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Message |
< Erlang ~ Calculate difference between two adjacent list elements |
| niglas |
 Posted: Wed Feb 02, 2011 2:21 pm |
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Joined: 31 Jan 2011
Posts: 12
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How do I calculate the difference of two adjacent sublists?
From:
Code:
[[51,"one"],
[55,"two"],
[57,"three"]]
To:
Code:
[[0,"one"],
[4,"two"],
[2,"three"]]
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| zajda |
| Posted: Wed Feb 02, 2011 8:26 pm |
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Joined: 22 Aug 2009
Posts: 83
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This is without std lib, however it would be possible to use mapfoldl.
Code: -module(xxx).
-export([diff/3, diff/1]).
diff(Input) ->
diff(Input, 0, []).
diff([[CurrentX, CurrentVal]|Tail], 0, []) ->
diff(Tail, CurrentX, [[0, CurrentVal]]);
diff([], _PrevX, Acc) ->
lists:reverse(Acc);
diff([[CurrentX, CurrentVal]|Tail], PrevX, Acc) ->
diff(Tail, CurrentX, [[CurrentX - PrevX, CurrentVal]|Acc]). |
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| koops.j@gmail.com |
| Posted: Thu Feb 03, 2011 12:02 pm |
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Joined: 31 Jan 2009
Posts: 12
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As suggested by zajda, a solution using lists:mapfoldl/3:
Code:
diff(L) ->
{ Result, _ } =
lists:mapfoldl(fun([ C, T ], nil) -> { [ 0, T ], C };
([C, T], Prev) -> { [ C-Prev, T ], C }
end, nil, L),
Result.
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| niglas |
| Posted: Fri Feb 04, 2011 9:00 am |
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Joined: 31 Jan 2011
Posts: 12
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Thanks for the all help.
I'm facing another similar problem too. It seems a little bit more tricky because it requires look ahead instead:
From:
Code:
[[51,"one"],
[55,"two"],
[57,"three"]]
To:
Code:
[[4,"one"],
[2,"two"],
[0,"three"]]
My first thought is that it could be achieved by reversing the list before putting it into the list, but is there a better way? |
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| rvirding |
| Posted: Sat Feb 05, 2011 5:41 pm |
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User
Joined: 30 Aug 2006
Posts: 452
Location: Stockholm, Sweden
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You need to do some lookahead here:
Code: diff ([[X1,V1],[X2,V2]=Two|Rest]) ->
[[X2-X1,V1]|diff([Two|Rest])];
diff([[_,V]]) -> [[0,V]].
Using the alias in the pattern, =, means we can have our cake an eat by both pulling apart the second element and keeping a reference to it, i.e. it is a bit more efficient. We could do it for the whole rest of the list after the 1st element but it becomes a bit more difficult to read:
Code: diff([[X1,V1]|[[X2,V2]|_]=Rest]) ->
[[X2-X1]|diff(Rest)];
Code not tested and hopefully I didn't misunderstand what you wanted. |
_________________
Robert Virding, Erlang Solutions Ltd. |
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| niglas |
| Posted: Tue Feb 08, 2011 11:01 am |
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Joined: 31 Jan 2011
Posts: 12
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I think I like the second one better.
The final function I used is then:
Code: diff([[X1,V1]|[[X2,_]|_]=Rem]) ->
[[X2-X1]|diff(Rem)];
diff([[_,V]]) -> [[0,V]]. |
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| rvirding |
| Posted: Tue Feb 08, 2011 11:19 am |
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Joined: 30 Aug 2006
Posts: 452
Location: Stockholm, Sweden
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Sorry, a mistake crept into my code! It should be:
Code: diff([[X1,V]|[[X2,_]|_]=Rem]) ->
[[X2-X1,V]|diff(Rem)];
diff([[_,V]]) -> [[0,V]].
I forgot to add the actual value not just the difference. |
_________________
Robert Virding, Erlang Solutions Ltd. |
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| niglas |
| Posted: Tue Feb 08, 2011 11:46 am |
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Joined: 31 Jan 2011
Posts: 12
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rvirding wrote: It should be:
Code: diff([[X1,V]|[[X2,_]|_]=Rem]) ->
[[X2-X1,V]|diff(Rem)];
diff([[_,V]]) -> [[0,V]].
Yeah, I made a typo too. It should have been what you now corrected it to. That's what I am using right now. Thanks! |
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| wuji |
| Posted: Wed Aug 15, 2012 6:30 am |
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| wuji |
| Posted: Wed Aug 15, 2012 6:30 am |
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Joined: 10 Aug 2012
Posts: 654
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| dongdongwu |
| Posted: Thu Sep 20, 2012 5:25 am |
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