Variables and Types

To solve problems, we need to store and work with data like numbers, letters, or words. We store data in variables.

Example

#include <iostream>
using namespace std;

int main() {
    int age = 15;
    double height = 1.70;
    char grade = 'A';
    string name = "Ali";

    cout << name << " is " << age << " years old.";
    return 0;
}

Output:

Ali is 15 years old.

Explanation

• int → whole numbers (like 3, -7, 2025)
• double → numbers with decimals (like 3.14 or -0.5)
• char → a single letter or symbol (like 'A', 'z', '?')
• string → text or sequence of characters (like "Hello")

More number types

long long

When numbers get too big for an int (for example, when you count up to billions or trillions), we use long long.

long long population = 8000000000; // 8 billion

Why? Because an int can usually store values up to about 2 billion (2,147,483,647). If we need larger numbers, we must use long long.

float and double

Both store *decimal numbers, but there is a difference in precision** (how many digits they can store correctly):

Type	Precision	Example
float	about 6–7 digits	3.141593
double	about 15–16 digits	3.141592653589793

    float f = 1.61;
    double d = 1.61;

    cout << setprecision(20); // show 20 decimal places

    cout << f << endl; // prints 1.6100000143051147461
    cout << d << endl; // prints 1.6100000000000000977

Caution

As you can see in the example, most real numbers cannot be stored exactly using floats or doubles. This is something you need to keep in mind whenever you work with these types.

Most of the time, programmers use double because it is more precise. However, double uses 8 bytes of memory, while float uses only 4. Use float only if you really need to save memory.

bool

A bool variable can take only two different values: true or false.

bool isEven = true;
bool isPrime = false;

You’ll often use it for conditions and logic in if statements:

int x = 7;
bool big = x > 10;
cout << big; // prints 0 (false)

true is represented internally as 1, and false as 0.

Example: Input and Output with Variables

int x, y;
cin >> x >> y;
cout << x + y;

Input:

4 9

Output:

13

That’s how many simple problems on platforms like Codeforces or AtCoder look!

Type summary table

Type	Meaning	Example	Memory (approx.)
int	whole number	5, -10	4 bytes
long long	large whole number	1000000000000	8 bytes
float	decimal (low precision)	3.14	4 bytes
double	decimal (high precision)	3.1415926535	8 bytes
bool	true/false	true	1 byte
char	one character	'A'	1 byte
string	text	"Hello"	depends on length

Type conversion

Sometimes we mix types:

int a = 3;
double b = 2.5;
cout << a + b; // 5.5

C++ automatically converts a to a double temporarily, so the sum is also a double.

Optional: auto

auto lets the computer guess the type for you:

auto x = 5;        // int
auto y = 3.14;     // double
auto word = "Hi";  // const char*

You’ll mostly use it in more advanced problems later.

Naming Variables

When you create variables in C++, you must choose a name for each one. This name tells the program (and humans!) what the variable represents.

Good names make your code easier to read and understand.

Rules for naming variables

Here are the basic rules you must follow:

You can: - Use letters, digits, and underscores (_) - Start with a letter or an underscore - Use both uppercase and lowercase letters (C++ is case-sensitive)

You cannot: - Start the name with a number - Use spaces - Use special symbols like @, $, %, !, #, ?, etc. - Use C++ keywords (like int, for, return, while, etc.)

Examples

Correct names:

int age;
double total_price;
bool isValid;
string firstName;

Incorrect names:

int 2cats;       // starts with a number
int my variable; // spaces not allowed
int total$;      // special character
int int;         // keyword

Case sensitivity

C++ treats uppercase and lowercase letters as different characters:

int score = 10;
int Score = 20;

cout << score; // prints 10

Here, score and Score are two different variables.

Naming style tips

There are different naming styles, but most C++ programmers use one of these:

Style	Example	Common use
snake_case	player_health	variables and functions in simple programs
camelCase	playerHealth	used often in larger projects or libraries
PascalCase	PlayerHealth	used for classes or structs

Pick one style and be consistent throughout your code.

Example

#include <iostream>
using namespace std;

int main() {
    int playerHealth = 100;
    int playerScore = 2500;
    bool isAlive = true;

    cout << "Health: " << playerHealth << endl;
    cout << "Score: " << playerScore << endl;
    cout << "Alive: " << isAlive << endl;

    return 0;
}

Output:

Health: 100
Score: 2500
Alive: 1

Quick summary

Rule	Allowed	Example
Start with letter or underscore	YES	_count, sum
Start with number	NO	9lives
Use space	NO	total price
Use underscore	YES	total_price
Use symbol ($, #, @, etc.)	NO	total$
Use C++ keyword	NO	int, while

Good variable names make your code cleaner and easier to debug!