📘 Section 5 — Functions

Learn how to define reusable code blocks, pass data through parameters, and build modular programs.

🎯 What You’ll Learn

• What functions are and why we use them
• How to define and call functions
• The difference between formal and actual parameters
• How to use return values
• Calling functions with call by value

1. What Is a Function?

A function is a block of code which only runs when it is called. Data can be passed to the function and this data is known as parameters. Define the code once and use it many times.

2. Your First Function

Here’s the simplest example — a function that prints a message:

#include <stdio.h>

void fun() {
    printf("hi");
}

int main() {
    fun();
    return 0;
}

3. Functions with Parameters

You can pass data to a function using parameters:

#include <stdio.h>

void fun(int a) {         // formal parameters
    printf("%d", a);
}

int main() {
    int a = 5;
    fun(a);               // actual parameters
    return 0;
}

Formal vs Actual Parameters

4. Calling Methods

📌 In this section we focus on Call by Value. You’ll learn Call by Reference when we cover Pointers.

5. Functions with Return Values

Some functions only do an action, like printing. These functions can be void.

Other functions calculate a value and send it back to the place where the function was called. These functions use return.

#include <stdio.h>

int square(int x) {
    return x * x;
}

int main() {
    int result = square(5);
    printf("%d", result);
    return 0;
}

Here, square(5) returns 25, and this value is stored in result.

return ends the function and sends a value back to the caller.

🧪 Exercises — Trace the Output

Practice reading code carefully. Try to predict the output before revealing the answer!

Exercise 1: Single Parameter

void Fun(int y) {
    printf("y = %d\n", y);
}

int main() {
    Fun(5);        // first call

    int x = 6;
    Fun(x);        // second call
    Fun(x + 1);    // third call

    return 0;
}

y = 5
y = 6
y = 7

Exercise 2: Two Parameters — Watch the Order!

void Fun(int y, int x) {
    printf("x = %d\ny = %d\n", x, y);
}

int main() {
    int x = 25, y = 13;
    printf("x = %d\ny = %d\n", x, y);
    Fun(x, y);

    return 0;
}

x = 25
y = 13
x = 13
y = 25

Exercise 3: Local Variable Scope

void Fun(int y) {
    int x = 100;
    printf("x = %d\ny = %d\n", x, y);
}

int main() {
    int x = 25, y = 13;
    printf("x = %d\ny = %d\n", x, y);
    Fun(x);

    return 0;
}

x = 25
y = 13
x = 100
y = 25

🛠️ Coding Exercises

Exercise 4: Hours to Seconds Converter

Task: Write a function called ToSeconds() that converts hours to seconds. The function should take in one integer parameter representing the number of hours and returns the equivalent number of seconds. Using the above function, calculate how many seconds are there in a day, week, and month.

#include <stdio.h>

int ToSeconds(int hours) {
    return hours * 60 * 60;
}

int main() {
    int hours = 24;
    int seconds = ToSeconds(hours);
    printf("%d hours = %d seconds\n", hours, seconds);

    int hoursInDay   = 24;       // 1 day = 24 hours
    int hoursInWeek  = 7 * 24;   // 1 week = 7 days = 7 * 24 hours
    int hoursInMonth = 30 * 24;  // 1 month = 30 days = 30 * 24 hours

    printf("Hours in 1 day = %d\n", ToSeconds(hoursInDay));
    printf("Hours in 1 week = %d\n", ToSeconds(hoursInWeek));
    printf("Hours in 1 month = %d\n", ToSeconds(hoursInMonth));

    return 0;
}

Exercise 5: Quad Root Calculator

Task: Using the built-in math function sqrt(), write the function Quadroot() that returns the quad root (⁴√) of an integer.

💡 Recall: ⁴√n = √(√n) — apply the square root twice.

#include <stdio.h>
#include <math.h>    // needed for the use of sqrt function

double Quadroot(int num) {
    return sqrt(sqrt(num));
}

int main() {
    int num;
    printf("Enter a number to calculate its quad root: ");
    scanf("%d", &num);

    if (num < 0) {
        printf("Negative number quad root is not a real number\n");
    } else {
        double quadroot = Quadroot(num);
        printf("Quad root = %.2f\n", quadroot);
    }

    return 0;
}

Enter a number to calculate its quad root: 81
Quad root = 3.00

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