How code works

Computers have a lot of constraints, and by themselves, they can only do a small number of things. The power that the computer programmer adds, through coding, is putting these small things together, in the right order, to produce something much bigger.

Coding is much like writing a recipe. You assemble ingredients (the data) and give the computer a step-by-step recipe for how to use them.

Here’s an example:

Step 1. Load photo from hard drive.

Step 2. Resize photo to 400 pixels wide by 300 pixels high.

Step 3. Apply sepia filter to photo. Step 4. Print photo.

This is what’s known as pseudo-code. It isn’t written in a valid computer programming language, but it represents the algorithm that you want to use. In this case, the algorithm takes a photo, resizes it, applies a filter and then prints it. It’s a relatively straightforward algorithm, but it’s an algorithm nonetheless!

Swift code is just like this: a step-by-step list of instructions for the computer. These instructions will get more complex as you read through this book, but the principle is the same: You are simply telling the computer what to do, one step at a time.

Each programming language is a high-level, pre-defined way of expressing these steps. The compiler knows how to interpret the code you write and convert it into instructions that the CPU can execute.

There are many different programming languages, each with its own advantages and disadvantages. Swift is an extremely modern language. It incorporates the strengths of many other languages while ironing out some of their weaknesses. In years to come, we’ll look back on Swift as being old and crusty, too. But for now, it’s an extremely exciting language because it is quickly evolving.

This has been a brief tour of computer hardware, number representation and code, and how they all work together to create a modern program. That was a lot to cover in one section! Now it’s time to learn about the tools you’ll use to write in Swift as you follow along with this book.