Unit 3: Practice

Practice
Challenges

Practice

Area Triangle

if statements

boolean logic

Math class

Create a class called AreaTriangle which prompts the user for 3 sides of a triangle. Given those sides, calculate and print the area. (Your program should also check to make sure that the side lengths given form a valid triangle. If they do not, print an error message instead of the triangle’s area.) Hint: Use Heron’s formula.

Example Output

Enter s1 s2 s3: 3 4 5 [Enter ↩]
The area of that triangle is 6.0

Template Code

public class AreaTriangle {
    public static void main(String[] args) {
        // write your code here
    }
}

Solution

Compute Distance

Scanner

Math class

Create a class called ComputeDistance which prompts the user to enter 2 (x, y) coordinates. Given those 2 points, calculate and print the distance between them.

Example Output

Enter x1 y1: 5 6 [Enter ↩]
Enter x2 y2: 12 -6.3 [Enter ↩]
The distance between those 2 points is 14.152384958020328

Template Code

public class ComputeDistance {
    public static void main(String[] args) {
        // write your code here
    }
}

Solution

Practice Arrays

arrays

for loops

Create a class called PracticeArrays with solution code for the exercises below.

Part 1

Initialize an array of Strings which contain words in your favorite quote. Use a for loop to print the entire quote in one line. Bonus points if you can also code a solution using a for each loop!

Example Quote to Array Quote: “The man dies in all who keep silent in the face of tyranny”. Array:

String[] quote = {
    "The",
    "man",
    "dies",
    "in",
    "all",
    "who",
    "keep",
    "silent",
    "in",
    "the",
    "face",
    "of",
    "tyranny"
};

Note: Remember that Java ignores whitespace (for the most part), so separating each element with a new line is valid. In fact, it is good practice in this case since the array is so long.

Note: Some developers prefer to add what is called a trailing comma at the end of the last element in the array. This may seem odd since there’s no comma afterward, which is why we normally don’t do this. However, there are some benefits.

Example Output

The man dies in all who keep silent in the face of tyranny

Part 2

Ask the user to enter the length of an array. Initialize an array of integers of the given length. Use a for loop to assign even numbers to the array. Use another for loop to print the result.

Example Output

Enter the length of the array: 6 [Enter ↩]
0
2
4
6
8
10

Template Code

public class PracticeArrays {
    public static void main(String[] args) {
        // write your code here
    }
}

Solution

Print Reverse

arrays

loops

Given an array of numbers (below), print the items in reverse order using a for loop. Your code should be in a class called PrintReverse.

Array

int[] numbers = { 1, 2, 3, 4, 5 };

Example Output

Template Code

public class PrintReverse {
    public static void main(String[] args) {
        // write your code here
    }
}

Solution

Repeat Print

loops

Create a class called RepeatPrint which prints a message of your choice to the console 5 times. Bonus points if you can solve this problem in 2 ways: one solution with a for loop, and the other with a while loop.

Example Output

Hail Hydra
Hail Hydra
Hail Hydra
Hail Hydra
Hail Hydra

Template Code

public class RepeatPrint {
    public static void main(String[] args) {
        // write your code here
    }
}

Solution

Challenges

Find Factor

while loop

break

boolean logic

Create a class called FindFactor which asks the user for a number greater than 2. Find and print the smallest factor of that number that is not 1.

Bonus points if you can do it in 3 ways:

Without using a boolean variable or break statement
Using a break statement
Using a boolean variable

Example Output

Enter a number greater than 2: 87 [Enter ↩]
The smallest factor of 87 is 3.

Template Code

public class FindFactor {
    public static void main(String[] args) {
        // write your code here
    }
}

Solution

Hailstone

if statements

while loops

arithmetic operators

Create a class called Hailstone which asks the user for a positive integer n and display their input (the first number in the hailstone sequence).

If n is even, divide n by 2. If n is odd, multiply n by 3 and add 1. Print the result (the current number in the hailstone sequence).

Keep doing this until n converges on 1 (it is the last item in the sequence, which you should also print) then print the number of items in the sequence and the average of all the numbers in the sequence.

Example Output

Enter starting positive number n: 10 [Enter ↩]
10
5
16
8
4
2
1
Number of items: 7
Average: 6.571428571428571

Template Code

public class Hailstone {
    public static void main(String[] args) {
        // write your code here
    }
}

Solution

Locker Puzzle

arrays

for loops

nested loops

logical operators

Create a class called LockerPuzzle which contains your solution code.

In a school, there are 100 lockers numbered 1 to 100. There are 100 students in the school. All of the lockers are initially closed.

Student 1 goes through each locker and opens them.

Student 2 goes through every other locker and opens them if they are closed, and closes them if they are open.

Student 3 goes through every 3 lockers and opens them if they are closed, and closes them if they are open.

Students 4-100 follow the same pattern, opening/closing every 4th locker, 5th locker, 6th locker, …, and 100th locker.

Visualization of Lockers (0 is closed, 1 is open)

Locker Numbers	1	2	3	4	5
Initial State (all closed)	0	0	0	0	0
After Student 1	1	1	1	1	1
After Student 2	1	0	1	0	1
After Student 3	1	0	0	0	1

Print the numbers of the lockers that are open after student 100 is done opening/closing their locker(s).

You should get the same output every time (it’s a series of special numbers).

Template Code

public class LockerPuzzle {
    public static void main(String[] args) {
        // write your code here
    }
}

Solution

Phone Word

loops

String methods

if statements

switch statements

Create a class called PhoneWord which asks the user to enter a phone word, which is then converted to a phone number if it is 7 characters or longer.

The program will only convert the first 7 characters to a phone number.

The program should be case insensitive (a is the same as A) and it should ignore any characters that aren’t letters or digits.

Use a for loop or while loop.

Hint 1: Wait, what’s a phone word? Look at your phone’s keypad. There are usually 3 letters that correspond with each number that could be combined to form a phone number. For example, the phone word for3V3R would translate into 367-3837.

(Basically, your program should preserve numbers as they are and convert letters to numbers based on a phone keypad.)

Hint 2: You might want to use regular expressions to get rid of all the characters that aren’t numbers or letters!

Bonus points if you can format the phone number such that there is a hyphen where there usually is (between the 3rd and 4th digit).

Note: To complete this challenge, you will need to know String methods in Java. You can read about them here.

Example Output

Enter a phone word: For3v3R [Enter ↩]
The number is 367-3837

Template Code

public class PhoneWord {
    public static void main(String[] args) {
        // write your code here
    }
}

Solution (For Loop)

Solution (While Loop)

Statistics

Scanner

while loop

arithmetic operators

Math class

boolean logic

Create a class called Statistics which asks the user to enter prices until they want to quit. After the user quits, display the number of items, average price (if possible), and sample standard deviation of the prices (if possible). Make sure the standard deviation is only up to 2 decimal points of precision (use System.out.printf and %.2f).

Note: How to calculate sample standard deviation for this problem (since you don’t know all the data at once)

Keep a count of the number of items (e.g. numItems)
Keep a running sum of each price squared (e.g. sumSquares) (ex. if the prices are 5, 6, and 1, the sum of each price squared is 25 + 36 + 1)
Keep a running sum of each price (e.g. sumItems) (ex. if the prices are 5, 6, and 1, the sum is 12)
Sample standard deviation will be (not in Java syntax): sqrt((numItems * sumSquares - sumItems * sumItems) / (numItems * (numItems - 1)))

Example Output 1

Enter the price, or -1 to quit: $4 [Enter ↩]
Enter the price, or -1 to quit: $5 [Enter ↩]
Enter the price, or -1 to quit: $6.45 [Enter ↩]
Enter the price, or -1 to quit: $-1 [Enter ↩]
Number of items: 3
Average price is $5.15
Standard deviation of prices is $1.23

Example Output 2

Enter the price, or -1 to quit: $5 [Enter ↩]
Enter the price, or -1 to quit: $-1 [Enter ↩]
Number of items: 1
Average price is $5.00
Cannot calculate standard deviation for one item

Example Output 3

Enter the price, or -1 to quit: $-1 [Enter ↩]
Number of items: 0
No data entered. Cannot calculate statistics.

Template Code

public class Statistics {
    public static void main(String[] args) {
        // write your code here
    }
}

Solution

Unit 3: Practice

Table of Contents

Practice

Area Triangle

Compute Distance

Practice Arrays

Part 1

Part 2

Print Reverse

Repeat Print

Challenges

Find Factor

Hailstone

Locker Puzzle

Phone Word

Statistics