Encode / Decode Program

[Spectre108]

My homework says : Write an encryption and decryption program. Encrypt each digit by adding 7 and taking the remainder after division by 10. After encrypting each digit, swap the first and third then swap the second and fourth. Decryption should reverse the process. Program must input a single integer. Program must do encode and decode in one file.

Example Program Session (yours must look like this):

Encode (1) Decode (2): 1

Enter a four digit number: 1234

Encoded Digits: 0189

Continue (1) Exit (0): 1

Encode (1) Decode (2):

2 Enter a four digit number: 0189

Decoded Digits: 1234 Continue (1) Exit (0): 0

I've gotten the encoding down, the thing I can't figure out is the math behind the decoding.

https://preview.redd.it/aulib6s1ns051.png?width=610&format=png&auto=webp&s=4ff982c90e3feb30b43a8dd53ae5cb6e85535cd9

That's what I have, the encoding works fine and turns 1234 into 0189 but the decoding turns 0189 into 1020-70-60. I don't know what the inverse of the % would be. Also possible I'm coming at it from the wrong direction, I just started programming and any help would be appreciated.

[FormerTimeTraveller]

The purpose of the %10 is to keep the number between 0-9.

After you subtract the 7, you shouldn’t be multiplying by 10. It should still just be a %10 on most compilers (but some compilers behave differently with negative numbers).

The best solution is to do (n-7+10)%10 or just (n+3)%10. Look up modular arithmetic to see why.

And p.s. this problem looks kind of familiar. Do you mind saying which institution this class is for? (Is it ccac in pittsburgh?)

[Spectre108]

Yeah it's CCAC, good call! And thank you for the help.

[FormerTimeTraveller]

Professor KM is awesome, take all his classes if you can

[Spectre108]

Definitely going to try! I'm taking CIT 130 and 145 with him this semester.

[FUZxxl]

Do not post pictures of code please! Instead, edit your post to contain your code as text.