如何在 Erlang 的列表中搜索项目？答案

【问题标题】：How to Search for an item in a List in Erlang?如何在 Erlang 的列表中搜索项目？
【发布时间】：2010-10-14 23:10:25
【问题描述】：

我正在为公司使用编写缓存生成服务器。我想知道如何从列表中搜索一个项目，因为我想要比较 erlang 中的各种数据结构（如 dict、orddict、List、元组、树、队列等）用于缓存程序的搜索成本。

例子：

List = [{"A1",["ankit","sush", "Hover", "x4", "a3","nilesh","mike","erlang" | ...]}|...].

现在，我想搜索 Key A1 并在列表中搜索“mike”。搜索上述列表的最佳方法是什么。

请提供一些例子。至少是伪的。

【问题讨论】：

标签： functional-programming erlang

【解决方案1】：

没有比这更简单的了：

fein(_, []) ->
    [];
fein(Element, [Item| ListTail]) ->
    case (Item =:= Element) of
    true -> [Element|fein(Element, ListTail)];
    false -> fein(Element, ListTail)
    end.

【讨论】：

【解决方案2】：

只是为了简化https://stackoverflow.com/a/15587565/56250 的示例：

listFind(Element, List) ->
  lists:member(Element, List).

lists:member 适用于空列表。快速查看源代码 (https://github.com/erlang/otp/blob/07b8f441ca711f9812fad9e9115bab3c3aa92f79/erts/emulator/beam/erl_bif_lists.c#L184) 表明它执行得很慢。

【讨论】：

【解决方案3】：

您也可以从http://erlang.org/doc/man/lists.html#any-2 使用any

lists:any(fun(X) -> X == <<"yourthing">> end, YourList)

如果找到则返回true，否则返回false。

【讨论】：

【解决方案4】：

如果您的列表是一个简单的基于术语的项目列表，那么最简单的解决方案是：

listFind ( Element, [] ) ->
    false;

listFind ( Element, [ Item | ListTail ] ) ->
    case ( Item == Element ) of
        true    ->  true;
        false   ->  listFind(Element, ListTail)
    end.

假设您使用 == 运算符检查列表中项目的相等性，上述内容将起作用。您可以根据需要修改该运算符以适应其他类型的比较/相等。

【讨论】：

【解决方案5】：

使用来自lists 模块的keyfind 函数。例如：

30> List = [{"A1",["ankit","sush", "Hover", "x4", "a3","nilesh","mike","erlang"]}].
[{"A1",
  ["ankit","sush","Hover","x4","a3","nilesh","mike",
   "erlang"]}]
31> lists:keyfind("A1", 1, List).
{"A1",
 ["ankit","sush","Hover","x4","a3","nilesh","mike","erlang"]}

【讨论】：

OP 询问如何在列表中搜索"mike"，而不是"A1"。 lists:keyfind/3 想要根据元组的“关键”元素搜索元组列表，但 OP 想要的是搜索任意谓词。即，他想要hd(element(1,partition(Pred,List)))，除非不创建那些额外的列表。

【解决方案6】：

感谢您的回答。我编写了一个 Erlang 代码来查找将日期插入和提取到各种数据结构（如数据结构的列表、DICT、QUEUE、SET 和 PARALLEL MAP）中的时间。只有 list 的 insert & fetch 完成，其余的有并行 map 插入。我想分享代码和代码的结果。以下是模块test.erl

-module(test).
-author('ankit@hover.in').
-export ([ 
       %%list
       create_list/1, fetch_list/2, test_list/1, parmap_create_list/1 , ins_list/3, parallel_list/1,
       para_fetch_list/2, pfetch_list/1,

       %%dict
       create_dict/1, fetch_dict/2, test_dict/1, parmap_create_dict/1 , ins_dict/3, parallel_dict/1,

       %queue
       create_q/1, fetch_q/2, test_q/1, parmap_create_q/1, ins_q/3, parallel_q/1,

       %set
       create_set/1, fetch_set/2, test_set/1, parmap_create_set/1 , ins_set/3, parallel_set/1,

       create/1, multi_create/0,
       pcreate/1, parallel_multi_create/0

       %test/1 , multi_test/0 
      ]).

%%creation tests

%% For List
create_list(N) ->
    lists:foldl ( fun(X,Prev) -> Prev ++ [X] end, [] , lists:seq(1,N)).

test_list(N) ->
    {R1,List} = timer:tc(?MODULE, create_list, [N]),
    %%Set = lists:foldl(fun(X,Prev) -> sets:add_element(X,Prev) end,SET, lists:seq(1,N)),
    {R2,_} = timer:tc(?MODULE, fetch_list,[ N, List ]),
    {list,inserT,R1,fetchT,R2}.

%% For Dict
create_dict(N) ->
    lists:foldl( fun(X,Prev)-> dict:append([],X,Prev) end, dict:new(), lists:seq(1,N)).

test_dict(N) ->
    {R1,Dict} = timer:tc(?MODULE, create_dict, [N]),
    %%Set = lists:foldl(fun(X,Prev) -> sets:add_element(X,Prev) end,SET, lists:seq(1,N)),
    {R2,_} = timer:tc(?MODULE, fetch_dict,[ N, Dict ]),
    {dict,inserT,R1,fetchT,R2}.

%% For Queue
create_q(N) ->
    lists:foldl(fun(X,Prev) -> queue:in(X,Prev) end, queue:new() , lists:seq(1,N)).

test_q(N)->
    {R1,Q}  = timer:tc(?MODULE, create_q,[N]),
    %%Q = lists:foldl(fun(X,Prev) -> queue:in(X,Prev) end,Q0, lists:seq(1,N)),
    {R2,_} = timer:tc(?MODULE, fetch_q, [ N,Q ] ),
    {queue,inserT,R1,fetchT,R2}.

%% For Set
create_set(N) ->
    SET = sets:new(),
    lists:foldl(fun(X,Prev) -> sets:add_element(X,Prev) end,SET, lists:seq(1,N)).

test_set(N) ->   
    {R1,Set} = timer:tc(?MODULE, create_set, [N]),
    %%Set = lists:foldl(fun(X,Prev) -> sets:add_element(X,Prev) end,SET, lists:seq(1,N)),
    {R2,_} = timer:tc(?MODULE, fetch_set,[ N, Set ]),
    {set,inserT,R1,fetchT,R2}.

create(N)->
    [   timer:tc(?MODULE, X , [N])  || X <- [ test_list, test_dict, test_q,test_set ] ].

xmulti_create()->
    [ ?MODULE:create(X) || X<- [10,100,1000,10000,100000] ].

multi_create() ->
    InputRange = [10,100,1000,10000,100000],
    lists:map(fun(X) ->
              ?MODULE:create(X)
          end,InputRange).

%%fetching midpoint tests
fetch_q(N,Q)->
    fetch_q(N,Q,100).

fetch_q(N,Q,Find) ->
    F = fun(I) -> queue:out_r(I) end,
    R = lists:foldl( fun(_X,{Bool,PrevQ} ) ->
                 {{value, Ele},QQ} = F(PrevQ),
                 Ret1 = case Ele of 
                       Temp when Temp =:= Find -> 
                       true;
                       _ ->
                       Bool
                   end,
                 Ret2 = QQ, 

                 {Ret1,Ret2}

             end,{false,Q}, 
             lists:seq(1,N)),
    R.

fetch_set(N,Set) ->
    fetch_set(N,Set,100).

fetch_set(_N,Set,Find) ->
    Return = sets:is_element(Find,Set),
    Return.

fetch_list(N,List) ->
    fetch_list(N,List,500).

fetch_list(_N,List,Find) ->
    Ret = lists:foldl(fun(X,Prev) when X =:= Find -> true; 
             (X, Prev) -> Prev 
              end,false,List),
    Ret.

fetch_dict(N,Dict) ->
    {ok,List} = dict:find([],Dict),
    fetch_dict(N,List,500).

fetch_dict(_N,List,Find) ->
    Ret = lists:foldl(fun(X,Prev) when X =:= Find -> true; 
             (X, Prev) -> Prev
              end,false,List),
    Ret.

%% parallel operation

%% Parallel Map for Queue
parallel_q(N) ->
    {R1,Set} = timer:tc(?MODULE, parmap_create_q, [N]),
    {parallel_q,pcreate,R1}.

parmap_create_q(N) ->

    PID = spawn(fun() ->
            ?MODULE:ins_q( queue:new(),0,N) end),
    [PID ! {self(),X} || X <- lists:seq(1,N) ],
    receive
    {ok, Q} ->
        %%io:format("~n Still insertin, Q till now ~p",[Q]),
        ok;
    {done, Q}  ->
        io:format("~n *******DONE*********~n ~p",[Q]);
    _E ->
        io:format("unexpected ~p",[_E])
    end.

ins_q(Q,Ctr, Max) ->    
    receive
    {From, N} ->
        %%io:format("~n insert ~p, ctr ~p, Q ~p ~n",[N,Ctr,Q]),
        NewQ = queue:in( N, Q) ,
        case Ctr
        of Temp when Temp < Max ->
            From ! {ok, NewQ};
        _-> 
            From ! {done, NewQ}
        end,
        ?MODULE:ins_q(NewQ,Ctr+1, Max);
    _E ->
        io:format("unexpected ~p",[_E]),
        ?MODULE:ins_q(Q, Ctr, Max)
    end.

%% Parallel Map for set
parallel_set(N) ->
    {R1,Set} = timer:tc(?MODULE, parmap_create_set, [N]),
    {parallel_set,pcreate,R1}.

parmap_create_set(N) ->

    PID = spawn(fun() ->
            ?MODULE:ins_set( sets:new(),0,N) end),
    [PID ! {self(),X} || X <- lists:seq(1,N) ],
    receive
    {ok, Sets} ->
        %%io:format("~n Still insertin, Sets till now ~p",[Sets]),
        ok;
    {done, Sets}  ->
        io:format("~n *******DONE*********~n ~p",[Sets]);
    _E ->
        io:format("unexpected ~p",[_E])
    end.

ins_set(Sets,Ctr, Max) ->    
    receive
    {From, N} ->
        %%io:format("~n insert ~p, ctr ~p, Sets ~p ~n",[N,Ctr,Sets]),
        NewSets = sets:add_element(N,Sets),

        case Ctr
        of Temp when Temp < Max ->
            From ! {ok, NewSets};
        _-> 
            From ! {done, NewSets}
        end,
        ?MODULE:ins_set(NewSets,Ctr+1, Max);
    _E ->
        io:format("unexpected ~p",[_E]),
        ?MODULE:ins_set(Sets, Ctr, Max)
    end.

%% Parallel Map for dict
parallel_dict(N) ->
    {R1,Set} = timer:tc(?MODULE, parmap_create_dict, [N]),
    {parallel_dict,pcreate,R1}.

parmap_create_dict(N) ->

    PID = spawn(fun() ->
            ?MODULE:ins_dict( dict:new(),0,N) end),
    [PID ! {self(),X} || X <- lists:seq(1,N) ],
    receive
    {ok, Dict} ->
        %%io:format("~n Still insertin, Sets till now ~p",[Dict]),
        ok;
    {done, Dict}  ->
        io:format("~n *******DONE*********~n ~p",[Dict]);
    _E ->
        io:format("unexpected ~p",[_E])
    end.

ins_dict(Dict,Ctr, Max) ->    
    receive
    {From, N} ->
        %%io:format("~n insert ~p, ctr ~p, Dict ~p ~n",[N,Ctr,Dict]),
        NewDict = dict:append([],N,Dict),

        case Ctr
        of Temp when Temp < Max ->
            From ! {ok, NewDict};
        _-> 
            From ! {done, NewDict}
        end,
        ?MODULE:ins_dict(NewDict,Ctr+1, Max);
    _E ->
        io:format("unexpected ~p",[_E]),
        ?MODULE:ins_dict(Dict, Ctr, Max)
    end.

%% Parallel Map for list
parallel_list(N) ->
    {R1,List} = timer:tc(?MODULE, parmap_create_list, [N]),
    {R2,_} = timer:tc(?MODULE, para_fetch_list, [N,List]),
    {parallel_list,pCreate,R1,pFetch,R2}.

parmap_create_list(N) ->

    PID = spawn(fun() ->
            ?MODULE:ins_list( [],0,N) end),
    [PID ! {self(),X} || X <- lists:seq(1,N) ],
    receive
    {ok, List} ->
        %%io:format("~n Still insertin, List till now ~p",[List]),
        ok;
    {done, List}  ->
        io:format("~n *******DONE*********~n ~p",[List]);
    _E ->
        io:format("unexpected ~p",[_E])
    end.

ins_list(List,Ctr, Max) ->    
    receive
    {From, N} ->
        %%io:format("~n insert ~p, ctr ~p, Sets ~p ~n",[N,Ctr,Dict]),
        NewList = List ++ [N] ,

        case Ctr
        of Temp when Temp < Max ->
            From ! {ok, NewList};
        _-> 
            From ! {done, NewList}
        end,
        ?MODULE:ins_list(NewList,Ctr+1, Max);
    _E ->
        io:format("unexpected ~p",[_E]),
        ?MODULE:ins_list(List, Ctr, Max)
    end.

para_fetch_list(List, FindN) ->
    Pid = spawn(fun() ->
            ?MODULE:pfetch_list(FindN) end),
    Self = self(),
    Now1 = now(),
    lists:map ( fun(X) ->
            Pid ! {Self, X},
            receive
                {true,Find} ->
                io:format("~n Found ~p ",[Find]),
                true;
                {false,Find}  ->
                %io:format("~n NOT FOUND ~p ",[Find]),
                false;
                _E ->
                io:format("para main unexpected ~p",[_E])
            after 4000 ->
                io:format("timerout ",[])

            end
        end, List ) ,
    Now2 = now(),
    timer:now_diff(Now2,Now1).

pfetch_list(Find) ->    
    receive
    {From, CurrN} ->

        _Ret = case CurrN of 
              Temp when Temp =:= Find ->
              From ! {true,Find},
              true;
              _ -> 
              From ! {false,Find},
              false
          end,
        %%io:format("~n insert ~p, ctr ~p, Sets ~p ~n",[N,Ctr,Dict]),
        ?MODULE:pfetch_list(Find);
    _E ->
        io:format("pfetch unexpected ~p",[_E]),
        ?MODULE:pfetch_list(Find)
    end.    

pcreate(N)->
    [   timer:tc(?MODULE, X , [N])  || X <- [ parallel_list ] ].

parallel_multi_create() ->
    InputRange = [10,100,1000,10000],
    lists:map(fun(X) ->
              ?MODULE:pcreate(X)
          end,InputRange).

结果如下： 对于并行地图 DS

(ankit@127.0.0.1)2> test:pcreate(1000).
[{5399,{parallel_list,pCreate,5352,pFetch,30}},
 {9886,{parallel_dict,pcreate,9875}},
 {87603,{parallel_q,pcreate,87593}},
 {42271,{parallel_set,pcreate,42223}}]

适用于所有 DS

    (ankit@127.0.0.1)3> test:multi_create().
 %% For 10   
[[{38,{list,inserT,14,fetchT,8}},
      {91,{dict,inserT,67,fetchT,11}},
      {47,{queue,inserT,13,fetchT,21}},
      {81,{set,inserT,61,fetchT,7}}],
%%for 100
     [{234,{list,inserT,191,fetchT,30}},
      {690,{dict,inserT,642,fetchT,35}},
      {218,{queue,inserT,60,fetchT,144}},
      {559,{set,inserT,540,fetchT,6}}],
%% for 1000
     [{13746,{list,inserT,13452,fetchT,262}},
      {96137,{dict,inserT,96009,fetchT,116}},
      {967,{queue,inserT,284,fetchT,674}},
      {5562,{set,inserT,5547,fetchT,5}}],
%% for 10000  
   [{438301,{list,inserT,437256,fetchT,1027}},
      {361450,{dict,inserT,360412,fetchT,1020}},
      {7937,{queue,inserT,2292,fetchT,5636}},
      {31293,{set,inserT,31279,fetchT,5}}],
% for 100000
     [{43750210,{list,inserT,43739956,fetchT,10238}},
      {51517971,{dict,inserT,51507134,fetchT,10819}},
      {92503,{queue,inserT,29676,fetchT,62811}},
      {682118,{set,inserT,682100,fetchT,5}}]]

希望此信息能帮助您确定哪个 DS 最适合用于不同目的。当我完成整个代码时，我会更新它。

【讨论】：

【解决方案7】：

如果您想要这种“列表”，您可以轻松手工制作自己的搜索：

search_key(Key, [{Key,_}=V|_]) -> {ok, V};
search_key(Key, [_|T]) -> search_key(Key, T);
search_key(_, []) -> not_found.

search_val(Val, [{_,L}=X|T]) ->
  case _search_val(Val, L) of
    ok -> {ok, X};
    not_found -> search_val(Val, T)
  end;
search_val(_, []) -> not_found.

_search_val(Val, [Val|_])-> ok;
_search_val(Val, [_|T])-> _search_val(Val, T);
_search_val(_,[])-> not_found.

但我不确定你到底想要什么。例如，如果您想在列表中搜索键“A1”而不是值“mike”，这将是不同的解决方案。如果您想知道如何以最佳结构存储这种类型，那只是另一个问题。您应该提供更多信息。

【讨论】：

【解决方案8】：

如果您想搜索列表，请使用 list module 中的函数，它是 Erlang 附带的 extensive documentation 的一部分。

如果您想知道要使用的最佳数据结构 - 这是一个稍微不同的问题，需要更多信息。

【讨论】：