Variables and Data Types

Why this matters

In Introduction to Programming, you learned that a program is a set of ordered instructions. In Setting up the Development Environment, you wrote, compiled, and ran C++ code.

Now we need programs that can remember information while they run.

That is what variables do.

A variable is like a labeled container in your program:

• a basketball score
• the voltage in an electronics project
• the cost of paint for a home renovation
• the tempo of a song

A data type tells C++ what kind of value the container is allowed to hold.

Declaring a variable

In C++, you create a variable by writing:

type name = value;

For example:

• int points = 21; stores a whole number
• double voltage = 3.7; stores a decimal number
• bool isPlaying = true; stores true or false

C++ wants to know the type early so it can reserve the right kind of space and catch mistakes before your program runs.

```cpp
#include <iostream>

int main() {
    int points = 21;
    double voltage = 3.7;
    bool gameStarted = true;

    std::cout << "Points: " << points << std::endl;
    std::cout << "Voltage: " << voltage << std::endl;
    std::cout << "Game started: " << gameStarted << std::endl;

    return 0;
}
```

Which line correctly creates a C++ variable named paintCans that stores the whole number 4?

Types are categories of values

Here are several beginner-friendly C++ types:

Type Stores Example int whole numbers 42, -7 double decimal numbers 3.14, 98.6 char one character 'A', 'x' bool true or false true, false

Notice that a char uses single quotes: 'A'.

Later, in Arrays and Strings, you will work more deeply with text. For now, focus on these basic building blocks.

You are storing the battery level of a device as 87.5 percent. Which type is the best fit?

Variables can vary

The word variable means the value can change while the program runs.

Think of a basketball score: it starts at 0, then changes as points are added.

You can assign a new value using =. In C++, = means store the value on the right into the variable on the left.

This is not the same as mathematical equality. It is an instruction.

```cpp
#include <iostream>

int main() {
    int homeScore = 0;

    std::cout << "Starting score: " << homeScore << std::endl;

    homeScore = 2;
    std::cout << "After a basket: " << homeScore << std::endl;

    homeScore = homeScore + 3;
    std::cout << "After a three-pointer: " << homeScore << std::endl;

    return 0;
}
```

True or false: In C++, the line score = score + 1; means take the current value of score, add 1, and store the result back into score.

Good names make programs readable

C++ variable names should clearly describe what they store.

Good examples:

• paintGallons
• songTempo
• playerScore
• circuitVoltage

Avoid names like:

• x when the meaning is not obvious
• thing
• number1

Basic naming rules:

• names can contain letters, digits, and underscores
• names cannot start with a digit
• names cannot contain spaces
• names are case-sensitive: score and Score are different

Fill in the blank to create a variable named songTempo that stores the whole number 120.

_____ songTempo = 120;

Some values should not change

Sometimes you want a named value, but you do not want it to vary.

For example, if you are estimating renovation costs, the number of square feet in a room might be fixed for that calculation.

C++ lets you use const to say: this variable must not be changed after it is created.

This helps prevent accidental mistakes.

```cpp
#include <iostream>

int main() {
    const double roomWidth = 12.5;
    const double roomLength = 10.0;
    double area = roomWidth * roomLength;

    std::cout << "Room width: " << roomWidth << std::endl;
    std::cout << "Room length: " << roomLength << std::endl;
    std::cout << "Area: " << area << " square feet" << std::endl;

    return 0;
}
```

What you can do now

You now know how to create named pieces of memory in C++.

Key ideas:

• a variable stores a value
• a data type tells C++ what kind of value is stored
• common types include int, double, char, and bool
• = assigns a value
• variable names should explain their purpose
• const protects values that should not change

Next, in Input and Output, you will learn how programs can communicate more directly with users instead of only printing fixed messages.