Week 3: Control Flow

Exercise #3.1 Harmonic Numbers

Harmonic numbers are a sequence of numbers that arises in various fields of mathematics. The n-th harmonic number, denoted as Hn, is defined as the sum of the reciprocals of the first n natural numbers: Write a Java program HarmonicNumber that on input a positive integer n, prints the n -th harmonic number.

Test case 1:

Test case 2:

My code:

import java.util.*;
public class hns {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        double n = sc.nextInt(), sum = 0, num = 1;
        while ( num <= n ) {
            sum += 1/num;
            num ++;
        }
        System.out.println(sum);
    }
}
​

Exercise #3.2 Black Jack

In Black Jack card game, we call a value busts if it exceeds a sum of 21. Write a Java program BlackJack that on input 2 integer values greater than 0, prints a single integer of whichever value is nearest to 21 without going bust. Print -1 if both values bust.

For example:

My code:

import java.util.*;
​
public class blkjack {
    public static void main(String[] args) {
        int a, b;
        Scanner sc = new Scanner(System.in);
        a = sc.nextInt();
        b = sc.nextInt();
        if(a>=b){
            if(a<22)
                System.out.println(a);
            else if(b<22)
                System.out.println(b);
            else System.out.println(-1);
        }
        else if(a<b) {
            if(b<22)
                System.out.println(b);
            else if(a<22)
                System.out.println(a);
                else System.out.println(-1);
        }
    }
}
​

Exercise #3.3 Pandemic Spread

Write a Java program PandemicSpread to simulate how fast a pandemic spread with the following input: init : the initial number of infected persons on day 1numInfect : the average number of healthy people that one infected person newly infects per daypopulation : the total number of people in the areaReturn the day on which the entire population will be infected.

For example:

My code:

import java.util.*;
​
public class pandsp {
    public static void main(String[] args) {
        int init, numInfect, population, day = 1;
        Scanner sc = new Scanner(System.in);
        init = sc.nextInt();
        numInfect = sc.nextInt();
        population = sc.nextInt();
        while(init < population){
            init += numInfect*init;
            day ++;
        }
        System.out.println(day);
    }
}
​

Exercise #3.4 Right Triangle

Write a Java program RightTriangle that takes three integer input numbers and determines whether they make up the side lengths of a right triangle .

Your program must print output true if and only if the following two conditions are true :

1. All three integers are positive (greater than zero).

2. You can find two integers where the sum of the squares of those two integers is equal to the square of the third integer.

For example:

My code:

(a bit strange tho, but it works)

import java.util.Scanner;
public class RT {
    public static void main(String[] args) {
        int a, b, c;
        Scanner sc = new Scanner(System.in);
        a = sc.nextInt();
        b = sc.nextInt();
        c = sc.nextInt();
        System.out.println((a>0 && b>0 && c>0 )&&(a > b && a > c && a * a == b * b + c * c || b > a && b > c && b * b == a * a + c * c || c > a && c > b && c * c == b * b + a * a));
    }
}
​

Lab Exercise #3.1 Armstrong Number

An Armstrong number is an n-digit number that equals the sum of the nth power of its digits. For example,

• 153 is a three-digit number where the sum of the cubes of the individual digits equal

• 1634 is a four-digit number where the sum of the 4th power of the individual digits equal 1634

Write a Java program Armstrong that on input two positive integers a and k, prints the first k Armstrong numbers greater than or equal to a.

For example:

My code:

import java.util.Scanner;
​
public class Armstrong {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int k = sc.nextInt();
        int sum = 0;
        while (k != 0) {
            int num = a;
            String str = Integer.toString(num);
            int length = str.length();
            while (num > 0) {
                int digit = num % 10;
                num = num / 10;
                sum += Math.pow(digit, length);
            }
            if (sum == a) {
                k--;
                System.out.println(sum);
            }
            a ++;
            sum = 0;
        }
    }
}

Lab 3 Challenge - Hailstone Sequence Length

The Hailstone sequence is computed as follows:

• Start with an integer number nIf n is even, the next number in the sequence is n/2

• If n is odd, the next number is 3n+1\

• Continue until it reaches 1For example, for n = 3, the sequence is 3, 10, 5, 16, 8, 4, 2, 1 and for n = 12, the sequence is 12, 6, 3, 10, 5, 16, 8, 4, 2, 1

Write a Java program HailStone that on input a positive integer n, prints the length of the Hailstone sequence starting at n.

Note that, interestingly, we still do not know whether the Hailstone sequence will ever reaches 1 for all starting n ! This is also known in the literature as the Collatz conjecture.

For example:

My code:

import java.util.Scanner;
​
public class Hailstone {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt(), l = 1;
        while (n != 1) {
            if (n % 2 == 0) {
                n /= 2;
                l ++;
            }
            else {
                n = 3 * n + 1;
                l ++;
            }
        }
        System.out.println(l);
    }
}
​