Imagine that you are writing a piece of scientific software, and you need to calculate primes up to a limit, as we did in the previous chapter. You have a nice algorithm to calculate them, so you copy and paste it to wherever you need. One day, though, your friend, B. Riemann, gives you a better algorithm to calculate primes, which will save you a lot of time. At this point, you need to go over your whole code base and replace the old code with the new one.

This is actually a bad way to go about it. It's error-prone, you never know what lines you are chopping out or leaving in by mistake, when you cut and paste code into other code, and you may also risk missing one of the places where prime calculation is done, leaving your software in an inconsistent state where the same action is performed in different places in different ways. What if, instead of replacing code with a better version of it, you need to fix a bug, and you miss one of the places? That would be even worse.

So, what should you do? Simple! You write a function, get_prime_numbers(upto), and use it anywhere you need a list of primes. When B. Riemann comes to you and gives you the new code, all you have to do is replace the body of that function with the new implementation, and you're done! The rest of the software will automatically adapt, since it's just calling the function.

Your code will be shorter, it will not suffer from inconsistencies between old and new ways of performing a task, or undetected bugs due to copy-and-paste failures or oversights. Use functions, and you'll only gain from it, I promise.