Lab 11 – Active Pointers & Memory

PSUC Lab 11 — Memory Addresses & Pointers

Active Logic: Pointers & Memory Dereferencing

Master C pointer variables: address-of operator (&var), indirection/dereference operator (*ptr), pointer arithmetic, pointer array traversal, and substring pointer search (strstr).

Pointer Rules

Concept	Syntax / Rule	Key Trap / Pitfall
Address Operator	`&var` (gives memory address)	Do not dereference uninitialized NULL pointers!
Dereference Operator	`*ptr` (fetches stored value)	`*ptr` gets value; `ptr` gets memory address
Pointer Arithmetic	`ptr++` (advances by `sizeof(type)`)	`(int*)ptr + 1` adds 4 bytes to memory address!
Array Equivalence	`arr[i] == *(arr + i)`	Array name acts as a constant pointer to 0th element

🎯 Learning Philosophy: Active Logic Building

Test your pointer logic across 3 Interactive Learning Modes:

🧠 Tab 1 (Logic & Hints): Memory layout & pointer addresses.
🧩 Tab 2 (Logic Puzzle): Complete missing dereferences *ptr & test live address visualizers!
💻 Tab 3 (C Solution): Reference verified C code solutions.

Solve pointer logic puzzles before peeking at C reference code!

Exercise 1 of 5

Access & Add Two Integers Using Pointers

Stores addresses of two integers in pointer variables p1 and p2 and computes sum via dereferencing (*p1 + *p2).

Address & Dereference Syntax

Address assignment: int *p1 = &num1;
Dereference addition: sum = *p1 + *p2;

Interactive Logic Hints

💡 Hint 1: What does %p print in printf()? ▼

%p prints the hexadecimal memory address stored in the pointer variable!

🧩 Complete Pointer Dereference Logic

int num1 = 15, num2 = 25;
int *p1 = num1;
int *p2 = &num2;
int sum = ;

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📍 Pointer Address Simulator

num1:

num2:

Memory addresses & dereferenced sum...

add_pointers.cReference C Code

#include <stdio.h>

int main() {
    int num1 = 15, num2 = 25, sum;
    int *p1 = &num1;
    int *p2 = &num2;

    sum = *p1 + *p2;

    printf("p1 = %p, *p1 = %d\n", (void*)p1, *p1);
    printf("p2 = %p, *p2 = %d\n", (void*)p2, *p2);
    printf("Sum (*p1 + *p2) = %d\n", sum);
    return 0;
}

Exercise 2 of 5

Greatest & Smallest Among Three Numbers

Compares three numbers using pointers *pa, *pb, and *pc.

Pointer Comparison Rules

Compare values: if (*pa >= *pb && *pa >= *pc) max = *pa;
Store pointer to max: int *pMax = pa;

Interactive Logic Hints

💡 Hint 1: What is the difference between comparing pa vs *pa? ▼

pa > pb compares memory addresses! *pa > *pb compares the actual numbers stored at those addresses! Always use *!

🧩 Complete Pointer Min/Max Comparison

int *pMax = pa;
if ( > *pMax) pMax = pb;
if (*pc > ) pMax = pc;
printf("Max = %d", );

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📊 Pointer Min/Max Comparator

a:

b:

c:

Min/Max pointer result...

pointer_min_max.cReference C Code

#include <stdio.h>

int main() {
    int a = 45, b = 89, c = 23;
    int *pa = &a, *pb = &b, *pc = &c;

    int *pMax = pa;
    if (*pb > *pMax) pMax = pb;
    if (*pc > *pMax) pMax = pc;

    int *pMin = pa;
    if (*pb < *pMin) pMin = pb;
    if (*pc < *pMin) pMin = pc;

    printf("Greatest = %d, Smallest = %d\n", *pMax, *pMin);
    return 0;
}

Exercise 3 of 5

Determine String Length Using Pointer

Calculates string length by advancing a character pointer ptr++ until reaching null terminator '\0'.

Pointer Subtraction Length Formula

Initialize: char *ptr = str;
Advance: while (*ptr != '\0') ptr++;
Length: length = ptr - str; (Pointer subtraction!)

Interactive Logic Hints

💡 Hint 1: How does ptr - str calculate length? ▼

Subtracting base pointer str from end pointer ptr gives the exact number of elements between them!

🧩 Complete Pointer Strlen Loop

char *ptr = str;
while ( != '\0') {
;
}
int len = ptr str;

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📏 Pointer Strlen Engine

String:

Length status...

ptr_strlen.cReference C Code

#include <stdio.h>

int main() {
    char str[] = "Pointer Memory Traversal";
    char *ptr = str;

    while (*ptr != '\0') {
        ptr++;
    }

    printf("Length of '%s' = %ld\n", str, ptr - str);
    return 0;
}

Exercise 4 of 5

Compute Sum of Array Elements Using Pointer

Traverses an array using pointer arithmetic (*(ptr + i)) to calculate total sum.

Array Pointer Traversal

int *ptr = arr;
Accumulation: sum += *(ptr + i) or sum += *ptr; ptr++;

Interactive Logic Hints

💡 Hint 1: What is the difference between *ptr++ vs (*ptr)++? ▼

*ptr++ advances pointer to NEXT memory location! (*ptr)++ increments value in current memory location!

🧩 Complete Pointer Array Sum Loop

int *ptr = arr, sum = 0;
for (i = 0; i < n; i++) {
sum += ;
}

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➕ Pointer Array Sum Visualizer

Array Elements (comma separated):

Sum result...

ptr_array_sum.cReference C Code

#include <stdio.h>

int main() {
    int arr[] = {10, 20, 30, 40, 50};
    int n = 5, i, sum = 0;
    int *ptr = arr;

    for (i = 0; i < n; i++) {
        sum += *(ptr + i);
    }

    printf("Sum of array elements = %d\n", sum);
    return 0;
}

Exercise 5 of 5

Substring Pointer Search (Returning Pointer to Match)

Searches a substring pattern in text and returns a pointer pointing to the first character of occurrence.

Pointer strstr() Function

Function: char *ptr = strstr(str, sub);
Returns NULL if substring not found!
Index position = ptr - str

Interactive Logic Hints

💡 Hint 1: What does strstr() return on match? ▼

It returns a pointer pointing to the exact memory address where the substring starts!

🧩 Complete Substring Pointer Function

char *match = (str, sub);
if (match NULL) {
int pos = match - str;
printf("Found at index %d", pos);
}

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🔎 Substring Pointer Match Visualizer

Main Text:

Substring Pattern:

Substring match result...

ptr_substring.cReference C Code

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

int main() {
    char str[] = "learning c pointers is fun";
    char sub[] = "pointers";

    char *match = strstr(str, sub);

    if (match != NULL) {
        printf("Substring found at index %ld: '%s'\n", match - str, match);
    } else {
        printf("Substring NOT found.\n");
    }
    return 0;
}

Knowledge Check

Test Your Pointer & Memory Mastery

Question 1 of 5

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