## Chapter 2: Variables and Data Types (Variables and Data Types in C++) The first practical thing we learn in programming is how to store data. In C++, this is done using Variables. ### What is a variable? (Simple Language) Think of a variable like this: A labelled box - Each box has a name - Each box can hold a specific type of data - The data inside the box can be changed Examples: - A box to hold age - Another box to hold a name - A separate box to hold a price ### Why are variables necessary? Because in a program: - Every value can change repeatedly - Every value has a different meaning Without variables: - the program would be hard - confusing - and useless ### Basic Data Types Now let's look at some of the most common data types in C++ and understand each one in simple terms. Example Code: Basic Data Types C++ ``` #include <iostream> #include <string> using namespace std; int main() { int years = 22; // whole number float cost = 149.75; // decimal (less precise) double gravity = 9.80665; // decimal (more precise) char section = 'B'; // a single character bool passed = true; // true or false string studentName = "Amit"; // text cout << years << endl; cout << cost << endl; cout << gravity << endl; cout << section << endl; cout << passed << endl; cout << studentName << endl; return 0; } ``` Now let's understand each data type separately **Integer (int):** int is used to store whole numbers. - It can hold values ​​from approximately -2 billion to +2 billion - It does not have decimals - Best for counting purposes Where is it used? - Number of students - Days, months - Count of items Example C++ ``` int numberOfApples = 10; ``` Here: - A variable named `numberOfApples` - It stores the value 10 **Floating-point types (float and double):** When we need to store numbers with decimal points, we use `float` or `double`. **float** - For decimal numbers - Less precise - Up to approximately 7 decimal digits - Uses less memory C++ ``` float temperature = 36.5; ``` **double** - More accurate than float - Up to approximately 15 decimal digits - Used more often in real-world programs C++ ``` double accountBalance = 1543.75; ``` Rule of thumb: If in doubt → use `double` **Character (char):** `char` stores only a single character or symbol. - Always written in single quotes (' ') - Internally, each character is stored as a number (ASCII value) C++ ``` char firstInitial = 'J'; ``` Use cases: - Grade (A, B, C) - Section - Single symbol **Boolean (bool):** `bool` can only hold two values: - true - false That is: Yes / No True / False On / Off C++ ``` bool isRaining = false; ``` Use cases: - Condition checking - Decision making - Loops and if-else statements **String (string):** `string` is used to store text. - Holds multiple characters together - Always in double quotes (" ") - It is a class from the C++ standard library For this: C++ ``` #include <string> ``` is necessary. C++ ``` string message = "Welcome to C++"; ``` Use cases: - Names - Cities - Messages ### Variable Naming Rules: There are some rules for naming variables in C++. Correct Rules Must start with a letter (a-z, A-Z) or an underscore (_). In the middle: - Letters - Numbers - Underscores are allowed. - Cannot use C++ keywords. - Case-sensitive. `age` and `Age` are different variables. **Good Variable Names** C++ ``` int studentAge; double accountBalance; string firstName; ``` - Easy to understand at a glance. - Professional code. **Bad (but legal) Names** C++ ``` int x; // meaning not clear int a1b2c3; // confusing ``` The code will run, but it will be difficult to understand. **Illegal Variable Names** C++ ``` int 2fast; // cannot start with a number int my-name; // hyphen not allowed int class; // is a keyword ``` These will prevent the program from compiling. **Beginner Tip (Golden Rule)** Choose variable names that make their meaning clear without needing comments.