Pointers

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main() {
    // Pointer basics
    int x = 42;
    int *ptr = &x;  // ptr stores address of x

    printf("Value: %d\n", x);       // 42
    printf("Address: %p\n", &x);     // 0x7ffee...
    printf("Pointer: %p\n", ptr);    // Same address
    printf("Dereferenced: %d\n", *ptr); // 42

    *ptr = 100;  // Modify x through the pointer
    printf("x is now: %d\n", x);  // 100

    // Pointer arithmetic
    int arr[] = {10, 20, 30, 40, 50};
    int *p = arr;  // Array name IS a pointer to first element

    printf("%d\n", *p);       // 10 (first element)
    printf("%d\n", *(p + 1)); // 20 (second element)
    printf("%d\n", *(p + 4)); // 50 (fifth element)
    // p + 1 advances by sizeof(int) bytes, not 1 byte!

    // Dynamic memory allocation
    int *dynamicArr = (int *)malloc(5 * sizeof(int));
    if (dynamicArr == NULL) {
        printf("Memory allocation failed!\n");
        return 1;
    }

    // Use the memory
    for (int i = 0; i < 5; i++) {
        dynamicArr[i] = i * 10;
    }

    // Resize with realloc
    dynamicArr = (int *)realloc(dynamicArr, 10 * sizeof(int));

    // ALWAYS free when done
    free(dynamicArr);
    dynamicArr = NULL;  // Prevent dangling pointer

    // calloc: allocate + zero-initialize
    int *zeros = (int *)calloc(100, sizeof(int));
    // All 100 ints are initialized to 0
    free(zeros);

    // Double pointer (pointer to pointer)
    int val = 5;
    int *pVal = &val;
    int **ppVal = &pVal;
    printf("Value: %d\n", **ppVal);  // 5

    return 0;
}