lists:sort/2
Erik Stenman
Erik.Stenman@REDACTED
Tue Jun 3 18:06:22 CEST 2003
I can't see that any of your functions could be right
Take the elements {1,2} and {4,1}
{1,2} < {4,1} AND {4,1} < {1,2} !!!
You have to make a total order like:
F1 = fun ({X1,Y1},{X2,Y2}) -> if (X1 < 3) or (X2 < 3) -> X1 < X2; true -> Y1
< Y2 end end.
Or
F2 = fun ({X1,Y1},{X2,Y2}) -> if (X1 < 3) and (X2 < 3) -> X1 < X2; true ->
Y1 < Y2 end end.
Depending on what you mean by "I wanted a list sorted on P for all P < 3,
and sorted on R for the remainder." (From your example I guess you want
solution 1).
6> lists:sort(F1,L).
[{1,11.2000},{1,10.1000},{2,10.2000},{3,10.3000},{4,10.4000}]
> -----Original Message-----
> From: owner-erlang-questions@REDACTED
> [mailto:owner-erlang-questions@REDACTED] On Behalf Of Ulf Wiger
> Sent: den 3 juni 2003 17:47
> To: erlang-questions@REDACTED
> Subject: lists:sort/2
>
>
>
> 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.
> I tried two different funs on the same list. The second one
> below produced the wanted result. The first one didn't. There
> is no help from the documentation in explaining why this
> happens, or indeed that there is a difference.
>
> Does anyone feel acquainted enough with the sorting
> algorithm to explain the phenomenon? Am I using the function
> in a way that was never intended?
>
> /Uffe
>
> L = [{1, 10.1},
> {4, 10.4},
> {1, 11.2},
> {3, 10.3},
> {2, 10.2}].
> --> [{1,10.1000},{4,10.4000},{1,11.2000},{3,10.3000},{2,10.2000}]
>
> lists:sort(
> fun({P1,_}, {P2,_}) when P1 < 3 ->
> P1 < P2;
> ({_,R1}, {_,R2}) ->
> R1 < R2
> end, L).
> --> [{1,10.1000},{4,10.4000},{1,11.2000},{2,10.2000},{3,10.3000}]
>
> lists:sort(
> fun({P1,_}, {P2,_}) when P2 < 3 ->
> P1 < P2;
> ({_,R1}, {_,R2}) ->
> R1 < R2
> end, L).
> --> [{1,11.2000},{1,10.1000},{2,10.2000},{3,10.3000},{4,10.4000}]
>
>
> /Uffe
> --
> Ulf Wiger, Senior Specialist,
> / / / Architecture & Design of Carrier-Class Software
> / / / Strategic Product & System Management
> / / / Ericsson AB, Connectivity and Control Nodes
>
>
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