如何在Elixir中重写Erlang组合算法? [英] How to rewrite Erlang combinations algorithm in Elixir?
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问题描述
Erlang版本:
comb(0,_) - > ;
[[]];
comb(_,[]) - >
[];
comb(N,[H | T]) - >
[[H | L] || (N-1,T)] ++梳(N,T)。
Elixir版本我想出了这一点,但这是不正确的:
def组合(0,_),do:[[]]
def组合(_,[]),do:[] $ b $对于y <组合(n-1,xs),b def组合(n,[x | xs])do
,do:[x | y] ++组合(n,xs)
结束
示例用法,结果不正确:
IEX>组合(2,[1,2,3])
[[1,2,[3],[2,3]]]
任何关于我做错什么的指针?
谢谢!
Sean
解决方案
您需要将表达式包含在括号中,如Erlang代码。
def组合(n,[x | xs])do
(对于y <组合(n - 1,xs),do:[x | y])+ +组合(n,xs)
end
演示:
iex(1)> defmodule Foo do
...(1)> def组合(0,_),do:[[]]
...(1)> def组合(_,[]),do:[]
...(1)> def组合(n,[x | xs])do
...(1)> (对于y <组合(n-1,xs),do:[x | y])++组合(n,xs)
...(1)>结束
...(1)> end
{:module,Foo,
< 70,79,82,49,0,6,6,100,66,69,65,77,69,120,68,99 ,0,0,0,133,131,104,2,110,0,14,101,108,105,120,105,114,95,100,111,99,115,95,118,49,108 ,0,0,0,2,104,2,...>>>
{:组合,2}}
iex(2) Foo.combination 2,[1,2,3]
[[1,2],[1,3],[2,3]]
I've been tinkering with Elixir for the last few weeks. I just came across this succinct combinations algorithm in Erlang, which I tried rewriting in Elixir but got stuck.
Erlang version:
comb(0,_) ->
[[]];
comb(_,[]) ->
[];
comb(N,[H|T]) ->
[[H|L] || L <- comb(N-1,T)]++comb(N,T).
Elixir version I came up with this, but it's not correct:
def combination(0, _), do: [[]]
def combination(_, []), do: []
def combination(n, [x|xs]) do
for y <- combination(n - 1, xs), do: [x|y] ++ combination(n, xs)
end
Example usage, with incorrect results:
iex> combination(2, [1,2,3])
[[1, 2, [3], [2, 3]]]
Any pointers on what I'm doing wrong?
Thanks!
Sean
解决方案
You need to wrap the for expression in parentheses like the Erlang code.
def combination(n, [x|xs]) do
(for y <- combination(n - 1, xs), do: [x|y]) ++ combination(n, xs)
end
Demo:
iex(1)> defmodule Foo do
...(1)> def combination(0, _), do: [[]]
...(1)> def combination(_, []), do: []
...(1)> def combination(n, [x|xs]) do
...(1)> (for y <- combination(n - 1, xs), do: [x|y]) ++ combination(n, xs)
...(1)> end
...(1)> end
{:module, Foo,
<<70, 79, 82, 49, 0, 0, 6, 100, 66, 69, 65, 77, 69, 120, 68, 99, 0, 0, 0, 137, 131, 104, 2, 100, 0, 14, 101, 108, 105, 120, 105, 114, 95, 100, 111, 99, 115, 95, 118, 49, 108, 0, 0, 0, 2, 104, 2, ...>>,
{:combination, 2}}
iex(2)> Foo.combination 2, [1, 2, 3]
[[1, 2], [1, 3], [2, 3]]
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