Variables: Boxes for Your Data

What is a variable?

A variable is a named place in the computer's memory where you keep a value, so you can use it later.

Think of it like this: A variable is like a labelled box. The label is the name (age), and inside the box is the value (25). In C++ every box also has a type: a box made for whole numbers can only hold whole numbers.

Creating a variable

To create a variable you write its type, its name, and (usually) a starting value:

#include <iostream>
using namespace std;

int main() {
    int age = 25;
    cout << "My age is " << age << "\n";
    return 0;
}

Output

My age is 25

• int is the type: a whole number (integer).
• age is the name we chose.
• = 25 puts the value 25 into the box.
• When we print age (no quotes!), C++ prints the value inside: 25.

Changing a variable

A variable can change – that's why it is called variable. Use = again to put a new value in. The old value is thrown away:

#include <iostream>
using namespace std;

int main() {
    int score = 10;
    cout << "Score: " << score << "\n";

    score = 50;               // no "int" this time: the box already exists
    cout << "Score: " << score << "\n";

    score = score + 5;        // take the old value, add 5, store it back
    cout << "Score: " << score << "\n";
    return 0;
}

Output

Score: 10
Score: 50
Score: 55

Remember: You write the type (int) only once, when you create the variable. After that you just use its name.

Many variables

#include <iostream>
using namespace std;

int main() {
    int apples = 3;
    int oranges = 5;
    int total = apples + oranges;
    cout << "Fruit in the bowl: " << total << "\n";
    return 0;
}

Output

Fruit in the bowl: 8

Common mistake: Using a variable before giving it a value. int x; cout << x; prints a random leftover number from memory. Always give your variables a starting value.