Structures in C

01What is a Structure?

A structure (struct) is a user-defined data type that allows you to group different data types together under one name. Think of it as creating your own custom data type.

🆚 Arrays vs Structures

Array

• All elements same type
• Access by index [0], [1], [2]...
• Example: 5 integers

main.c

int scores[5];  // 5 ints only

Structure

• Different types together
• Access by name .age, .name
• Example: Person data

main.c

struct Person { 
  int age; 
  char name[50]; 
};

Real-World Examples

• Student: name, ID, grades, age
• Book: title, author, price, pages
• Point: x coordinate, y coordinate
• Date: day, month, year

02Defining a Structure

structure_basic.c

1#include <stdio.h>
2
3// Define a structure (blueprint)
4struct Student {
5    int id;              // Member 1: integer
6    char name[50];       // Member 2: string (char array)
7    float gpa;           // Member 3: float
8    int age;             // Member 4: integer
9};  // Don't forget the semicolon!
10
11int main() {
12    // Create a structure variable
13    struct Student student1;
14    
15    // Assign values to members using dot (.) operator
16    student1.id = 101;
17    student1.gpa = 3.85;
18    student1.age = 20;
19    // For strings, use strcpy
20    // strcpy(student1.name, "Alice");
21    
22    // Or initialize at declaration
23    struct Student student2 = {102, "Bob", 3.92, 21};
24    
25    // Print values
26    printf("Student ID: %d\n", student2.id);
27    printf("Name: %s\n", student2.name);
28    printf("GPA: %.2f\n", student2.gpa);
29    printf("Age: %d\n", student2.age);
30    
31    return 0;
32}

Output:

Student ID: 102
Name: Bob
GPA: 3.92
Age: 21

🔍 How This Program Works

struct Student {...} — Defines a blueprint (template) for Student data. No memory allocated yet.

struct Student student1; — Creates an actual variable using the blueprint. Memory is now allocated.

student1.id = 101 — The . operator accesses member "id" and assigns value 101 to it.

{102, "Bob", 3.92, 21} — Initialize all members at once in order they were declared.

student2.name — Access the name member. Since it's a char array, use %s to print.

Key Points

• struct keyword defines the structure
• Members are listed inside curly braces {}
• Semicolon ; required after closing brace
• Use . (dot) operator to access members

03Structure Memory Layout

Structure members are stored in contiguous memory in the order they are declared. However, the compiler may add padding bytes for alignment.

Memory Layout: struct Point { int x; int y; }

x (int)

4 bytes

y (int)

4 bytes

Total: 8 bytes (contiguous memory)

Memory Padding & Alignment

The compiler adds padding bytes to align data on memory boundaries for faster access:

struct Example { char a; int b; char c; }

pad

b (int)

4 bytes

pad

Total: 12 bytes (not 6!) due to padding

struct_padding.c

1#include <stdio.h>
2
3struct WithPadding {
4    char a;     // 1 byte + 3 padding
5    int b;      // 4 bytes
6    char c;     // 1 byte + 3 padding
7};  // Total: 12 bytes
8
9struct Optimized {
10    int b;      // 4 bytes
11    char a;     // 1 byte
12    char c;     // 1 byte + 2 padding
13};  // Total: 8 bytes (better!)
14
15int main() {
16    printf("WithPadding size: %zu bytes\n", sizeof(struct WithPadding));   // 12
17    printf("Optimized size: %zu bytes\n", sizeof(struct Optimized));       // 8
18    
19    return 0;
20}

💡 Optimization Tip

Order structure members from largest to smallest to minimize padding and save memory.

04typedef with Structures

typedef creates an alias (shorter name) for a type, so you don't have to write struct every time.

typedef_struct.c

1#include <stdio.h>
2
3// Without typedef - must use "struct Student" everywhere
4struct Student {
5    int id;
6    char name[50];
7};
8
9// With typedef - can use just "Person"
10typedef struct {
11    int id;
12    char name[50];
13    int age;
14} Person;  // "Person" is now the type name
15
16int main() {
17    // Without typedef
18    struct Student s1 = {101, "Alice"};
19    
20    // With typedef (cleaner!)
21    Person p1 = {201, "Bob", 25};
22    Person p2 = {202, "Carol", 30};
23    
24    printf("Student: %s\n", s1.name);
25    printf("Person 1: %s, Age: %d\n", p1.name, p1.age);
26    printf("Person 2: %s, Age: %d\n", p2.name, p2.age);
27    
28    return 0;
29}

Without typedef

main.c

struct Student s1;
struct Student s2;

With typedef ✓

main.c

Person p1;
Person p2;

05Nested Structures

A structure can contain another structure as a member — this is called nesting.

nested_struct.c

1#include <stdio.h>
2
3// Define a Date structure
4typedef struct {
5    int day;
6    int month;
7    int year;
8} Date;
9
10// Define a Person structure with nested Date
11typedef struct {
12    char name[50];
13    int age;
14    Date birthdate;  // Nested structure!
15} Person;
16
17int main() {
18    // Initialize with nested values
19    Person person1 = {
20        "Alice",
21        25,
22        {15, 6, 1999}  // birthdate: June 15, 1999
23    };
24    
25    // Access nested members with multiple dots
26    printf("Name: %s\n", person1.name);
27    printf("Age: %d\n", person1.age);
28    printf("Birthdate: %d/%d/%d\n", 
29           person1.birthdate.day,
30           person1.birthdate.month,
31           person1.birthdate.year);
32    
33    // Modify nested member
34    person1.birthdate.year = 1998;
35    
36    return 0;
37}

Output:

Name: Alice
Age: 25
Birthdate: 15/6/1999

06Array of Structures

You can create an array where each element is a structure — perfect for storing lists of records.

array_of_struct.c

1#include <stdio.h>
2#include <string.h>
3
4typedef struct {
5    int id;
6    char name[30];
7    float gpa;
8} Student;
9
10int main() {
11    // Array of 3 students
12    Student students[3] = {
13        {101, "Alice", 3.85},
14        {102, "Bob", 3.92},
15        {103, "Carol", 3.78}
16    };
17    
18    // Print all students
19    printf("Student Records:\n");
20    printf("%-5s %-15s %s\n", "ID", "Name", "GPA");
21    printf("----------------------------\n");
22    
23    for (int i = 0; i < 3; i++) {
24        printf("%-5d %-15s %.2f\n", 
25               students[i].id, 
26               students[i].name, 
27               students[i].gpa);
28    }
29    
30    // Find highest GPA
31    float maxGpa = 0;
32    int topStudent = 0;
33    for (int i = 0; i < 3; i++) {
34        if (students[i].gpa > maxGpa) {
35            maxGpa = students[i].gpa;
36            topStudent = i;
37        }
38    }
39    printf("\nTop Student: %s with GPA %.2f\n", 
40           students[topStudent].name, maxGpa);
41    
42    // Memory: 3 * sizeof(Student) bytes
43    printf("\nSize of one Student: %zu bytes\n", sizeof(Student));
44    printf("Size of array: %zu bytes\n", sizeof(students));
45    
46    return 0;
47}

Output:

Student Records:
ID Name GPA
----------------------------
101 Alice 3.85
102 Bob 3.92
103 Carol 3.78

Top Student: Bob with GPA 3.92

Size of one Student: 40 bytes
Size of array: 120 bytes

Array of Structures Memory Layout

id, name, gpa

students[0]

id, name, gpa

students[1]

id, name, gpa

students[2]

Each struct stored contiguously in memory

07Common Mistakes

❌ Forgetting Semicolon After struct

main.c

struct Point {
    int x;
    int y;
}  // ERROR! Missing semicolon
struct Point {
    int x;
    int y;
};  // Correct!

❌ Assigning Strings with =

main.c

struct Student s;
s.name = "Alice";  // ERROR! Can't assign strings like this
// Correct way:
strcpy(s.name, "Alice");  // Use strcpy()

❌ Comparing Structures with ==

main.c

struct Point p1 = {1, 2};
struct Point p2 = {1, 2};
if (p1 == p2) {  // ERROR! Can't compare structs with ==
}
// Must compare member by member:
if (p1.x == p2.x && p1.y == p2.y) {  // Correct!
}

08Summary

What You Learned:

✓Structures: Group different data types under one name
✓Access: Use dot operator (.) to access members
✓Memory: Members stored contiguously with possible padding
✓typedef: Creates shorter type aliases (no "struct" keyword needed)
✓Nesting: Structures can contain other structures
✓Arrays: Create arrays of structures for record lists

09Next Steps

You've learned the main derived data types! Continue with:

Next: Unions→Back to Data Types

What You Will Learn

01What is a Structure?

🆚 Arrays vs Structures

Array

Structure

Real-World Examples

02Defining a Structure

Output:

🔍 How This Program Works

Key Points

03Structure Memory Layout

Memory Layout: struct Point { int x; int y; }

Memory Padding & Alignment

struct Example { char a; int b; char c; }

💡 Optimization Tip

04typedef with Structures

Without typedef

With typedef ✓

05Nested Structures

Output:

06Array of Structures

Output:

Array of Structures Memory Layout

07Common Mistakes

❌ Forgetting Semicolon After struct

❌ Assigning Strings with =

❌ Comparing Structures with ==

08Summary

What You Learned:

09Next Steps