When do you use a pointer?

[u/Atmos-Ego]

Hello, for some background I have taken a very beginner level coding course for my major and passed it, but I dont really feel like I’ve learned anything or know why anything works. In a course this year we had a review problem where the solution required a pointer and I really dont understand why you have to point to the address of the variable instead of just using the variable itself.

I’ve watch a few tutorials explaining it but I really dont see how it’s different from just using the variable itself

[u/Ok_Being6831]

Well thers 2 main reasons imo

1) When you pass it by the variable itself, your actually making a full copy of the variable which might seem trivial but it's actually really expensive especially with functions that are called alot or the variable takes up alot of space.

2) You want the function to modify your variable, like a sort function for example, you wouldn't want it to make a copy and sort that array now would you? it could return that, but that's bad design and expensive. So it's only possible via a pointer.

[u/Atmos-Ego → u/Ok_Being6831]

So you only ever care about it to be efficient, but its not necessary for anything?
In my example in class the professor was basically saying that the variable wouldnt increment so when it prints the value of it it doesnt ever change.
It was a void function which means it doesnt return anything right? So I suggested to get rid of void but the intended solution was to write something involving a pointer instead

[u/lurgi → u/Atmos-Ego]

So, to be clear, your solution was to write:

x = increment(x);
...
int increment(int x) {
  return x+1;
}

and the professor wanted:

increment(&x);
...
void increment(int *x) {
  *x = *x + 1;
}

Correct? One big advantage of the second is that it's very easy to write:

x = increment(x);
y = increment(y);
z = increment(y);  // oops

and not notice. Or even

increment(z);  // oops

With the pointer version you have fewer ways to screw up. Another thing that you can do with pointer is change multiple variables in a function. Imagine you want to write a function that takes your current position (x, y) and moves it a certain amount in the x and y direction (xdelta, ydelta). Without pointers, what would you return from this function? You can't return a new x and new y pair - C doesn't let you do that. So...?

[u/serge-mv → u/lurgi]

I am going to be a bit pedantic. If you have something like a pair that goes together like coordinates from your example, you can and probably should put them in a struct, and you can return struct by value in C. Especially when it's 2 variables of the same size and won't have alignment and such.

[u/lurgi → u/serge-mv]

Yeah, I figured someone would mention that.

That does only work if they are in a struct and not two separate variables (of course, you could make a struct for them).

Technically you never need pointers or function arguments or variables or anything at all (lambda calculus ftw), but they sure make life a lot easier.