Chapter 1a - Variables & Printing

Your First C++ Program

Every C++ program needs two things: the iostream header (for input/output) and a main() function.

#include <iostream>
using namespace std;

int main() {
    cout << "Hello, World!" << endl;
    return 0;
}

Part	What it does
#include <iostream>	Loads the input/output library
using namespace std;	Lets you write cout instead of std::cout
int main()	Entry point — where the program starts
cout <<	Prints to the screen
endl	Ends the line (like pressing Enter)
return 0;	Tells the OS the program finished successfully

🧪 Try it yourself

#include <iostream>
using namespace std;

int main() {
    cout << "Hello, World!" << endl;
    cout << "My name is C++!" << endl;
    return 0;
}

Challenge: Change the messages to print your own name and a fun fact about yourself.

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Variables and Data Types

A variable stores a value in memory. In C++ you must declare the type of data before using a variable.

int age = 17;
double price = 9.99;
string name = "Alice";
bool isStudent = true;
char grade = 'A';

Common Data Types

Type	What it stores	Example
int	Whole numbers	42, -5, 0
double	Decimal numbers	3.14, -0.5
string	Text (requires <string> or <iostream>)	"Hello"
bool	True or false	true, false
char	A single character	'A', '9'

🧪 Variables in action

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

int main() {
    int age = 17;
    double gpa = 3.85;
    string name = "Alice";
    bool isStudent = true;

    cout << "Name: " << name << endl;
    cout << "Age: " << age << endl;
    cout << "GPA: " << gpa << endl;
    cout << "Student: " << isStudent << endl;

    return 0;
}

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Printing with cout

Use << to chain multiple values in one cout statement.

string name = "Alice";
int age = 17;

cout << "My name is " << name << " and I am " << age << " years old." << endl;

My name is Alice and I am 17 years old.

You can also use "\n" instead of endl — it is faster for large outputs.

cout << "Line 1\n";
cout << "Line 2\n";

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Arithmetic Operators

int a = 10;
int b = 3;

cout << a + b << endl;   // 13  — addition
cout << a - b << endl;   // 7   — subtraction
cout << a * b << endl;   // 30  — multiplication
cout << a / b << endl;   // 3   — integer division (truncates)
cout << a % b << endl;   // 1   — modulo (remainder)

Integer Division

10 / 3 in C++ gives 3, not 3.33... because both operands are int. Use double if you need decimals:

double result = 10.0 / 3;   // 3.333...

🧪 Arithmetic practice

#include <iostream>
using namespace std;

int main() {
    int apples = 24;
    int friends = 5;

    int each = apples / friends;
    int leftover = apples % friends;

    cout << "Each friend gets " << each << " apples." << endl;
    cout << "There are " << leftover << " apples left over." << endl;

    return 0;
}

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User Input with cin

cin >> reads input from the keyboard and stores it in a variable.

#include <iostream>
using namespace std;

int main() {
    int age;
    cout << "Enter your age: ";
    cin >> age;
    cout << "You are " << age << " years old." << endl;
    return 0;
}

Reading strings with spaces

cin >> stops at a space. Use getline(cin, variable) to read a full line:

string fullName;
cout << "Enter your full name: ";
getline(cin, fullName);
cout << "Hello, " << fullName << "!" << endl;

🧪 Input example

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

int main() {
    string name;
    int birthYear;
    int currentYear = 2025;

    cout << "What is your name? ";
    getline(cin, name);

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

    int age = currentYear - birthYear;
    cout << "Hello, " << name << "! You are approximately " << age << " years old." << endl;

    return 0;
}

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Exercises

Activity: Temperature Converter

Write a C++ program that asks the user for a temperature in Celsius and prints the Fahrenheit equivalent.

Formula: F = (C × 9/5) + 32

Expected output:

Enter temperature in Celsius: 100
100 Celsius = 212 Fahrenheit

💡 Hint

Use double for the temperature variables so you get decimal precision. Read the Celsius value with cin, compute the formula, then print with cout.

📒 Solution

#include <iostream>
using namespace std;

int main() {
    double celsius;
    cout << "Enter temperature in Celsius: ";
    cin >> celsius;

    double fahrenheit = (celsius * 9.0 / 5.0) + 32;
    cout << celsius << " Celsius = " << fahrenheit << " Fahrenheit" << endl;

    return 0;
}

Activity: Area Calculator

Write a program that reads the length and width of a rectangle from the user and prints the area and perimeter.

Expected output:

Enter length: 5
Enter width: 3
Area: 15
Perimeter: 16

📒 Solution

#include <iostream>
using namespace std;

int main() {
    double length, width;

    cout << "Enter length: ";
    cin >> length;

    cout << "Enter width: ";
    cin >> width;

    cout << "Area: " << length * width << endl;
    cout << "Perimeter: " << 2 * (length + width) << endl;

    return 0;
}