HOW THE CODE WORKS
You may have noticed that this calculator isn't like many of the others you have seen. It allows you to enter expressions like
1+3*4 and then executes the expression when you press the equals key. The calculator also follows the mathematical rules for expressions containing multiplication and division operators. The answer to
16. Multiplication and division of terms is done before addition and subtraction.
So, how is this magic achieved? It is done using a technique called Top-down, recursive descent parsing. That's quite a mouthful, isn't it? It is also rather tedious to explain and this article would be far too long if I attempted to do so.
If you're interested in learning the technique, there is a series of articles written by Jack Crenshaw called Let's Build A Compiler. One of the articles in this series covers the development of a top-down, recursive descent expression parser, which is what you are after if you are thinking of building your own calculator. The article is very readable and you don't need a degree in Computer Science to understand it like you do with so many other articles on the subject on the Internet. It is located here: https://compilers.iecc.com/crenshaw/tutor2.txt
Don't worry if it takes you more than a few attempts to understand what Jack is talking about. Parsing is hard and requires study. And bare this in mind: it took them years in the 50s to work out how to do this stuff. "So, how long did it take you?" Not telling! :)
BTW if you build a calculator, drop me a message below and let me know where it is. I enjoy looking at them. Good luck!