Data Types and Variables

From program shape to program data

In Introduction to C++, you learned how a C++ program is compiled and why C++ programmers think carefully about performance.

Now we zoom into the smallest building blocks your programs manipulate: data types and variables.

A variable answers two questions:

• What kind of value is this? int, double, char, bool, etc.
• Where can I store and reuse it? a named place in memory

This matters whether you're counting users on a website, storing a stock price, tracking an AI model's accuracy, or representing a single character in a file.

Common primitive types

C++ gives you several built-in, primitive types. These are the backbone of larger structures you'll meet later.

Type Meaning Example value int whole number 42 double decimal number 3.14159 float smaller decimal number 0.75f char single character 'A' bool true/false value true

A variable declaration combines a type, a name, and often an initial value:

int users = 1200;

Read it as: “Create a variable named users that stores an integer.”

```cpp
#include <iostream>

int main() {
    int users = 1200;
    double modelAccuracy = 0.9275;
    char grade = 'A';
    bool marketOpen = true;

    std::cout << "Users: " << users << "\n";
    std::cout << "Model accuracy: " << modelAccuracy << "\n";
    std::cout << "Grade: " << grade << "\n";
    std::cout << "Market open? " << marketOpen << "\n";

    return 0;
}
```

True or false?

In C++, every variable has a type that the compiler knows before the program runs.

A variable is a typed label

A useful mental model:

name        type       value
---------   -------    --------
balance     double     1050.75
shares      int        20
symbol      char       'A'
isActive    bool       true

The name lets you refer to the value later.

The type tells C++:

• how much memory may be needed
• what kind of operations make sense
• how to interpret the raw bits stored there

This is one reason C++ can be fast: it does not need to guess what kind of data a variable holds while the program is running.

You need to store the number of completed coding challenges a learner has solved. Which type is the best everyday choice?

Give variables a starting value

In C++, a variable can be declared first, then assigned later—but when possible, prefer initializing it right away.

Common styles:

• int a = 10; assignment-style initialization
• int b(10); parentheses initialization
• int c{10}; brace initialization

Brace initialization is popular in modern C++ because it helps prevent accidental narrowing, such as trying to squeeze a decimal value into an integer.

Also notice const: it marks a variable as read-only after initialization. Use it when a value should not change, like a tax rate, physics constant, or configuration limit.

```cpp
#include <iostream>

int main() {
    int challengeCount{42};
    double price{99.95};
    const double taxRate{0.0825};

    std::cout << "Challenges solved: " << challengeCount << "\n";
    std::cout << "Price: $" << price << "\n";
    std::cout << "Tax rate: " << taxRate << "\n";

    return 0;
}
```

Fill in the blank with the best type name.

To store a value like 99.95, use a variable of type _____.

Types have sizes

C++ types occupy memory. The exact size can depend on your system and compiler, but some relationships are common:

• char is usually 1 byte
• int is commonly 4 bytes
• double is commonly 8 bytes
• bool is often 1 byte, though it represents only true or false

You can ask your compiler using sizeof.

For finance and AI work, precision matters. A double can represent many decimal values, but floating-point math is still approximate. Later, when programs become more complex, this becomes important for money, statistics, and model scores.

```cpp
#include <iostream>

int main() {
    std::cout << "sizeof(char): " << sizeof(char) << " byte(s)\n";
    std::cout << "sizeof(int): " << sizeof(int) << " byte(s)\n";
    std::cout << "sizeof(float): " << sizeof(float) << " byte(s)\n";
    std::cout << "sizeof(double): " << sizeof(double) << " byte(s)\n";
    std::cout << "sizeof(bool): " << sizeof(bool) << " byte(s)\n";

    return 0;
}
```

char vs std::string

A char stores one character:

• 'A'
• '7'
• '$'

Notice the single quotes.

For multiple characters, C++ commonly uses std::string. A string is not a primitive type, but it is so useful that you will use it constantly.

Use:

• char for one symbol
• std::string for names, messages, IDs, and labels

This distinction is similar to frontend work: a single character and a full text field are related, but not the same thing.

```cpp
#include <iostream>
#include <string>

int main() {
    char rating = 'A';
    std::string company = "AlgoDaily";
    std::string ticker = "AAPL";

    std::cout << "Rating: " << rating << "\n";
    std::cout << "Company: " << company << "\n";
    std::cout << "Ticker: " << ticker << "\n";

    return 0;
}
```

Which initialization style is especially helpful for catching accidental narrowing conversions?

Types can convert, but be careful

C++ can convert between some types automatically. Sometimes that is useful. Sometimes it hides a bug.

Example: storing a decimal value in an int drops the fractional part.

9.99  ->  9

That might be fine for counting whole items, but terrible for prices, probabilities, or measurements.

The takeaway: choose the type that matches the meaning of the data. In the next lesson, Operators, Conditions, and Loops, you'll use these typed values to compute results and control program flow.