d.i = 65;
    printf("Union as int: %d\n", d.i);
    d.f = 10.5f;
    printf("Union as float: %.2f\n", d.f);

    printf("Size of Student: %lu\n", sizeof(Student));
    printf("Size of Data union: %lu\n", sizeof(Data));
    return 0;
}

Dry Run Insight

• Student retains all fields simultaneously.
• In union Data, writing d.f overwrites previously stored d.i.

Common Mistakes

• Expecting union members to hold independent values.
• Forgetting . operator for structure member access.
• Missing typedef consistency.

Practice

1. Create Employee structure.
2. Create union for sensor value (int or float).
3. Compare memory sizes of both.