Personally, I would feel trying to find the solution by parsing other people's code would be way more complicated than just coming up with your own solution.

I would forget about C, though, and try to do it in a language you already know to begin with.

To be honest I also find it very difficult to relate, as especially the Cesar Chiffre seems like such an easy problem. I only have to read it and my brain already maps out the solution. (I guess you could call it a No-Brainer, except that it seems to be a "Brainer" :-)

I honestly can not imagine how you usually code.

However, I want to say I remember my first year of studying mathematics. Many of the exercises also seemed very hard back then, but looking back on them the following year, they actually seemed quite easy (well, most of them). So practice and experience really can work. No guarantees, though.

Personally I like "Project Euler" for programming problems: https://projecteuler.net/ I actually had questions pop up in interviews that I had previously solved on project Euler.

I would be very interested to hear about some examples of real-world contexts that have proven tricky. As much detail as possible is preferable.

Incidentally I describe my problems with math almost the exact same way - virtually nonexistent fundamental understanding, and so huge issues with abstracting out even the simplest real-world tasks. (My wake-up call was when I was basically just mashing buttons on my calculator one day when I realized I didn't know how to compute how much of X I could buy given that it was $Y per weight.)

Have you tried writing it out on paper as a flowchart? Sounds old school, but can help with visualising what's happening.

Loops become super simple when you see them as series of steps with a yes/no question of whether to stop or go through your steps again.

Draw up some boxes for every variable you think you're going to need, give them labels and add the labels to the flowchart.

This then allows you to run through the flowchart step by step changing the values in each box as you go.