📘 Section 14 — Structs in C

A struct lets you bundle together several variables — possibly of different types — into a single named unit. Instead of carrying around 5 separate variables for a student, you carry one struct Student.

🎯 What You’ll Practice

Declaring a struct and listing its members
Creating struct variables and initializing them at declaration
Accessing members using the dot operator (.)
Copying one struct into another with simple assignment
Passing structs to functions and returning values from them
Working with arrays of structs

1. Why Do We Need Structs?

Imagine you want to store data about a student: their GPA, their letter grade, their age, their name. Using normal variables, you’d write:

float gpa;
char grade;
int age;
char name[50];

That works for one student. For 100 students, you’d need 100 of each — and nothing ties them together. A struct solves this by bundling related fields into a single type you can reuse.

2. Declaring a Struct

#include <stdio.h>

struct student {
    float gpa;
    char grade;
};

This defines a new type called struct student. No memory is reserved yet — we’ve only described what a student looks like. To actually create one, we declare a variable of that type.

3. Creating and Using a Struct

#include <stdio.h>

struct student {
    float gpa;
    char grade;
};

int main() {
    struct student s1;        // declaration — like "int s1;" but for our new type

    s1.gpa = 3.9;             // access members with the dot operator
    s1.grade = 'A';

    printf("GPA: %.2f, GRADE: %c\n", s1.gpa, s1.grade);
    return 0;
}

Output:

GPA: 3.90, GRADE: A

💡 The dot operator . reads as “of” — so s1.gpa reads as “the gpa of s1”.

4. Initializing a Struct at Declaration

You can fill in all the members the moment you create the variable, using curly braces:

struct student s1 = {3.9, 'A'};        // gpa = 3.9, grade = 'A'

The values are matched in order to the members listed in the struct definition.

#include <stdio.h>

struct student {
    float gpa;
    char grade;
};

int main() {
    struct student s1 = {3.9, 'A'};
    printf("GPA: %.2f, GRADE: %c\n", s1.gpa, s1.grade);
    return 0;
}

5. Copying a Struct

You can copy all the members of one struct into another with a single assignment — C handles each field for you:

#include <stdio.h>

struct student {
    float gpa;
    char grade;
};

int main() {
    struct student s1 = {3.9, 'A'};
    struct student s2;

    s2 = s1;        // copies every member of s1 into s2

    printf("GPA: %.2f, GRADE: %c\n", s2.gpa, s2.grade);
    return 0;
}

💡 Struct assignment copies all members at once. This is one of the few cases where C lets you “assign” a whole block of data with =.

6. Arrays of Structs

To store many students, declare an array of structs:

struct student students[2];        // 2 students

students[0].gpa = 3.9;
students[0].grade = 'A';

students[1].gpa = 2.7;
students[1].grade = 'C';

Each element of the array is a complete struct student — with all its fields.

7. Reading Strings into a Struct Member — A Common Pitfall

When a struct contains a char name[50] and you mix fgets with scanf, you’ll usually hit a small annoyance: fgets reads the leftover \n from the previous scanf, or it keeps the trailing \n in your string.

printf("Enter student name: ");
fgets(students[i].name, sizeof(students[i].name), stdin);
students[i].name[strlen(students[i].name) - 1] = '\0';   // strip the trailing '\n'

printf("Enter student age: ");
scanf("%d", &students[i].age);

printf("Enter total marks: ");
scanf("%f", &students[i].totalMarks);
fflush(stdin);     // clear the leftover '\n' so the next fgets isn't skipped

⚠️ fflush(stdin) is undefined behavior in standard C, but on most compilers (like the ones used for MS Windows) it works as expected — it clears the input buffer.

🧪 Exercises

Exercise 1: Student Records

Task: Create a structure called Student with members name, age, and totalMarks. Write a C program to:

Input data for two students.
Display each student’s information.
Calculate and print the average of their total marks.

💡 Hint

Define the struct with three members: char name[50], int age, float totalMarks.
Use an array of struct Student of size 2.
Read names with fgets (so spaces are allowed), then strip the trailing '\n'.
Sum both totalMarks and divide by 2.0 to get the average.

🟢 Click to Show Solution

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

struct Student
{
    char name[50];
    int age;
    float totalMarks;
};

void displayStudentData(struct Student s)
{
    printf("\nStudent Data:\n");
    printf("Name: %s\n", s.name);
    printf("Age: %d\n", s.age);
    printf("Total Marks: %.2f\n", s.totalMarks);
}

int main()
{
    struct Student students[2];
    double sumOfAllMarks = 0;
    for (int i = 0; i < 2; i++)
    {
        printf("\nEnter student %d data:\n", i + 1);

        printf("Enter student name: ");
        fgets(students[i].name, sizeof(students[i].name), stdin);
        students[i].name[strlen(students[i].name) - 1] = '\0'; // remove the \n which is read by fgets after the user enters the string and press enter

        printf("Enter student age: ");
        scanf("%d", &students[i].age);

        printf("Enter total marks: ");
        scanf("%f", &students[i].totalMarks);
        fflush(stdin); // very important! empty the buffer so the already read \n (when you enter the input and press enter) don't be read by the next fgets
    }
    for (int i = 0; i < 2; i++)
    {
        displayStudentData(students[i]);
        sumOfAllMarks = sumOfAllMarks + students[i].totalMarks;
    }
    double averageMarks = sumOfAllMarks / 2;
    printf("--------\nAverage of marks: %.2f\n", averageMarks);
    return 0;
}

Sample run:

Enter student 1 data:
Enter student name: Ahmed
Enter student age: 20
Enter total marks: 85.5

Enter student 2 data:
Enter student name: Mona
Enter student age: 21
Enter total marks: 92.0

Student Data:
Name: Ahmed
Age: 20
Total Marks: 85.50

Student Data:
Name: Mona
Age: 21
Total Marks: 92.00
--------
Average of marks: 88.75

Exercise 2: Circles

Task: Define a structure named Circle with one member, radius. Write a C program to:

Input the radius for two circles.
Calculate the area and perimeter of each.
Display the results.

Use the formulas:

Quantity	Formula
Area	`π × radius × radius`
Perimeter	`2 × π × radius`

For π, use 22 / 7.0 (or 3.14159).

💡 Hint

Define struct Circle with one double radius member.
Write two functions: calculateArea(struct Circle c) and calculatePerimeter(struct Circle c).
⚠️ Use 22 / 7.0 (not 22 / 7), otherwise C does integer division and gives 3 instead of ~3.14.

🟢 Click to Show Solution

#include <stdio.h>

struct Circle
{
    double radius;
};

double calculateArea(struct Circle c)
{
    return 22 / 7.0 * c.radius * c.radius;
}

double calculatePerimeter(struct Circle c)
{
    return 2 * 22 / 7.0 * c.radius;
}

int main()
{
    struct Circle circles[2];

    for (int i = 0; i < 2; i++)
    {
        printf("\nEnter circle %d radius: ", i + 1);
        scanf("%lf", &circles[i].radius);
    }

    for (int i = 0; i < 2; i++)
    {
        double area = calculateArea(circles[i]);
        double perimeter = calculatePerimeter(circles[i]);

        printf("\nCircle %d:\n", i + 1);
        printf("Radius: %.2f\n", circles[i].radius);
        printf("Area: %.2f\n", area);
        printf("Perimeter: %.2f\n", perimeter);
    }

    return 0;
}

Sample run:

Enter circle 1 radius: 7
Enter circle 2 radius: 3.5

Circle 1:
Radius: 7.00
Area: 154.00
Perimeter: 44.00

Circle 2:
Radius: 3.50
Area: 38.50
Perimeter: 22.00

Exercise 3: Employees & Highest Salary

Task: Create a structure called Employee to store id, name, and salary. Write a program to:

Input data for three employees.
Display the details of all three.
Find and print the employee with the highest salary.

💡 Hint

Define struct Employee with int id, char name[50], double salary.
While reading input, keep track of an highestSalaryIndex variable. After each new employee, compare their salary to employees[highestSalaryIndex].salary and update if larger.
After all input is read, the variable points to the highest-paid employee.

🟢 Click to Show Solution

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

struct Employee
{
    int id;
    char name[50];
    double salary;
};

int main()
{
    struct Employee employees[3];
    int highestSalaryIndex = 0;

    for (int i = 0; i < 3; i++)
    {
        printf("\nEnter employee %d details:\n", i + 1);
        printf("Employee ID: ");
        scanf("%d", &employees[i].id);
        fflush(stdin);
        printf("Name: ");
        fgets(employees[i].name, sizeof(employees[i].name), stdin);
        employees[i].name[strlen(employees[i].name) - 1] = '\0';
        printf("Salary: ");
        scanf("%lf", &employees[i].salary);

        if (employees[i].salary > employees[highestSalaryIndex].salary)
        {
            highestSalaryIndex = i;
        }
    }

    printf("\nDetails of all employees:\n");
    for (int i = 0; i < 3; i++)
    {
        printf("\nEmployee %d:\n", i + 1);
        printf("Employee ID: %d\n", employees[i].id);
        printf("Name: %s\n", employees[i].name);
        printf("Salary: %.2f\n", employees[i].salary);
    }

    printf("\nEmployee with the highest salary:\n");
    printf("Employee ID: %d\n", employees[highestSalaryIndex].id);
    printf("Name: %s\n", employees[highestSalaryIndex].name);
    printf("Salary: %.2f\n", employees[highestSalaryIndex].salary);

    return 0;
}

Why track the index, not the salary? By saving the index, we keep a reference to the full record (id, name, and salary) — not just the salary number. That way we can print all three fields at the end without searching the array again.

📋 Quick Reference

Concept	Syntax
Define a struct	`struct Name { type member1; type member2; };`
Declare a variable	`struct Name v;`
Initialize at declaration	`struct Name v = {value1, value2};`
Access a member	`v.member`
Copy whole struct	`v2 = v1;`
Array of structs	`struct Name arr[10];` then `arr[i].member`
Pass to a function	`void f(struct Name s)` called with `f(v);`

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