CP2 · Arrays, Pointers, Recursion

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Lecture Reviewer

Grouped by lecture — Arrays · Pointers · Recursion

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Arrays

Arrays.pptx · Prepared by: Veronica Louise L. Piando

What is an Array?

An array is a collection of data elements that are all the same type, stored consecutively in memory. Arrays provide a way to name a collection and reference its individual elements — they behave like a list of variables with a uniform naming mechanism.

Examples: a collection of integers, doubles, or characters.

Array Declaration

Syntax: <type> <arrayName>[<dimension>];

<dimension> must be an integer constant or constant expression.

int score[5]; // array of 5 integers int A[10]; // array of 10 uninitialized integers char B[80]; // array of 80 characters double C[800]; // array of 800 doubles

Subscripting / Indexing

Individual variables that make up an array are called: indexed variables, subscripted variables, or elements.

The first element always has index 0. The last element has index dimension - 1.

int A[10]; A[3] = 1; // assign value 1 to index 3 int x = A[3]; // read element at index 3 int i=7, j=2, k=4; A[0] = 1; A[i] = 5; // A[7] = 5 A[j] = A[i] + 3; // A[2] = 8 A[j+1] = A[i] + A[0]; // A[3] = 6

Array Initialization

Fewer values than size? Remaining positions auto-set to zero (0).

Omit size? Compiler sets minimum size needed for initializers.

int c[] = {6, 12, 16}; // size = 3 automatically int d[5] = {1, 2}; // d[2], d[3], d[4] = 0

2-D Arrays

Declaration: int A[rows][cols]; — e.g., int A[3][10]; creates 30 integers.

Access with two subscripts: A[row][col]. Manipulated using nested for-loops.

char A[3][10]; A[1][2] = 'a'; // row 1, column 2 char c = A[1][2]; // c = 'a'

Searching Arrays

Method	Array Type	How it Works
Unordered Linear	Unsorted	Scans every element. Returns index or -1.
Ordered Linear	Sorted	Stops early if element exceeds target.
Binary Search	Sorted (required)	Checks middle; eliminates half each step.

// Binary Search Set lower = 0, upper = size-1 while (lower <= upper AND not found): m = (lower + upper) / 2 if A[m] == searchValue → FOUND at m if A[m] > searchValue → upper = m - 1 else → lower = m + 1

Binary search requires a sorted array.

Sorting Arrays

Four sorting methods: Selection Sort, Bubble Sort, Shell Sort, Quick Sort.

Selection Sort — repeatedly finds the largest element in the unsorted portion and swaps it to the end.

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Pointers

CC103_-Lecture_Pointers_.pptx · Prepared by: Veronica Louise L. Piando

Memory Addresses

Every variable has a memory address. The & operator yields the address. Print with %p.

char ch = 'A'; printf("%p", &ch); // prints address of ch

What is a Pointer?

A pointer is a variable that stores the memory address of another variable.

char* cPtr; // pointer to char int *numPtr; // pointer to int float * xPtr; // pointer to float

Always initialize pointers — an uninitialized pointer holds a garbage address.

int *numPtr = NULL; // safe initialization

4 Steps to Using Pointers

Step 1 — Declare the variable: int num;

Step 2 — Declare the pointer: int *numPtr = NULL;

Step 3 — Assign the address: numPtr = #

Step 4 — Dereference to use: *numPtr = 42;

int num; int *numPtr = NULL; numPtr = # *numPtr = 42; printf("%d", num); // prints 42

Pass by Value vs. Reference

❌ Pass by Value

Function receives a copy. Changes do NOT affect original.

✅ Pass by Reference

Function receives the address. Changes DO affect original.

// GOOD: swap by reference void swap(int* a, int* b) { int tmp = *a; *a = *b; *b = tmp; } swap(&x, &y); // x and y actually swapped

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Recursion

Lecture_5_-_Recursion.ppt · Prepared by: Joshua Martinez, PhD Candidate

What is a Recursive Function?

A recursive function is one that calls itself. Requires a base case to stop.

void message(int times) { if (times > 0) { printf("This is a recursive function.\n"); message(times - 1); } }

Without a base case → stack overflow / infinite recursion.

Depth of Recursion

Each call creates a new instance of local variables on the call stack.

For message(5): depth = 6 (calls for times = 5, 4, 3, 2, 1, 0).

Classic Example — Recursive Factorial

int factorial(int num) { if (num > 0) return num * factorial(num - 1); else return 1; // base case: 0! = 1 } // factorial(4) = 4 × 3 × 2 × 1 = 24

Direct vs. Indirect Recursion

Direct

Function calls itself directly. void f() { f(); }

Indirect

A calls B, B calls A back. void A(){B();} void B(){A();}

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Laboratory Exercises

Based on your submitted lab files · run & practice C code in-browser

🖥️ In-Browser C Code Runner

Each exercise below has a code editor where you can write and run actual C code. Type your solution, provide sample input, and click ▶ Run to execute. Use Hint if you're stuck, or Solution to see the answer. Your submitted lab files are shown at the bottom as reference.

🔹 Arrays Exercises

Arrays

A1 · Even & Odd Counter

Your submitted lab · count even and odd numbers in an array

Easy ▾

Task: Ask the user to enter 7 integers. Count how many are even and how many are odd, then print both counts.

This is your submitted Arrays.c — try modifying it below!

Sample Input

1 2 3 4 5 6 7

Expected Output

Even numbers: 3
Odd numbers: 4

#include <stdio.h>

int main() {
    const int size = 7;
    int num[size];
    int i, odd = 0, even = 0;

    printf("Enter 7 integers:\n");

    for (i = 0; i < size; i++) {
        scanf("%d", &num[i]);

        if (num[i] % 2 == 0) {
            even++;
        } else {
            odd++;
        }
    }

    printf("Even numbers: %d\n", even);
    printf("Odd numbers: %d", odd);
    return 0;
}

📥 Input space or newline separated

Output

Click ▶ Run to execute your code...

Use the modulo operator % to check if a number is even (num % 2 == 0) or odd (num % 2 != 0). Use two counter variables and a for-loop.

Arrays

A2 · Array Sum & Average

Compute the total and average of elements in an array

Easy ▾

Task: Read 5 integers from the user into an array. Compute and print the sum and the average (as a decimal). Format average to 2 decimal places.

Sample Input

10 20 30 40 50

Expected Output

Sum: 150
Average: 30.00

#include <stdio.h>

int main() {
    // Write your solution here
    // Declare an array of 5 integers
    // Read 5 numbers from input
    // Compute sum and average
    // Print both

    return 0;
}

📥 Input

Output

Click ▶ Run to execute your code...

Accumulate the sum in a variable inside the for-loop. Compute average as sum / 5.0 (use .0 to force floating-point division). Use %.2f to print 2 decimal places.

Arrays

A3 · Reverse Print an Array

Print elements of an array in reverse order

Easy ▾

Task: Read 6 integers into an array and print them in reverse order, each on its own line.

Sample Input

1 2 3 4 5 6

Expected Output

#include <stdio.h>

int main() {
    // Declare array, read 6 ints, print in reverse

    return 0;
}

📥 Input

Output

Click ▶ Run to execute your code...

After filling the array with a forward loop for(i=0; i<6; i++), use a backward loop for(i=5; i>=0; i--) to print in reverse.

Arrays

A4 · Find Maximum Element

Search an array for the largest value

Medium ▾

Task: Read 8 integers. Find and print the maximum value and its index.

Sample Input

3 7 1 9 5 2 8 4

Expected Output

Max value: 9
Found at index: 3

#include <stdio.h>

int main() {
    // Read 8 ints, find max and its index

    return 0;
}

📥 Input

Output

Click ▶ Run to execute your code...

Initialize max = arr[0] and maxIdx = 0. Loop from index 1. If arr[i] > max, update both max and maxIdx.

🔸 Pointers Exercises

Pointers

P1 · Pointer Basics

Your submitted lab · address, value, and modification via pointer

Easy ▾

Task: Declare int value = 24, create a pointer to it, print the address and original value, then change the value to 51 through the pointer and print the new value. This is your submitted Pointers.c.

No Input Required

(no stdin needed)

Expected Output

Address of value: 0xffe4a0
Original value: 24
Value after modification: 51

* Address shown as simulated in browser runner

#include <stdio.h>

int main() {
    int value = 24;
    int *p = NULL;

    p = &value;

    printf("Adress of value: %p\n", p);
    printf("Original value: %d\n", *p);

    *p = 51;
    printf("Value after modification through pointers: %d\n", *p);

    return 0;
}

📥 Input

Output

Click ▶ Run to execute...

Use int *p = NULL; to declare, p = &value; to assign the address, *p to dereference (read/write the value). Print address with %p.

Pointers

P2 · Swap Two Numbers Using Pointers

Classic swap via pass-by-reference

Medium ▾

Task: Write a swap function that takes two int* parameters and swaps the values. Call it from main() to swap two numbers entered by the user.

Sample Input

5
10

Expected Output

Before: a=5, b=10
After swap: a=10, b=5

#include <stdio.h>

// Function prototype
void swap(int* a, int* b);

int main() {
    int a, b;
    printf("Enter two numbers:\n");
    scanf("%d", &a);
    scanf("%d", &b);

    printf("Before: a=%d, b=%d\n", a, b);
    swap(&a, &b);
    printf("After swap: a=%d, b=%d\n", a, b);

    return 0;
}

// Function definition
void swap(int* a, int* b) {
    // use a temp variable
}

📥 Input

Output

Click ▶ Run to execute...

Inside swap: int tmp = *a; *a = *b; *b = tmp; — use the dereference operator to actually modify the values at the addresses.

Pointers

P3 · Pointer Arithmetic on Arrays

Traverse an array using only pointer arithmetic

Hard ▾

Task: Declare int arr[5] = {10, 20, 30, 40, 50};. Using a pointer and pointer arithmetic only (no subscript []), print each element and its address.

No Input Required

(values hardcoded)

Expected Output

Element: 10  Address: 0xffe4a0
Element: 20  Address: 0xffe4a4
Element: 30  Address: 0xffe4a8
Element: 40  Address: 0xffe4ac
Element: 50  Address: 0xffe4b0

* Addresses simulated in browser runner

#include <stdio.h>

int main() {
    int arr[5] = {10, 20, 30, 40, 50};
    int *p = arr; // pointer to first element

    // Use p and pointer arithmetic to traverse
    // No arr[i] allowed! Use *p and p++

    return 0;
}

📥 Input

Output

Click ▶ Run to execute...

Use a for-loop: for(int i=0; i<5; i++, p++). Access value with *p and address with p. Remember: p++ moves the pointer to the next integer in memory (4 bytes ahead).

🌀 Recursion Exercises

Recursion

R1 · Factorial

Your submitted lab · compute n! using recursion

Easy ▾

Task: Compute n! recursively. Base case: factorial(0) = factorial(1) = 1. This is your submitted Recursion.c.

Sample Input

Expected Output

Factorial of 5 is 120

#include <stdio.h>

int factorial(int n);

int main() {
    int n, result;

    printf("Enter a number: ");
    scanf("%d", &n);

    result = factorial(n);
    printf("Factorial of %d is %d", n, result);
    return 0;
}

int factorial(int n) {
    if (n == 0 || n == 1) {
        return 1;
    }
    return n * factorial(n - 1);
}

📥 Input

Output

Click ▶ Run to execute...

The base case n == 0 || n == 1 returns 1. The recursive case returns n * factorial(n-1). Try tracing: 5 × 4 × 3 × 2 × 1 = 120.

Recursion

R2 · Sum of N Natural Numbers

Add 1+2+3+...+n using recursion

Easy ▾

Task: Write a recursive function sumN(int n) that returns the sum 1 + 2 + 3 + ... + n. Base case: sumN(0) = 0.

Sample Input

Expected Output

Sum of 1 to 10 = 55

#include <stdio.h>

// Function prototype
int sumN(int n);

int main() {
    int n;
    scanf("%d", &n);
    printf("Sum of 1 to %d = %d", n, sumN(n));
    return 0;
}

// Function definition
int sumN(int n) {
    // Write your recursive function here
}

📥 Input

Output

Click ▶ Run to execute...

Base case: if (n == 0) return 0;. Recursive case: return n + sumN(n - 1);. Similar structure to factorial, but addition instead of multiplication.

Recursion

R3 · Fibonacci Sequence

Generate Fibonacci numbers recursively

Medium ▾

Task: Write a recursive function fib(int n). Base cases: fib(0) = 0, fib(1) = 1. Recursive: fib(n) = fib(n-1) + fib(n-2). Print the nth Fibonacci number.

Sample Input

Expected Output

Fibonacci(7) = 13

#include <stdio.h>

// Function prototype
int fib(int n);

int main() {
    int n;
    scanf("%d", &n);
    printf("Fibonacci(%d) = %d", n, fib(n));
    return 0;
}

// Function definition
int fib(int n) {
    // Two base cases and one recursive case
}

📥 Input

Output

Click ▶ Run to execute...

Two base cases: if (n==0) return 0; if (n==1) return 1;. Then: return fib(n-1) + fib(n-2);. Sequence: 0 1 1 2 3 5 8 13 ... so fib(7) = 13.

Recursion

R4 · Power Function (x^n)

Compute x raised to n using recursion

Hard ▾

Task: Write a recursive function power(int base, int exp) that returns base^exp. Base case: power(x, 0) = 1. Recursive: power(x, n) = x * power(x, n-1).

Sample Input

2
8

Expected Output

2 ^ 8 = 256

#include <stdio.h>

// Function prototype
int power(int base, int exp);

int main() {
    int b, e;
    scanf("%d", &b);
    scanf("%d", &e);
    printf("%d ^ %d = %d", b, e, power(b, e));
    return 0;
}

// Function definition
int power(int base, int exp) {
    // Base case: anything^0 = 1
    // Recursive: base * power(base, exp-1)
}

📥 Input

Output

Click ▶ Run to execute...

Base case: if (exp == 0) return 1;. Recursive: return base * power(base, exp - 1);. Think of it as multiplying base by itself exp times.

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Identification

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Find the Error & Correct It

15 items — spot the bug, then reveal fix

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True or False

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Multiple Choice

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Key Terms & Concepts

arrays · pointers · recursion · errors