What does (_,[]) mean? - erlang

Question

I was given a question which was:
given a number N in the first argument selects only numbers greater than N in the list, so that
greater(2,[2,13,1,4,13]) = [13,4,13]
This was the solution provided:
member(_,[]) -> false;
member(H,[H|_]) -> true;
member(N,[_,T]) -> member(N,T).
I don't understand what "_" means. I understand it has something to do with pattern matching but I don't understand it completely. Could someone please explain this to me

Answer 1

This was the solution provided:
I think you are confused: the name of the solution function isn't even the same as the name of the function in the question. The member/2 function returns true when the first argument is an element of the list provided as the second argument, and it returns false otherwise.
I don't understand what "_" means. I understand it has something to do with pattern matching but I don't understand it completely. Could someone please explain this to me
_ is a variable name, and like any variable it will match anything. Here are some examples of pattern matching:
35> f(). %"Forget" or erase all variable bindings
ok
45> {X, Y} = {10, 20}.
{10,20}
46> X.
10
47> Y.
20
48> {X, Y} = {30, 20}.
** exception error: no match of right hand side value {30,
20}
Now why didn't line 48 match? X was already bound to 10 and Y to 20, so erlang replaces those variables with their values, which gives you:
48> {10, 20} = {30, 20}.
...and those tuples don't match.
Now lets try it with a variable named _:
49> f().
ok
50> {_, Y} = {10, 20}.
{10,20}
51> Y.
20
52> {_, Y} = {30, 20}.
{30,20}
53>
As you can see, the variable _ sort of works like the variable X, but notice that there is no error on line 52, like there was on line 48. That's because the _ variable works a little differently than X:
53> _.
* 1: variable '_' is unbound
In other words, _ is a variable name, so it will initially match anything, but unlike X, the variable _ is never bound/assigned a value, so you can use it over and over again without error to match anything.
The _ variable is also known as a don't care variable because you don't care what that variable matches because it's not important to your code, and you don't need to use its value.
Let's apply those lessons to your solution. This line:
member(N,[_,T]) -> member(N,T).
recursively calls the member function, namely member(N, T). And, the following function clause:
member(_,[]) -> false;
will match the function call member(N, T) whenever T is an empty list--no matter what the value of N is. In other words, once the given number N has not matched any element in the list, i.e. when the list is empty so there are no more elements to check, then the function clause:
member(_,[]) -> false;
will match and return false.
You could rewrite that function clause like this:
member(N, []) -> false;
but erlang will warn you that N is an unused variable in the body of the function, which is a way of saying: "Are you sure you didn't make a mistake in your function definition? You defined a variable named N, but then you didn't use it in the body of the function!" The way you tell erlang that the function definition is indeed correct is to change the variable name N to _ (or _N).

Answer 2

It means a variable you don't care to name. If you are never going to use a variable inside the function you can just use underscore.
% if the list is empty, it has no members
member(_, []) -> false.
% if the element I am searching for is the head of the list, it is a member
member(H,[H|_]) -> true.
% if the elem I am searching for is not the head of the list, and the list
% is not empty, lets recursively go look at the tail of the list to see if
% it is present there
member(H,[_|T]) -> member(H,T).
the above is pseudo code for what is happening. You can also have multiple '_' unnamed variables.

Answer 3

According to Documentation:
The anonymous variable is denoted by underscore (_) and can be used when a variable is required but its value can be ignored.
Example:
[H, _] = [1,2] % H will be 1
Also documentation says that:
Variables starting with underscore (_), for example, _Height, are normal variables, not anonymous. They are however ignored by the compiler in the sense that they do not generate any warnings for unused variables.

Answer 4

Sorry if this is repetitive...
What does (_,[]) mean?
That means (1) two parameters, (2) the first one matches anything and everything, yet I don't care about it (you're telling Erlang to just forget about its value via the underscore) and (3) the second parameter is an empty list.
Given that Erlang binds or matches values with variables (depending on the particular case), here you're basically looking to a match (like a conditional statement) of the second parameter with an empty list. If that match happens, the statement returns false. Otherwise, it tries to match the two parameters of the function call with one of the other two statements below it.