# Maps:to_list/1 element order.

Valentin Micic v@REDACTED
Fri Feb 28 11:53:51 CET 2020

```Thanks, this clarifies it.
Much obliged.

V/

> On 28 Feb 2020, at 11:56, Jesper Louis Andersen <jesper.louis.andersen@REDACTED> wrote:
>
> No, and it is subtle:
>
> L1 = [{X, X} || X <- lists:seq(1,32)].
> L1 == maps:to_list(maps:from_list(L1)).
> true
>
> L2 = [{X, X} || X <- lists:seq(1,33)].
> L2 == maps:to_list(maps:from_list(L2)).
> false
>
> At 32+ elements, Maps change their representation order to a hash array mapped trie. These are not generally ordered, but provide very fast lookups. In practice you are O(1) bound with a couple of memory reads. In theory you are O(lg n) bound width a branch factor of 32.
>
> If you need the order, you need to use a representation which maintains the order.
>
> On Fri, Feb 28, 2020 at 10:45 AM Valentin Micic <v@REDACTED <mailto:v@REDACTED>> wrote:
> Hi
>
> Even though documentation indicates that:
>
> to_list(Map) -> [{Key, Value}]
> Types
>
> Map = map()
> Key = Value = term()
>
>
> Returns a list of pairs representing the key-value associations of Map, where the pairs [{K1,V1}, ..., {Kn,Vn}] are returned in arbitrary order.
>
> It appears that the list returned has been sorted in ascending order, using value of the key as a criterion for sorting.
> I did some testing using a bit modified example offered in documentation:
>
>
> (x@REDACTED)47> f(Map), Map = #{42 => value_three,1337 => "value two","a" => 1, 1 => "Last entered"},maps:to_list(Map).
> [{1,"Last entered"},
>  {42,value_three},
>  {1337,"value two"},
>  {"a",1}]
>
>
>
> When I changed 1337 to -1337, I get the result that indicates that the list of pairs is *not* returned in arbitrary order, but actually sorted:
>
>
> (x@REDACTED)48>
> (x@REDACTED)48> f(Map), Map = #{42 => value_three, -1337 => "value two","a" => 1, 1 => "Last entered"},maps:to_list(Map).
> [{-1337,"value two"},
>  {1,"Last entered"},
>  {42,value_three},
>  {"a",1}]
>
>
> Could one relay on this always being the case?
> A I need this list to be sorted, I would hate to attempt sorting the already sorted list.
>
> Thanks in advance
>
> V/
>
>
>
> --
> J.

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