# lists:sort/2

Ulf Wiger etxuwig@REDACTED
Tue Jun 3 18:10:52 CEST 2003

```I will try thinking before posting next time.
Maybe it's time to call it a day.  :)

/Uffe

On Tue, 3 Jun 2003, Erik Reitsma (ETM) wrote:

>> I encountered some surprising results when using
>> lists:sort/2. Given a list [{P,R}], I wanted a list sorted
>> on P for all P < 3, and sorted on R for the remainder.
>
>What result would you like when you compare {P1,R1} and {P2,R2}, and P1<3 and P2>=3? And what if P1>=3 and P2<3? It looks like you have not specified this.
>
>> lists:sort(
>>   fun({P1,_}, {P2,_}) when P1 < 3 ->
>> 	  P1 < P2;
>>      ({_,R1}, {_,R2}) ->
>> 	  R1 < R2
>>   end, L).
>
>If this function even anti-symmetrical? Consider X1={1,10} and X2={4,9}. Then F(X1,X2) = true and F(X2,X1) = true. Your function should be such, that F(A,B) = not F(B,A) for all A and B.
>
>When you have defined the relation you want, I think you will be able to find a fun that really implements that relation.
>
>*Erik.
>

--
Ulf Wiger, Senior Specialist,
/ / /   Architecture & Design of Carrier-Class Software
/ / /    Strategic Product & System Management
/ / /     Ericsson AB, Connectivity and Control Nodes

```