When to make class and function?

This is a stylistic and preference question and depending on how formal a place you work at it could be a matter of standards. I follow a couple of rules.

• Classes
  • Sets of related data belong in classes together
  • Functions to operate on that data should be in the classes together
    The classic Example is the Car class functions would be things like Drive and AddGass
• Functions
  • If you are going to use it more then once it should be in a function
  • Most functions should be no more then one screen of code
  • Functions Should do one thing well not a bunch of things poorly

there Are a ton of opinions, but over time you must develop your own style.

A very general question, so just a few rules of thumb:

• code reuse: when you have the same or very similar piece of code in two places, it should be moved to a function

• readibility: if a function spans more than a single page on screen, you may want to break it apart into several functions

• focus: every class or function should do only one specific task. Everything that is not core to this purpose should be delegated to other classes/functions.

I think the canonical answer here is that you should organize your code so that it's readable and maintainable. With that said, it's also important to consider the cost of organizing your code, and how long you expect your code to live.

More directly in response to your question: functions should be used to replace repetitive or otherwise well contained pieces of code. If you apply the same 10 operations over and over again on the same kinds of elements/data you might want to think about collecting all that information into a more concise and clear function. In general, a function needs well defined inputs and outputs.

Classes, in essence, collect functions and data together. Much like you should use a function to collect operations into concise, well defined collections of operations, classes should organize functions and data relevant to be stored together. That is, if you have a bunch of functions that operate on some things like a steering wheel, brakes, accelerators, etc. you should think about having a Vehicle class to organize these relevant functions and data/objects.

Beyond acting as an organizational element, classes should be used to enable easy reuse and creation of multiple "things" - suppose you wanted a collection of those Vehicles. Classes allow you to tie meaning or at least some semantics to your program.

The point of all this, though, is to make your life and the lives of others easier hen it comes to authoring and maintaining your program. So, by all means, when you need a solution to a problem in less than ten minutes and you think it's a one-time use program, ignore all this if you think it'll let you accomplish what you need to faster. Bear in mind, all this organization, semantics and ease of repetitve operation exists to make it easier to accomplish your objectives.

It's actually very simple nicky!

The purpose of splitting code into methods is simply to allow its reuse. When you create a method you allow your program to invoke it at any time from several places instead of repeating the code again and again.

So every time you write lines and think... 'hey, I might need this functionality again somewhere in my program', then you need to put it in a method.

As for classes, you will try to group similar functionalities together. And try to keep classes short and simple. If you need several classes, you'll also group them in packages and so on.

When I write code, I usually have a pretty good idea what I'll be using again. But often I will start to write a few lines of code and realize that I wrote something quite similar in the past. So I'll find it and put it in a method then the two or more locations can now just invoke it. That is reuse at its best!

You can often use analyzers to find various metrics which will "put a grade" on your reuse and code duplication.

Happy learning!