Luke Hartley is ready for a certificate

Luke, Hi!

Thanks for your kind words and your "strike" through our problemset :) For one who "started recently... for a hobby" your feat is quite impressive - it took just about 1 month! I wonder how recently you get acquainted with coding - is it about few months or few years - and it looks you use at least two languages freely enough!

Your certificate is ready and attached to your profile! Congratulations!

If you don't mind I dare to share small thought I had when browsing some of your solutions: it seems you are already confident enough with general usage of your chosen languages - but probably it is the time to start concentrating on, eh, "finding better approaches". E.g. in "Instrument Tuner" code you use long if-else chain - there is nothing (almost) bad about it, but the code could be just two lines instead.

    if (note == 0 || note == 1)
    {
        ch = 'C';
    }
    else if (note == 2 || note == 3)
    {
        ch = 'D';
    }
    else if (note == 4)
    {
        ch = 'E';
    }
    else if (note == 5 || note == 6)
    {
        ch = 'F';
    }
    else if (note == 7 || note == 8)
    {
        ch = 'G';
    }
    else if (note == 9 || note == 10)
    {
        ch = 'A';
    }
    else if (note == 11)
    {
        ch = 'B';
    }

If you have a math degree you should definitly look Project Euler. The site has 731 question and most of them are extremely hard and requires good math knowledge to solve.

https://projecteuler.net/archives

I'd say that Project Euler was the inspiration for this site :) but it is quite well-known resource and Luke probably have seen it too :)

after solving your problems?

Much depends on your goals. There are people who mainly like to solve programming problems for the sake of fun - here is a field of "Competitive Programming" and most popular resource (with regular contests) is CodeForces, I believe.

On the other hand most people are interested in writing some larger projects, rather than doing exercises. Well, for this one mainly should start implementing any such projects which came to mind. You can find lists of "project ideas" just by googling if it is hard to come up with ideas (here is one of them) - or you can look at hyperskill.org, there are exercises arranged as medium-size projects.

I suppose putting the characters in an array and then looking them up

Yep, it is what came to my mind, though perhaps there is even some subtle way to write direct formula (but though it may be more clever, it won't be more clear, I believe).