Debugging

Debugging is trying to find why your code doesn’t behave the way you want it to. Maybe it’s generating some errors. Maybe it’s not generating errors, but the output isn’t what you want it to be. Either way, there’s a bug in the code: a bug is a mistake or problem that is causing the code to fail at its goal. Debugging is the process of finding and removing those bugs.

Types of Error

• Compilation Errors compilation errors are those that occur when compiling our code in the first place. Not every language has compilation, and so, not every language has com- pilation errors. Those languages that don’t require compilation, like Python and JavaScript, can sometimes have tools that simulate this process, letting us know before we execute our code if there are errors present within it.

Errors can differ significantly from language to language, but there are some common ones. For compilation errors, some things you might often encounter are: • Syntax errors. You’ve written code that just doesn’t make sense in the current programming language. This is akin to a grammatical error in an essay. • Name errors. You’ve written code that tries to use something that doesn’t exist. Imagine, for example, I asked you, “Give her the book,” but never specified who “her” is. The command doesn’t make sense because I’m including a person that doesn’t exist. • Type errors. You’ve written code that tries to do something that doesn’t make sense, like requesting the smell of True or the color of the number 5.

• Runtime Errors Runtime errors are errors that we encounter only when actually running the code. Languages that don’t have compilation will only have runtime errors, and even languages that do require compilation can have runtime errors because we can’t anticipate every error just by looking at the code.

Runtime errors most often occur because of something specific to the results that code generates when it runs. Some of the common runtime errors you will encounter are: • Divide by zero errors. Your code contains a number being divided by another, but when those numbers actually have values, it turns out you’re trying to divide by zero! • Null errors. Null errors are like name errors: you’re referring to something that doesn’t exist. Here, though, the variable would exist, but it wouldn’t have any value. Imagine your code said, “Grab the twelfth book on the shelf,” but the shelf only has six books. The request makes sense in the directions until you see the shelf: then you realize you’re trying to use something that doesn’t exist, but you only “see” that at runtime. • Memory errors. Your computer can only remember a certain amount of stuff at a time. If you try to require it to remember more than that, you’ll hit a memory error.