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x00
Posted: Thu Mar 03, 2011 11:44 am Reply with quote
Joined: 03 Mar 2011 Posts: 2
I am new to functional programming, however I've always quite liked pattern matching type things.

I have been following learn you some Erlang for the great good

I have been finding it quite a helpful tutorial, filled in quite few gaps I didn't get from the getting started guide.

Anyway I'm at recursion.
http://learnyousomeerlang.com/recursion
It was suggested try I make the following two examples tail recursive

Code:
zip([],[]) -> [];
zip([X|Xs],[Y|Ys]) -> [{X,Y}|zip(Xs,Ys)].


Code:
lenient_zip([],_) -> [];
lenient_zip(_,[]) -> [];
lenient_zip([X|Xs],[Y|Ys]) -> [{X,Y}|lenient_zip(Xs,Ys)].


This is what I cam up with

Code:
tail_zip(A,B) -> zip(A,B,[]).

tail_zip([],[], Acc) -> Acc;
tail_zip([X|Xs],[Y|Ys], Acc) ->
   tail_zip(Xs,Ys, Acc++[{X,Y}]).


And

Code:
lenient_zip(A,B) -> lenient_zip(A,B,[]).

tail_lenient_zip([],Y,Acc) -> Acc++Y;
tail_lenient_zip(X,[],Acc) -> Acc++X;
tail_lenient_zip([X|Xs],[Y|Ys], Acc) ->
   tail_lenient_zip(Xs,Ys, Acc++[{X,Y}]).


Note the second isn't meant to be exactly the same function otherwise I could do it like so:

Code:
tail_lenient_zip(A,B) -> lenient_zip(A,B,[]).

tail_lenient_zip([],_,Acc) -> Acc;
tail_lenient_zip(_,[],Acc) -> Acc;
tail_lenient_zip([X|Xs],[Y|Ys], Acc) ->
   tail_lenient_zip(Xs,Ys, Acc++[{X,Y}]).


Anyway it worked however his code is like so:
Code:
tail_zip(X,Y) -> reverse(tail_zip(X,Y,[])).

tail_zip([],[],Acc) -> Acc;
tail_zip([X|Xs],[Y|Ys], Acc) ->
    tail_zip(Xs,Ys, [{X,Y}|Acc]).

Code:

tail_lenient_zip(X,Y) -> reverse(tail_lenient_zip(X,Y,[])).

tail_lenient_zip([],_,Acc) -> Acc;
tail_lenient_zip(_,[],Acc) -> Acc;
tail_lenient_zip([X|Xs],[Y|Ys], Acc) ->
    tail_lenient_zip(Xs,Ys,[{X,Y}|Acc]).


Note reverse was done in another example. I Know there is already a reverse for lists.

Anyway is the way I did it wrong? Advantages/disadvantages?
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zajda
Posted: Thu Mar 03, 2011 6:40 pm Reply with quote
User Joined: 22 Aug 2009 Posts: 83
the second set of your answers (with reverse) is better. It is better to add element in front of the list.
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x00
Posted: Fri Mar 04, 2011 12:37 am Reply with quote
Joined: 03 Mar 2011 Posts: 2
Yep I'm kind of getting that now.

However in that example he could have used his tail recursive reverse, but he used the initial one.
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dongdongwu
Posted: Thu Sep 20, 2012 6:08 am Reply with quote
User Joined: 19 Sep 2012 Posts: 236
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