Erlang lists:index_of function?

Question

I'm looking for an Erlang library function that will return the index of a particular element in a list.

So, if

X=[10,30,50,70]

then

lists:index_of(30, X)

would return 1, etc., just like java.util.List's indexOf() method.

Does such a method exist in the Erlang standard lib? I tried looking in the lists module but no luck. Or should I write it myself?

Thanks.

Answer 1

You'll have to define it yourself, like this:

index_of(Item, List) -> index_of(Item, List, 1).

index_of(_, [], _)  -> not_found;
index_of(Item, [Item|_], Index) -> Index;
index_of(Item, [_|Tl], Index) -> index_of(Item, Tl, Index+1).

Note however that accesing the Nth element of a list is O(N), so an algorithm that often accesses a list by index will be less efficient than one that iterates through it sequentially.

Answer 2

As others noted, there are more efficient ways to solve for this. But if you're looking for something quick, this worked for me:

string:str(List, [Element]).

Answer 3

This function is very uncommon for Erlang and this is may be reason why it is not in standard library. No one of experienced Erlang programmers need it and is discourage to use algorithms using this function. When someone needs it, can write for own purpose but this very rare occasions are not reason to include it to stdlib. Design your data structures in proper way instead of ask for this function. In most cases need of this function indicates error in design.

Answer 4

Other solutions (remark that these are base-index=1):

index_of(Value, List) ->
   Map = lists:zip(List, lists:seq(1, length(List))),
   case lists:keyfind(Value, 1, Map) of
      {Value, Index} -> Index;
      false -> notfound
   end.

index_of(Value, List) ->
   Map = lists:zip(List, lists:seq(1, length(List))),
   case dict:find(Value, dict:from_list(Map)) of
      {ok, Index} -> Index;
      error -> notfound
   end.

At some point, when the lists you pass to these functions get long enough, the overhead of constructing the additional list or dict becomes too expensive. If you can avoid doing the construction every time you want to search the list by keeping the list in that format outside of these functions, you eliminate most of the overhead.

Using a dictionary will hash the values in the list and help reduce the index lookup time to O(log N), so it's better to use that for large, singly-keyed lists.

In general, it's up to you, the programmer, to organize your data into structures that suit how you're going to use them. My guess is that the absence of a built-in index_of is to encourage such consideration. If you're doing single-key lookups -- that's really what index_of() is -- use a dictionary. If you're doing multi-key lookups, use a list of tuples with lists:keyfind() et al. If your lists are inordinately large, a less simplistic solution is probably best.

Answer 5

The following function returns a list of indices of a given element in a list. Result can be used to get the index of the first or last occurrence of a duplicate element in a list.

indices_of(Element, L) ->                                                                                                                                                          
    Indices = lists:zip(lists:seq(1,length(L)), L),                                                                                                                                
    [ I || {I, E} <- Indices, E == Element ].   

Answer 6

I think the writer makes a valid case. Here is my use case from a logging application. The objective is to check the severity of an error against the actions to be performed against various levels of error response.

get_index(A,L) ->
    get_index(A,L,1).
get_index(A,[A|_],N) ->
    N;
get_index(A,[_|T],N) ->
    get_index(A,T,N+1).

get_severity(A) ->
    Severity=[debug,info,warn,error],
    get_index(A,Severity).