Writing functions

A fundamental principle of programming is Don't Repeat Yourself (DRY).

While programming, if you find yourself writing the same statements over and over again, then turn those statements into a function. Functions are like tiny programs that complete one small task. For example, you might write a function to calculate sales tax and then reuse that function in many places in a financial application.

Like programs, functions usually have inputs and outputs. They are sometimes described as black boxes, where you feed some raw materials in one end and a manufactured item emerges at the other. Once created, you don't need to think about how they work.

Let's say that you want to help your child learn their times tables, so you want to make it easy to generate a times table for a number, such as the 12 times table:

1 x 12 = 12
2 x 12 = 24
...
12 x 12 = 144

You previously learned about the for statement earlier in this book, so...