3. C Fundamentals

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:

#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:

#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;
}