Colorado State University Logo | Spring 21: CS 150 - Culture and Coding (AUCC 3B/GT-AH3) Colorado State University Logo | Spring 21: CS 150 - Culture and Coding (AUCC 3B/GT-AH3)
Spring 21: CS 150 - Culture and Coding (AUCC 3B/GT-AH3)
Computer Science

Lab 05 - Caesar Cipher


For this lab, you are going to implement parts of a Caesar Cipher. While a full implementation will take into account spaces, we will not worry about that, and will use the ASCII table and manipulate characters directly.

In this lab, you will learn

  • char manipulation using an int
  • concatenating Strings together
  • Basic loops over Strings

Step 1 - Warm Up Method: charAtGame

For your first method, you will write a method from scratch, that checks to see if you guessed the right character, the String.charAt(int) method returns. For reference, you can read more about the charAt method by looking at this resource.

Writing the Method Signature

The method signature is essential for any method. You can look at all the other methods in the class to see examples of different signatures. This method will need the following:

  • public
  • static
  • returns a boolean
  • It needs the following parameters to work
    • String str - a simple string that you will run charAt on
    • int loc - a location for that you are looking at in the string
    • char guess - the character you expect to be returned!

Work with your TA on making sure the method signature is correct, or your code will not compile. Often this part is about learning how to read the instructions. I personally just returned true until I got the code compiling!

Writing the charAtGame body

The body of the charAtGame is straight forward, and often, you can get yourself in trouble by over thinking. Follow these steps when writing the method body.

  • Get the character from str based on the location! One way to do this is:
char myChar = str.charAt(loc);
  • Then compare that character to the guess that is passed in. That comparison will tell you if you need to return true or false (or for pros, you can return the comparison directly).

That is it!

Testing charAtGame

At this point, your code should be compiling. While the other tests won’t work correctly, it is good to test the method you just wrote! (Hint: always test each method after writing it!!!)

Find the runTests() method, and uncomment the lines under the “Testing charAtGame” When you run your program, you should see

Testing charAtGame

What other tests can you add to this line? We don’t grade the tests, so add what you want to make sure you understand how it is working!

Step 2 - Warm Up Method: loopPrintExample

Let’s continue our warm-up, by practicing writing a loop. Go to the loopPrintExample method. You will see the signature is already in place and since this is a void method, nothing needs to be returned.

For this method, you will write a loop that loops through an entire string. For every character in the string, it will print out the character on unique line.

For example, if you call the method with:


The following will print to System.out


Writing loopPrintExample

First off, you know you are looping through a line, so you know you are going to need a for loop. You also know you are looking at every location in the line, so you are starting at 0 and looping until the end of the line. What method helps us get the total number of characters in a line? .length()

This means the loop you are creating, could look like the following

for(int i = 0; i < line.length(); i++) {
// add your print with charAt in here

Hint: I would compile after writing that line to make sure it compiles.

Now inside of your loop, you want to print out every character on a unique line. To get a single character based on the location in the string, we used charAt. To print out to the System.out with a newline after each print, would you use System.out.print or System.out.println?

Take those two concepts and combine them together.

Testing loopPrintExample

Go to the runTests method, and uncomment the lines that have loopPrintExample in them. You may also want to add your own method calls with words you want to print out. For example


Step 3 - Warm Up Method: buildRepeatingString

Yet another warmup, and this one focuses on teaching you how to build a new string to return. Go to the buildRepeatingString method. You will notice it creates an empty string, and returns that

public static String buildRepeatingString(String str, int times) {
    String rtn = "";
    // your loop here
    return rtn;

The goal of this method is to build a new String (rtn), that is made up of the String str repeated a number of times equal to times with a space in between each one. For example,

buildRepeatingString("a", 2); // "a a"
buildRepeatingString("a", 3); // "a a a"
buildRepeatingString("ab", 2); // "ab ab"

Remember, to concatenate to a String, you just add to it, so

String myString = "a" + " " + "a";   // sets myString equal to "a a"

Writing buildRepeatingString

First, write a loop that goes from 0 to times. I would compile to make sure the loop is running correctly. Second, inside the loop concatenate onto rtn the value of str with an space added.

For the purposes of this assignment, you do not need to worry about a trailing space at the end of the line.

Testing buildRepeatingString

Once again, go to the runTests method. Uncomment the lines associated with buildRepeatingString and add your own lines to see if it is working as expected.

Step 4 - Writing Shift (Part of the Caesar Cipher)

For the first part of the Cipher, you are going to write a simple shift method, that takes in a character & int and shifts the character down the ASCII table by adding the int value to it. For example:

shift('A', 3); // returns 'D'
shift('A', 2); // return 'C'
shift('A', 32); // returns 'a'

When you add an int to a char, it converts them both to ints. How do you convert it back to a character? The answer: use (char)

char myCharacter = (char)65; // gives us 'A'
char myOtherCharacter = (char) (11+55); // gives us 'B'

Writing shift

Shift can be a single line method. Obviously, take out the method stub that just returns ‘x’ and put in your own line.

Testing shift

Once again, go to the runTests method. Uncomment the lines associated with shift and add your own lines to see if it is working as expected.

Step 5 - Encrypt

Find the encrypt method. You will see that it is already got a stub, that looks very similar to the buildRepeatingString method stub. This is because for encrypt, you are going to look at every character in the line, apply a shift and build a String with the return value of shift. This method combines what you have learned in he previous methods, and utilizes the shift method you build. Divide-Conquer-Glue.

Writing Encrypt

First write a loop that goes from 0 to the length of your line. Think about the loopPrintExample method you wrote. What were the conditions for the loop? You are probably dealing with similar conditions here. I would compile and run it maybe with a print, just to make sure you are looping through your line correctly.

Second, grabbing the character at a set location (charAt) call the shift method passing in the character and the shift value. This will apply the CaesarCipher shift for you. You will notice the shift returns a character, and you have to do something with that character - or you will loose it.

Third, take that character and concatenate it onto your return string. Think about how you concatenated a string in buildRepeatingString. Same idea but without the extra space!

Last, return the string you built (already part of the method stub)

Testing Encrypt

Go to your runTests() method, and uncomment the lines for testing encrypt. You will notice the decrypt method is really simple for the cipher, and is already implemented for you.

Build a working application

Last but not least, go to to the top of CaesarCipher file, and change

public static boolean RUN_TESTS = true;


public static boolean RUN_TESTS = false;

You will want to go explore CaesarMain now, and look through the provided code. We have actually built an interactive application that relies on your code to work! You can play with learning about charAt or the cipher when you run the application. I would do that now, and have fun playing the games.

Turning in

Once you have run the program and are satisfied it is working, you may submit for grading. Note that you only get five submission attempts, so make sure it is mostly working before you submit. We will bypass the main method in our tests, running our own tests on each method you wrote.

Computer Science Department

279 Computer Science Building
1100 Centre Avenue
Fort Collins, CO 80523
Phone: (970) 491-5792
Fax: (970) 491-2466

Spring 21: CS 150 - Culture and Coding (AUCC 3B/GT-AH3)

Survey of computer science, formal logic, and computational thinking. Explores the historical, gender, and cultural perspectives on the role of technology in society. Includes learning a basic programming language. Students will be expected to write small programs, and construct written arguments on ways in which technology influences our modern culture. Previous computer science experience not necessary.