Global Variables Bad?

[S3THST4]

So, I've heard from people that using global variables is bad technique/practice. If this is true, why are they bad? I find them quite useful.

[AJ32]

Well, I never really thought they were so bad, in fact in php I use them all the time. But if I had to guess:

1. They take up a lot of memory
2. They can be changed from anywhere, from any class or file, so they can't be depended on as much as static variables.

Just my guess at it, if you want to change data from anywhere that the whole program relies on (like I do with PHP) I see no problem other than the memory issue. 

[gbertoli3]

I had no idea that they were considered a 'bad' practice, but then again I haven't really used global variables that much, maybe just once or twice.

[KYA]

Inherently, they aren't bad, but generally you'll be working on a project with multiple people and everyone doesn't know your thought process like you. Another rule of thumb is to keep scope as small as possible. This allows for more efficient bug hunting. If you get an error, wrong result, etc... you can quickly narrow it down to a module or function to step through to figure out what the issue is. With a global variable, it could be modified anywhere, so the hunting takes longer.

[Rayne]

They introduce dependencies in your code that are hard to untangle. If you have a global variable that's modified in about 10 different modules, how will (you in the future) anyone else know what's modifying it at whatever time? It becomes a big mess. This is why I love the way Clojure works. It discourages mutable state, and provides ways around it most of the time. It allows you to use mutable state when you really need it, but you have to be explicit about it with refs, atoms, and agents.

[S3THST4]

Thanks for the responses, guys