Deleting old data

When you first learned how to program, one of the earliest concepts that you were taught was the variable. A variable is something that's used to store a value, so of course it's important! When you create a variable and assign its initial value, it feels like you're creating new data. This is because you are creating new data.

Then, later on in your code, you assign a new value to your variable. It's a variable, so naturally it varies. This might also seem like you're creating new data, which you are; if you assign the value 5 to your variable, then that's a new value that didn't exist before. Here's the catch: you're also destroying data. What if your variable had a value of 2 before you assigned it 5?

This might not seem like a big deal. Isn't a variable supposed to store whatever value you assign to it? Yes, variables work perfectly fine that way. They do exactly what we ask them to do. So, the problem isn't strictly that variables can change their values. Rather, it's a human limitation—we can't reason our way through the huge number of variable changes that happen in our code. This leads to some scary bugs.