# [erlang-questions] List to proplist?

Edwin Fine erlang-questions_efine@REDACTED
Mon Oct 20 06:56:03 CEST 2008

```Hi all,

I was wondering what the most elegant and efficient way would be to take a
list containing an even number of elements and convert it into a proplist,
using only Erlang library functions? This is really a learning thing for me
(how to think in Erlang) rather than an urgent question, but it is based on
a real need (having a flat list of key/value pairs and wanting it in
proplist form).

For example, the function would do this: f([1,2,3,4,5,6,7,8]) ->
[{1,2},{3,4},{5,6},{7,8}].

I know it would be easy to write a pattern-matching-style function, but I
wanted to do it without having to define a new function, in as compact a way
as possible while still keeping it reasonably efficient. I thought there
might be a proplists:from_list/1 function, but I couldn't find one.

The best I could come up with is this (it's ok to assume that there will be
an even number of elements in L):

element(1, lists:mapfoldl(fun(_,[X,Y|T]) -> {{X,Y}, T} end, L,
lists:seq(1, length(L) div 2))).

It seems like a bit of a perversion of the idea of foldl, being used to
"unaccumulate". I'm also not really happy with having to generate another
list using lists:seq, especially if L is long, but I can't think of a better
way to drive the foldl part with the right number of elements. I thought of
splitting L in half for this, but what if L has large elements? Wouldn't I
be using much more memory than a simple list of integers, or would it
internally be a list of pointers to half of the elements of L? Actually,
that would almost always be bigger than a list of integers on a 64-bit
system...

Is there a better way to do this given the stated constraints? If so, I'd
appreciate it if you'd share it with me.

Regards,
Edwin Fine
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