In C++, variables are used to store data that can be manipulated throughout the program. Each variable is associated with a data type that determines the kind of values it can store and how much memory is allocated for that variable.

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Declaring Variables

In C++, you declare a variable by specifying its data type followed by the variable name.

Syntax:

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data_type variable_name;

Example:

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int age;   // Declares an integer variable called age

Variables can also be initialized (assigned a value) when they are declared:

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int age = 25;   // Declares and initializes age with a value of 25

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Basic Data Types

1. Integer Types:

   • Stores whole numbers (both positive and negative).
   • Example: int, short, long, long long

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   int number = 10;       // Integer variable
   long bigNumber = 1000; // Long integer variable
   

2. Floating-Point Types:

   • Used to store decimal numbers.
   • Example: float, double

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   float temperature = 36.5f;  // Floating-point variable
   double pi = 3.14159;        // Double-precision floating-point variable
   

3. Character Type:

   • Used to store a single character.
   • Example: char

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   char grade = 'A';  // Character variable
   

4. Boolean Type:

   • Stores true or false.
   • Example: bool

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   bool isAdult = true;  // Boolean variable
   

5. String Type:

   • Stores a sequence of characters. Although not a built-in type, strings are commonly used via the std::string class.

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   std::string name = "John";  // String variable
   

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Variable Naming Rules

1. Names must begin with a letter or an underscore.

   • Example: int age;, int _number;

2. Subsequent characters can be letters, digits, or underscores.

   • Example: int count_123;

3. Case-sensitive:

   • Example: int Age; and int age; are two different variables.

4. Cannot use reserved keywords (such as int, class, return).

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Variable Scope

• Local Variables: Declared inside a function or block. Only accessible within that function or block.

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  void example() {
      int x = 10;  // Local variable
      std::cout << x;
  }
  

• Global Variables: Declared outside of all functions. Accessible by any function within the program.

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  int x = 10;  // Global variable
  
  void example() {
      std::cout << x;  // Accessing the global variable
  }
  

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Variable Initialization

Variables can be initialized in multiple ways:

1. Direct Initialization:

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   int x = 5;
   

2. Brace Initialization (C++11):

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   int x{5};
   

3. Copy Initialization:

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   int x(5);
   

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Constant Variables

Use the const keyword to declare constant variables, whose values cannot be changed after initialization.

Example:

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const float PI = 3.14159;  // Constant variable
PI = 3.14;  // Error: Cannot modify a constant variable

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Example Code Using Variables

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#include 
#include 

int main() {
    int age = 25;                // Integer variable
    float height = 5.9f;         // Floating-point variable
    char grade = 'A';            // Character variable
    bool isStudent = true;       // Boolean variable
    std::string name = "John";   // String variable

    // Output the values of the variables
    std::cout << "Name: " << name << std::endl;
    std::cout << "Age: " << age << std::endl;
    std::cout << "Height: " << height << std::endl;
    std::cout << "Grade: " << grade << std::endl;
    std::cout << "Is Student: " << isStudent << std::endl;

    return 0;
}

Output:

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Name: John
Age: 25
Height: 5.9
Grade: A
Is Student: 1

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Summary

• Variables store data that can be used and modified in the program.
• They have a data type (like int, float, char, bool) which defines the kind of data they hold.
• Variables can be local (within a function) or global (outside all functions).
• Use the const keyword to declare variables that should not change during program execution.
• Proper variable naming and initialization are crucial for readability and avoiding bugs.