Implementation of split_binary function of Erlang - erlang

Question

I'm new in the Erlang world. I'm trying to implement the function split_binary. The function takes as input (list, index) and it splits the list in two lists according to the index.
split(Lst, N) when N>=list:lenght(Lst) -> Lst;
split(Lst, N) when N<list:lenght(Lst) -> splitHelper(list:reverse(Lst), 0, N, []).
splitHelper([H|T], X, N, Acc) ->
if
X>=N ->
(list:reverse([H|T]), list:reverse(Acc));
X<N ->
splitHelper(T, X+1, N, [H|Acc])
end.
How can I improve my code?

Answer 1

I'm new in the Erlang world. I'm trying to implement the function
split_binary. The function takes as input (list, index) and it splits
the list in two lists according to the index.
According to the erlang docs for split_binary/2, the two arguments are a binary, which is not a list, and the number of bytes where you want to split the binary.
First, you need to have a basic understanding of what a binary is. A binary is a sequence of bytes, where each byte is 8 bits representing some integer, e.g.
0010 0001
which is 33. Here is an example of a binary:
<<1, 2, 3>>
When you don't specify a size for each integer, by default each integer will occupy one byte. If you wanted the 2 to occupy two bytes instead, i.e. 0000 0000 0000 0010, which is 16 bits, then you could write:
<<1, 2:16, 3>>
which the shell would display as:
<<1,0,2,3>>
Huh? Where did that 0 come from? The shell displays a binary byte by byte, and the first byte of the integer 0000 0000 0000 0010 is 0000 0000, which is 0.
Next, you can step through a binary just like you can for a list, extracting any number of bits at a time from the front of the binary. It so happens that split_binary/2 extracts 8 bits, or 1 byte, at a time from the head of the binary.
There are a couple of tricks to learning how to step through a binary:
For lists, [] means an empty list, and for binaries <<>> means an empty binary.
For lists you write [Head|Tail] to extract the head of the list, and for binaries you write <<Bits:3, Rest/binary>> to extract 3 bits from the front of the binary. In your case, you need to extract 8 bits from the front of the binary.
Here is an example of what you can do:
-module(a).
-compile(export_all).
split_b(Bin, N) ->
split_b(Bin, N, _Acc = <<>>).
split_b( Bin, _N = 0, Acc) -> [Acc, Bin];
split_b(<<Bits:8, Rest/binary>>, N, Acc) ->
split_b(Rest, N-1, <<Acc/binary, Bits>>).
In the shell:
40> c(a).
a.erl:2: Warning: export_all flag enabled - all functions will be exported
{ok,a}
41> a:split_b(<<5,6,7>>, 1).
[<<5>>,<<6,7>>]
42> a:split_b(<<5,6,7>>, 2).
[<<5,6>>,<<7>>]
Note that when constructing a binary one of the segments of the binary can be another binary:
23> Bin = <<1, 2, 3>>.
<<1,2,3>>
24> Acc = <<Bin/binary, 4>>.
<<1,2,3,4>>
If you are actually trying to implement lists:split/2, you can do this:
-module(a).
-compile(export_all).
split_l(N, List) ->
split_l(N, List, _Acc=[]).
split_l(_N=0, List, Acc) ->
[lists:reverse(Acc), List];
split_l(N, [H|T], Acc) ->
split_l(N-1, T, [H|Acc]).
In the shell:
2> c(a).
a.erl:2: Warning: export_all flag enabled - all functions will be exported
{ok,a}
3> a:split_l(1, [10, 20, 30]).
["\n",[20,30]]
4> shell:strings(false).
true
5> a:split_l(1, [10, 20, 30]).
[[10],[20,30]]
6> a:split_l(2, [10, 20, 30]).
[[10,20],[30]]

Answer 2

I think #7stud's answer is the best one, but I wanted to add a few minor details about your code, without actually checking if it works or not…
list:lenght/1 doesn't exist (unless you also created your own list module.
If you created your own list module, you can't use it in guards. Only BIFs are allowed there.
If you're trying to use stdlib's function to check the length of a list, then you should use erlang:length/1 or just length/1.
It's more idiomatic in Erlang to use snake_case (e.g. split_helper) instead of camelCase (e.g. splitHelper) for module names, function names and atoms in general.
You can use pattern-matching directly instead of writing an if as the sole expression of your function…
split_helper([H|T], X, N, Acc) when X > N ->
(list:reverse([H|T]), list:reverse(Acc));
split_helper([H|T], X, N, Acc) when X<N ->
split_helper(T, X+1, N, [H|Acc]).
Tuples are denoted with curly braces and not parentheses: {list:reverse([H|T]),…. BTW… This should have prevented your code from compiling at all. The error should've looked like syntax error before: ','
Also, you might have written your own list module, but if not and if you're trying to use stdlib functionality, it's lists:reverse/1 not list:reverse/1.
Finally, out of that list, I would strongly recommend you to write some simple tests for your code. This article may help you with that.