📝 Lesson 2: C++ Syntax Basics

Comments: Leaving Notes in Your Code

Comments are notes for humans. The compiler ignores them completely, so they never change what your program does — they exist to explain why the code does what it does, for you and for anyone who reads it later (including future-you).

// This is a single-line comment — everything after // is ignored.

/* This is a multi-line comment.
   It can span as many lines as you like,
   and ends at the closing marker. */

int score = 0;   // comments can also sit at the end of a line of code

The Anatomy of a C++ Program

Almost every C++ program shares the same skeleton. Here is the classic first program, and below it a breakdown of every piece:

#include <iostream>   // 1. bring in the input/output library

int main() {            // 2. the program's entry point
    std::cout << "Hello, World!" << std::endl;   // 3. a statement
    return 0;           // 4. report success to the operating system
}

1. #include <iostream> — a preprocessor directive that pulls in the standard input/output library so you can use std::cout and std::cin.
2. int main() — execution always begins here. The int means main hands an integer back to the operating system when it finishes.
3. Statements — the actual work, written between the { } braces. Each statement ends with a semicolon.
4. return 0; — 0 conventionally means "finished successfully." Any non-zero value signals an error.

Basic Syntax Rules

C++ is strict about a few mechanical rules. Learn these four now and you'll avoid the most common beginner compiler errors:

⚠️ Watch out: because whitespace is ignored, the compiler won't complain about messy formatting — but a human reviewer will. Indent the code inside every pair of braces; it makes the structure obvious at a glance.

Variables & Types

A variable is a named container for a value. In C++ you must state the type of data it holds when you declare it, and the type never changes afterward.

int age = 25;              // whole number
double price = 19.99;      // number with a decimal point
char initial = 'R';        // a single character, in single quotes
bool isActive = true;      // true or false
std::string name = "Ray";  // text, in double quotes (needs #include <string>)

Here are the fundamental types you'll use constantly:

Declaration vs. Initialization

Declaring a variable creates it; initializing it gives it a starting value. Always initialize — an uninitialized variable holds an unpredictable "garbage" value.

int count;        // declared but NOT initialized — value is garbage
count = 0;        // now assigned

int score = 0;    // declared AND initialized in one step (preferred)
int lives{3};     // modern brace-initialization, also fine

Naming Rules

• Names may use letters, digits, and underscores, but cannot start with a digit.
• They are case-sensitive and cannot be a reserved keyword (int, return, …).
• Choose descriptive names: studentCount beats sc, and both beat x.

Input & Output

Console programs talk to the user through two objects from <iostream>: std::cout (character out) prints, and std::cin (character in) reads.

std::cout << "Enter your name: ";   // << sends text TO the console
std::string userName;
std::cin  >> userName;                // >> reads input INTO the variable
std::cout << "Hello, " << userName << "!" << std::endl;

Two things worth noticing:

• Chaining. You can string several << together in one statement, as in the last line above, to print pieces one after another.
• Direction of the arrows. << points toward cout (data flows out); >> points away from cin (data flows into your variable).

Reading numbers works the same way — std::cin converts the typed text into the variable's type:

std::cout << "Enter your age: ";
int age;
std::cin >> age;
std::cout << "Next year you'll be " << age + 1 << ".\n";

Putting It All Together

Here's a complete program that uses everything from this lesson — comments, the program skeleton, variables, and input/output:

#include <iostream>
#include <string>

int main() {
    // Greet the user and collect some information
    std::string name;
    int birthYear;

    std::cout << "What's your name? ";
    std::cin  >> name;

    std::cout << "What year were you born? ";
    std::cin  >> birthYear;

    const int CURRENT_YEAR = 2026;
    int age = CURRENT_YEAR - birthYear;   // a little arithmetic

    std::cout << "Nice to meet you, " << name << "!\n";
    std::cout << "You are about " << age << " years old.\n";

    return 0;
}

Read it top to bottom: it includes the libraries it needs, enters main(), declares variables, prompts and reads input, computes a value, prints the result, and returns 0. That shape will feel familiar within a week.

Practice Exercise: Interactive Calculator

#include <iostream>

int main() {
    double a, b;

    // TODO: prompt for and read the first number into a
    // TODO: prompt for and read the second number into b
    // TODO: print the sum, difference, product, and quotient

    return 0;
}

#include <iostream>

int main() {
    double a, b;

    // Ask for the two numbers
    std::cout << "Enter the first number: ";
    std::cin  >> a;

    std::cout << "Enter the second number: ";
    std::cin  >> b;

    // Print every operation, each with a label
    std::cout << "Sum:        " << a + b << "\n";
    std::cout << "Difference: " << a - b << "\n";
    std::cout << "Product:    " << a * b << "\n";
    std::cout << "Quotient:   " << a / b << "\n";

    return 0;
}

#include <iostream>

int main() {
    double a, b;
    char op;

    std::cout << "Enter the first number: ";
    std::cin  >> a;

    std::cout << "Enter an operator (+ - * /): ";
    std::cin  >> op;

    std::cout << "Enter the second number: ";
    std::cin  >> b;

    // Pick the calculation based on the operator the user typed
    if (op == '+') {
        std::cout << "Result: " << a + b << "\n";
    } else if (op == '-') {
        std::cout << "Result: " << a - b << "\n";
    } else if (op == '*') {
        std::cout << "Result: " << a * b << "\n";
    } else if (op == '/') {
        if (b != 0) {
            std::cout << "Result: " << a / b << "\n";
        } else {
            std::cout << "Error: cannot divide by zero.\n";
        }
    } else {
        std::cout << "Unknown operator: " << op << "\n";
    }

    return 0;
}

🎯 Quick Quiz

Summary

📚 Additional Resources

• cppreference — Comments
• cppreference — Basic concepts (program structure)
• LearnCpp — Introduction to iostream: cout, cin, and endl

🚀 What's Next?

You can now write, comment, and structure a basic C++ program, store values in variables, and get data in and out. Next, in Lesson 3: Data Types & Operators, you'll go deeper into the type system you just met — numeric types in detail, type conversion, and the arithmetic, comparison, and logical operators that let your programs actually compute.