Example: Area Calculator using Functions in C++

// Area Calculator using Functions
#include <iostream>
#include <cmath> // Required for M_PI and pow()

// Define PI if not defined by cmath (some compilers might require it)
#ifndef M_PI
#define M_PI 3.14159265358979323846
#endif

using namespace std;

// Function to calculate the area of a circle
double calculateCircleArea(double radius) {
    return M_PI * pow(radius, 2);
}

// Function to calculate the area of a rectangle
double calculateRectangleArea(double length, double width) {
    return length * width;
}

// Function to calculate the area of a triangle
double calculateTriangleArea(double base, double height) {
    return 0.5 * base * height;
}

int main() {
    // Step 1: Declare variables for user choice and dimensions
    int choice;
    double radius, length, width, base, height;

    // Step 2: Display menu and get user input for shape choice
    cout << "Choose a shape to calculate its area:" << endl;
    cout << "1. Circle" << endl;
    cout << "2. Rectangle" << endl;
    cout << "3. Triangle" << endl;
    cout << "Enter your choice (1-3): ";
    cin >> choice;

    // Step 3: Use a switch statement to handle different choices
    switch (choice) {
        case 1:
            cout << "Enter the radius of the circle: ";
            cin >> radius;
            cout << "The area of the circle is: " << calculateCircleArea(radius) << endl;
            break;
        case 2:
            cout << "Enter the length of the rectangle: ";
            cin >> length;
            cout << "Enter the width of the rectangle: ";
            cin >> width;
            cout << "The area of the rectangle is: " << calculateRectangleArea(length, width) << endl;
            break;
        case 3:
            cout << "Enter the base of the triangle: ";
            cin >> base;
            cout << "Enter the height of the triangle: ";
            cin >> height;
            cout << "The area of the triangle is: " << calculateTriangleArea(base, height) << endl;
            break;
        default:
            cout << "Invalid choice. Please enter a number between 1 and 3." << endl;
            break;
    }

    return 0;
}