# 3. Variables and Data Types

### 3.1 Variable Declaration

**Python vs C:**

| Python | C |
|--------|---|
| `x = 5` | `int x = 5;` |
| `name = "John"` | `char name[] = "John";` |
| `pi = 3.14` | `float pi = 3.14f;` |

### 3.2 Basic Data Types

#### 3.2.1 Integer Types

| Type | Size (bytes) | Range | Usage |
|------|-------------|-------|--------|
| `char` | 1 | -128 to 127 | Small integers, characters |
| `short` | 2 | -32,768 to 32,767 | Small integers |
| `int` | 4 | -2,147,483,648 to 2,147,483,647 | Standard integers |
| `long` | 4/8 | System dependent | Large integers |
| `long long` | 8 | Very large range | Very large integers |

**Unsigned Variants:**
```c
unsigned char uc;    // 0 to 255
unsigned int ui;     // 0 to 4,294,967,295
unsigned short us;   // 0 to 65,535
```

#### 3.2.2 Floating-Point Types

| Type | Size | Precision | Range |
|------|------|-----------|-------|
| `float` | 4 bytes | ~7 digits | ±3.4 × 10^±38 |
| `double` | 8 bytes | ~15 digits | ±1.7 × 10^±308 |
| `long double` | 12/16 bytes | Extended precision | System dependent |

#### 3.2.3 Character and String Types

**Single Characters:**
```c
char letter = 'A';        // Single character
char digit = '5';         // Character representation of digit
char newline = '\n';      // Escape sequence
```

**Strings (Character Arrays):**
```c
char name[20] = "John";           // Fixed-size array
char message[] = "Hello World";   // Size determined by initializer
char buffer[100];                 // Uninitialized array
```

**Python vs C String Comparison:**
| Python | C |
|--------|---|
| `name = "John"` | `char name[] = "John";` |
| `len(name)` | `strlen(name)` |
| `name[0]` | `name[0]` |
| `name + " Doe"` | `strcat(name, " Doe");` |

### 3.3 Variable Declaration Rules

1. **Must declare before use** (unlike Python)
2. **Case-sensitive** (`age` ≠ `Age`)
3. **Cannot start with digits** (`2x` is invalid)
4. **Cannot use keywords** (`int`, `if`, `while`, etc.)
5. **Should use meaningful names** (`student_count` not `sc`)

### 3.4 Constants

```c
// Method 1: #define preprocessor directive
#define PI 3.14159
#define MAX_SIZE 100

// Method 2: const keyword
const int ARRAY_SIZE = 50;
const float GRAVITY = 9.81f;
```

**Python vs C Constants:**
| Python | C |
|--------|---|
| `PI = 3.14159` | `#define PI 3.14159` |
| `PI = 3.14159` | `const float PI = 3.14159f;` |