Welcome! Today I am going to go through my solution on the Vigenere cipher that is in Problem Set 2 of Harvard’s CS50 course. If you have been following these posts, you will notice that this looks a lot like Caesar’s cipher from last week. That is because we will be starting this exercise by copying our code from the previous solution. So, if there is something in this example that I don’t cover, please refer to my previous post. Now, let’s get started.

The first difference that you will notice between Caesar and Vigenere is that I start by declaring three variables and I set one of them equal to 0. I will come back to these and explain their existence later. Next, with Caesar we were expecting a command line argument that was a non-negative number. In this case, we are expecting a string input from the user as our cipher. So, our string s variable is set equal to the second command line argument, which is our users input. Next, I use a for loop to enumerate through the string to make sure that each character is an alpha. This is important, as we do not want the user to type an any numbers or other characters. If the character is found to not meet this condition, the user will be asked to “Try Again” and the program will return 1 indicating that an error has occurred.

The main difference between Caesar and Vigenere comes in the for loop that encrypts our text. For the isupper and islower portions I have added three lines of code that were not present in Caesar. The first line is where we use modular arithmetic to loop back through our keyword if the text that we are encrypting is longer than the keyword. On the next line we set k equal to the the number that the current character in is in the alphabet. We do this by simply subtracting 97 from the character. So, “a” will move the text by one character, whereas “c” will move the text by three characters. The last line increments the variable l so that each time we pass through the loop we are using the next character in the keyword. And, as in Caesar, we print each character to the screen after the line “ciphertext: “.

And that is all there is to the Vigenere cipher. I am interested on your take on this problem. If you have completed the problem, comment below with a link to your GitHub profile. If you enjoyed this post, please share on your favorite social media platform by clicking one of the buttons below. As always…Happy Programming!




    I joined CS50 about 6 weeks ago because since I was about 12 years old I have been interested in learning how to code. I wish I had done it when I was 12 as I am now 50. I struggled through pset 1 but managed to do it without cheating (too much) but this challenge (vigenere) has been driving me insane. I know it’s very much like Caesar but to be fair it is much more difficult. I had been working on it for a week and still had no clue until I snapped and found your page. I don’t feel I was taught enough to tackle this problem (maybe I’m wrong) this early in the course. I mean it is only week 2. Anyway, I know that you took the same course and it kinda makes me feel like I must be missing something because there is no way, not in a million years I would have figured out all of the parts that go into solving this problem. I thank you for your code as it has put me out of my misery but the truth is I wish I could have worked it out myself as there is no pleasure in cheating.


    1. I’m glad that I could help. We all cheat a little. Everyone uses Google when they get stuck, so don’t stress about it. For this one it is really key that you understand modular arithmetic. There are some really good videos out there that go through the fundamentals. It’s not super difficult, it is just different, so it takes some getting used too. Besides, the main focus when you are working with C through this course is to understand the concepts. That is way more important than the coding part in my opinion. Because, once you move up to the higher level languages (which start later in the course), you will see how much easier a lot of this stuff will be to implement. Anyway, hope you keep working on it and learning as much as you can.


  2. Hi Jason, I’m new to programming and am taking the CS50 course. Thank you for posting your solutions, they help me understand the concepts a lot more clearly when I’m stuck. That being said, I’m confused by lines 45 and 53 in your code. What does the 97 and 65 represent? also im not exactly sure how the int l is being used here. Thanks!


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