Mapping over 2+ lists/variables?
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Mapping over 2+ lists/variables?
by cnb
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Går det långsamt? Skaffa dig en snabbare bredbandsuppkoppling. Sök och jämför priser hos Kelkoo. _______________________________________________ erlang-questions mailing list erlang-questions@... http://www.erlang.org/mailman/listinfo/erlang-questions |
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Re: Mapping over 2+ lists/variables?
by Richard Carlsson-2
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Circular Function wrote:
> in python I can do: > >>> map(lambda x,y:x+y,[1,2,3],[4,5,6]) > [5, 7, 9] > >>> > > 38> lists:map(fun(X,Y) -> X+Y end,[1,2],[3,4]). > ** exception error: undefined function lists:map/3 > 39> > > isnt there general map-function that map is derived from that I can use? > what about listcomprehensions? > 44> [X+Y || X,Y <- lists:seq(1,10),lists:seq(1,10)]. > * 1: variable 'X' is unbound > 45> Erlang's list comprehensions can't do that. The function you want is lists:zipwith(Fun,List1,List2). You could also do lists:zip(List1,List2) and then run map on the result, but zipwith does it in one pass. /Richard -- "Having users is like optimization: the wise course is to delay it." -- Paul Graham _______________________________________________ erlang-questions mailing list erlang-questions@... http://www.erlang.org/mailman/listinfo/erlang-questions |
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Re: Mapping over 2+ lists/variables?
by Lev Walkin
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Circular Function wrote:
> > in python I can do: > >>> map(lambda x,y:x+y,[1,2,3],[4,5,6]) > [5, 7, 9] > >>> > > 38> lists:map(fun(X,Y) -> X+Y end,[1,2],[3,4]). > ** exception error: undefined function lists:map/3 > 39> Try opening a manual page for lists module erl -man lists and look for zip, zipwith, zipwith3 > isnt there general map-function that map is derived from that I can use? > what about listcomprehensions? > 44> [X+Y || X,Y <- lists:seq(1,10),lists:seq(1,10)]. > * 1: variable 'X' is unbound > 45> [X + Y || X <- lists:seq(1,10), Y <- lists:seq(1,10)]. -- vlm _______________________________________________ erlang-questions mailing list erlang-questions@... http://www.erlang.org/mailman/listinfo/erlang-questions |
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Re: Mapping over 2+ lists/variables?
by Sylvain Rousseau
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Hi,
You can do : 1> lists:zipwith(fun(X,Y) -> X+Y end, [1,2,3], [4,5,6]). [5, 7, 9] Does it help ? Regards, -- Sylvain
2008/7/23 Circular Function <circularfunc@...>:
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Re: Mapping over 2+ lists/variables?
by Edwin Fine
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2> [X + Y || {X, Y} <- lists:zip([1,2,3], [4,5,6])].
[5,7,9] 2008/7/23 Circular Function <circularfunc@...>:
-- The great enemy of the truth is very often not the lie -- deliberate, contrived and dishonest, but the myth, persistent, persuasive, and unrealistic. Belief in myths allows the comfort of opinion without the discomfort of thought. John F. Kennedy 35th president of US 1961-1963 (1917 - 1963) _______________________________________________ erlang-questions mailing list erlang-questions@... http://www.erlang.org/mailman/listinfo/erlang-questions |
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Re: Mapping over 2+ lists/variables?
by Richard O'Keefe
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On 24 Jul 2008, at 4:24 am, Circular Function wrote: > 38> lists:map(fun(X,Y) -> X+Y end,[1,2],[3,4]). > ** exception error: undefined function lists:map/3 > 39> So write your own: map(F, [A|As], [B|Bs]) -> [F(A,B) | map(F, As, Bs)]; map(_, [], [] ) -> []. I mean, it's less typing to FIX the problem than to complain about it! > isnt there general map-function that map is derived from that I can > use? No. The code for map is just map(F, [A|As]) -> [F(A) | map(F, As)]; map(_, [] ) -> []. Erlang is open source, and if you have it, you also have all the source code, including <ERLHOME>/lib/stdlib/src/lists.erl, so you can easily see for yourself how this and other functions work. > > what about listcomprehensions? > 44> [X+Y || X,Y <- lists:seq(1,10),lists:seq(1,10)]. > * 1: variable 'X' is unbound Indeed it is. Like Haskell (yah boo chiz) Erlang list comprehensions do not permit "parallel" iteration, only "nested" iteration. This is one of the things I like about Clean. Hmm. List comprehension uses '||', I've just thought of a use for '&&'... The Haskell approach would be [x + y | (x,y) <- zip [1..10] [1..10]] or better still, zipWith (+) [1..10] [1..10] The Erlang equivalent of using zip here is, well, using zip: [X + Y | {X,Y} <- lists:zip(lists:seq(1,10), lists:seq(1,10))] Personally I loathe this. Some Haskell compilers (notably GHC and I think YHC) are smart enough to do 'deforestation' and NOT really build a list of pairs. The Erlang compiler is not. (For one thing, 'zip' is part of the Haskell 'standard prelude'; it really counts as part of the language. 'zip' is not in the erlang: module, and not even in any module in the kernel application. It's in the lists module, which is in the stdlib application. So the compiler is not going to make any assumptions at all about what it does.) I would prefer to see [X + Y || X <- lists:seq(1,10) && Y <- lists:seq(1,10)] Sadly, that's not legal now. -- If stupidity were a crime, who'd 'scape hanging? _______________________________________________ erlang-questions mailing list erlang-questions@... http://www.erlang.org/mailman/listinfo/erlang-questions |
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