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

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.