STEP 6 Review: Pointers, Structures, malloc, and Files

📍 Pointer basics

int x = 5;
int *p = &x;     // p holds x's address 
printf("%d\n", *p); // 5 (read the pointed-to value) 
*p = 100;          // x also becomes 100! 

• &x: x's address
• *p: the value pointed to by p (dereferencing)
• int *pin * is part of a declaration; *p = 5in * dereferences a pointer

🏢 Structures (struct)

struct Point {
    int x, y;
};

struct Point p = { 3, 4 };
printf("%d %d\n", p.x, p.y); // 3 4

struct Point *pp = &p;
printf("%d\n", pp->x);  // 3 (-> is shorthand for (*pp).x) 

• Related data is grouped together in a new type
• Member access: for a structure value use .; for a pointer use ->

💾 malloc / free: dynamic memory

#include <stdlib.h>

int *a = malloc(sizeof(int) * 10);
if (a == NULL) { /* Allocation failed */  }

a[0] = 42;
free(a);             // Release the memory! 
a = NULL;            // Help prevent accidental double-free through this variable 

• Allocate the amount needed at runtime (array length can be chosen at runtime)
• When finished, free the allocation (or memory can leak)
• On failure, NULL is returned

📁 File input and output

FILE *fp = fopen("data.txt", "r");
if (fp == NULL) { /* Could not open the file */  }

int n;
fscanf(fp, "%d", &n);
fclose(fp);          // Close the file! 

• mode: "r"(read) / "w"(write, truncating existing contents) / "a"(append)
• After fopen, always check for NULL ; when finished, call fclose

⚡ Bitwise operations

• Evenness test: (n & 1) == 0
• Multiply/divide by two: n << 1 / n >> 1
• Logical &&and bitwise &operators differ