Week 5: Functions)

Overview

Welcome to Week 5 of C programming! This week, we will explore functions that make your code simple and reusable. By the end of this tutorial, you will:

• Understand how to use functions
• Understand the difference between call by value and call by referecne
• Understand how to use recursive functions.

Time Breakdown

• Introduction to a function
• Call by Value, Call by reference
• Recursion Fuction
• Exercises & Q/A

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1. Understanding the Function

Function can make your code short and reusable

Example: How to define and use functions

#include<stdio.h>

int sum1(int a, int b) {
    return a + b;
}

int sum2(int a, int b);

void print_hello();

void main(){
    int num1,num2;

    printf("Please enter two numbers: ");
    scanf("%d %d", &num1, &num2);

    int sum_of_func1= sum1(num1, num2);

    printf("Sum1: %d\n", sum_of_func1);
    printf("Sum2: %d\n", sum2(num1, num2));

    print_hello();
}

int sum2(int a, int b) {
    return a + b;
}

void print_hello() {
    printf("Hello, World!\n");
}

Example Output:

Please enter two numbers: 2 3
Sum1: 5
Sum2: 5
Hello, World!

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**2. Call by value, Call by reference **

Example: Change numbers in the functions

#include<stdio.h>

void I_cannot_change_num(int a, int b) {
    a=20;
    b=30;
    printf("Inside I_cannot_change_num: %d, %d\n", a, b);
}

void I_can_change_num(int *a, int *b) {
    *a=20;
    *b=30;
    printf("Inside I_can_change_num: %d, %d\n", *a, *b);
}
void main(){
    int num1,num2;
    printf("Please enter two numbers: ");
    scanf("%d %d", &num1, &num2);

    printf("=====================================\n");
    
    printf("Before I_cannot_change_num: %d, %d\n", num1, num2);
    I_cannot_change_num(num1, num2);
    printf("After I_cannot_change_num: %d, %d\n", num1, num2);

    printf("=====================================\n");

    printf("Before I_can_change_num: %d,%d\n", num1, num2);
    I_can_change_num(&num1, &num2);
    printf("After I_can_change_num: %d, %d\n", num1, num2);

    printf("=====================================\n");

}

Example Output:

Please enter two numbers: 2 3
=====================================
Before I_cannot_change_num: 2, 3
Inside I_cannot_change_num: 20, 30
After I_cannot_change_num: 2, 3
=====================================
Before I_can_change_num: 2,3
Inside I_can_change_num: 20, 30
After I_can_change_num: 20, 30
=====================================

---

**3. Recursive Function **

It is a different way of expressing loop

Example: calculate the sum from 0 to n

#include<stdio.h>

// similar to recurrence relation
int recursive_sum(int n){
    if(n==0){
        return 0;
    }
    return n + recursive_sum(n-1);
}

void main(){
    int num;
    printf("Please enter a number: ");
    scanf("%d",&num);
    int sum = recursive_sum(num);

    printf("sum is: %d \n",sum);
}

Example Output:

Please enter a number: 5
sum is: 15

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Exercise 1: Make a swap function.

1. Input two numbers
2. change the value of each variable
3. print the result

   **Example Output **

   Please enter two numbers: 1 2
   Before swap num: num1 = 1, num2 = 2
   After swap num: num1 = 2, num2 = 1
   

**Exercise 2: Make a program hadling 1-dimension matrix (array) **

You need to write these functinos:

• void set_matrix(int ary[], int len);
  -set the matrix in ascending order from 0 to len

• void reverse_matrix(int ary[], int len);
  -Reverse the order of the array

• void print_matrix(int ary[], int len);
  -Print out all numbers in the array

Hint*

1. Size of ary varies everytime. Use a sizeof() function to get a length of array
2. Array is handled in the way of Call By Reference. You don’t need to care about pointer!

Example Output (for n = 9):

===== After set_matrix function =====
Matrix: 0 1 2 3 4 5 6 7 8 
===== After reverse_matrix function =====
Matrix: 8 7 6 5 4 3 2 1 0 

Exercise 3: Write a program that calculate the Factorial using recursive function and for/while loop.**

You need to make 2 functions to calculate the fatorial of input number. One uses recursive function and the other one uses for or while loop.

**Example Output **

Please enter a number: 5
Factorial of 5 using recursive function is 120
Factorial of 5 using for loop is 120

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Bonus Challenge: Make a program that check if the string is palindrome or not (회문)

Input could be entered both in lowercase and uppercase.

**Example Output **

Please enter a string: Kayak
It is a palindrome!

Please enter a string: superman
It is not a palindrome

Please enter a string: exex
It is not a palindrome!

Bonus Challenge: Tower of Hanoi

If you are interested in the recursive function, solve this problem!

Korean ver. https://www.acmicpc.net/problem/11729

Englih ver. https://leetcode.com/discuss/post/5272714/tower-of-hanoi-by-satvikmpatil-owd6/

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Summary & Wrap-Up

This week, we covered:

• Basic rules of functinos
• Defference between call by value and call by reference
• Recursive Function

Next Week:

Happy coding! 🚀