---
title: "3. C Fundamentals"
description: 'Variables, constants, data types, and scope -- the building blocks of every C program.'
---
## Variables and scope

- Declaration: `int x;`
- Initialization: `int y = 10;`
- Scope: block (local), file (static), global

## Constants

- Literals: `42`, `3.14`, `'a'`, `"hello"`
- `#define PI 3.14159`
- `const int DAYS = 7;`

## Data types

- Primary: `char`, `int`, `float`, `double`, `void`
- Qualifiers: `short`, `long`, `signed`, `unsigned`
- Derived: arrays, pointers, functions
- User-defined: `struct`, `union`, `enum`

## Examples

### Swap Two Numbers (`swap_temp.c`)

Demonstrates variable declaration, initialization, and assignment:

```c
#include <stdio.h>

int main(void) {
    int a, b, temp;
    printf("Enter two integers (a b): ");
    scanf("%d %d", &a, &b);

    temp = a;
    a = b;
    b = temp;

    printf("After swap: a=%d, b=%d\n", a, b);
    return 0;
}
```

### Add Two Numbers (`add_two_numbers.c`)

Shows basic data types (`int`), function return values, and formatted output:

```c
#include <stdio.h>

int add(int a, int b) { return a + b; }

int main(void) {
    printf("add(3,4) = %d\n", add(3,4));
    return 0;
}
```