Chapter 3a - Functions

What Is a Function?

A function is a named, reusable block of code. Instead of writing the same logic in multiple places, you write it once inside a function and call it whenever needed.

// Function definition
void greet() {
    cout << "Hello from a function!" << endl;
}

int main() {
    greet();   // Function call
    greet();   // Call it again
    return 0;
}

---

Function Anatomy

returnType functionName(parameterType paramName, ...) {
    // body
    return value;  // required if returnType is not void
}

Part	Example	Meaning
Return type	int, double, void	Type of value the function sends back
Function name	calculateArea	What you call to run the function
Parameters	double length, double width	Input values the function receives
Return statement	return length * width;	Sends a result back to the caller

---

void Functions (No Return Value)

Use void when the function performs an action but doesn't need to send a value back.

#include <iostream>
using namespace std;

void printSeparator() {
    cout << "--------------------" << endl;
}

void greet(string name) {
    cout << "Hello, " << name << "!" << endl;
}

int main() {
    printSeparator();
    greet("Alice");
    greet("Bob");
    printSeparator();
    return 0;
}

🧪 Try it

#include <iostream>
#include <string>
using namespace std;

void printBox(string message) {
    int len = message.length() + 4;
    for (int i = 0; i < len; i++) cout << "*";
    cout << endl;
    cout << "* " << message << " *" << endl;
    for (int i = 0; i < len; i++) cout << "*";
    cout << endl;
}

int main() {
    printBox("C++ is great");
    printBox("Hello");
    return 0;
}

---

Functions That Return Values

int add(int a, int b) {
    return a + b;
}

double circleArea(double radius) {
    return 3.14159 * radius * radius;
}

bool isEven(int n) {
    return n % 2 == 0;
}

The returned value can be stored in a variable, printed directly, or used in an expression:

int sum = add(3, 7);             // store it
cout << add(10, 20) << endl;     // print it directly
cout << circleArea(5.0) << endl; // use in expression

🧪 Return value examples

#include <iostream>
using namespace std;

int max(int a, int b) {
    if (a > b) return a;
    return b;
}

double average(double a, double b, double c) {
    return (a + b + c) / 3.0;
}

int main() {
    cout << "Max of 4 and 9: " << max(4, 9) << endl;
    cout << "Average of 80, 90, 95: " << average(80, 90, 95) << endl;
    return 0;
}

---

Parameters vs Arguments

Term	Definition	Example
Parameter	Variable in the function definition	int a, int b in add(int a, int b)
Argument	Actual value passed when calling	add(3, 7) — here 3 and 7 are arguments

---

Default Parameter Values

You can give parameters a default value so the caller can omit them.

void printGreeting(string name, string greeting = "Hello") {
    cout << greeting << ", " << name << "!" << endl;
}

int main() {
    printGreeting("Alice");            // Hello, Alice!
    printGreeting("Bob", "Hey");       // Hey, Bob!
    return 0;
}

note

Default parameters must come last in the parameter list.

---

Function Overloading

C++ lets you define multiple functions with the same name but different parameter types or counts. The compiler picks the right one.

int add(int a, int b) {
    return a + b;
}

double add(double a, double b) {
    return a + b;
}

int add(int a, int b, int c) {
    return a + b + c;
}

int main() {
    cout << add(1, 2) << endl;         // calls int version → 3
    cout << add(1.5, 2.5) << endl;     // calls double version → 4.0
    cout << add(1, 2, 3) << endl;      // calls 3-param version → 6
    return 0;
}

---

Scope

A variable declared inside a function only exists inside that function. It cannot be accessed from outside.

void doSomething() {
    int localVar = 10;  // only visible inside doSomething
    cout << localVar << endl;
}

int main() {
    doSomething();
    // cout << localVar;  // ERROR: localVar not in scope here
    return 0;
}

---

Recursion (Intro)

A function can call itself. This is called recursion. Every recursive function needs a base case that stops the recursion.

int factorial(int n) {
    if (n == 0) return 1;       // base case
    return n * factorial(n - 1); // recursive call
}

int main() {
    cout << factorial(5) << endl;  // 120
    return 0;
}

How it unwinds:

factorial(5)
  = 5 * factorial(4)
       = 4 * factorial(3)
            = 3 * factorial(2)
                 = 2 * factorial(1)
                      = 1 * factorial(0)
                           = 1

🧪 Recursion example — Fibonacci

#include <iostream>
using namespace std;

int fibonacci(int n) {
    if (n <= 1) return n;
    return fibonacci(n - 1) + fibonacci(n - 2);
}

int main() {
    for (int i = 0; i < 10; i++) {
        cout << fibonacci(i) << " ";
    }
    cout << endl;
    // Output: 0 1 1 2 3 5 8 13 21 34
    return 0;
}

---

Function Declarations (Prototypes)

In C++, a function must be declared before it is called. If you define helper functions below main(), you need a prototype at the top.

#include <iostream>
using namespace std;

// Prototype — tells the compiler the function exists
int square(int n);

int main() {
    cout << square(5) << endl;  // works because prototype is above
    return 0;
}

// Actual definition — can be below main
int square(int n) {
    return n * n;
}

---

Exercises

Activity: Number Classifier

Write a function classify(int n) that returns "positive", "negative", or "zero" based on the input. Call it from main for several values.

📒 Solution

#include <iostream>
#include <string>
using namespace std;

string classify(int n) {
    if (n > 0) return "positive";
    if (n < 0) return "negative";
    return "zero";
}

int main() {
    cout << classify(10) << endl;
    cout << classify(-5) << endl;
    cout << classify(0) << endl;
    return 0;
}

Activity: Power Function

Write a function power(double base, int exp) that returns base raised to exp using a loop (no pow() from <cmath>).

Expected: power(2, 10) → 1024

💡 Hint

Start with result = 1 and multiply by base exactly exp times in a loop.

📒 Solution

#include <iostream>
using namespace std;

double power(double base, int exp) {
    double result = 1.0;
    for (int i = 0; i < exp; i++) {
        result *= base;
    }
    return result;
}

int main() {
    cout << power(2, 10) << endl;   // 1024
    cout << power(3, 4) << endl;    // 81
    cout << power(1.5, 3) << endl;  // 3.375
    return 0;
}

Activity: isPrime

Write a function bool isPrime(int n) that returns true if n is a prime number, false otherwise. Use it to print all primes from 2 to 50.

💡 Hint

A number is prime if it has no divisors other than 1 and itself. Loop from 2 to n-1 and check if any divide evenly.

📒 Solution

#include <iostream>
using namespace std;

bool isPrime(int n) {
    if (n < 2) return false;
    for (int i = 2; i < n; i++) {
        if (n % i == 0) return false;
    }
    return true;
}

int main() {
    cout << "Primes from 2 to 50: ";
    for (int i = 2; i <= 50; i++) {
        if (isPrime(i)) {
            cout << i << " ";
        }
    }
    cout << endl;
    return 0;
}