erlang小技巧

.列表操作 

lists:foreach(fun(X) -> io:format("E=~p~n",[X]) end, [1,2,3]). 
lists:duplicate(10, 16#f).  % [15,15,15,15,15,15,15,15,15,15] 

"abc-123" -> "abc" 
no_vsn(Name) -> lists:takewhile(fun($-)->false;(_)-> true end,Name). 

"abc-123" -> "123" 
vsn(Name) -> 
    case lists:dropwhile(fun($-)->false;(_)->true end,Name) of 
    [_Sep|Vsn] -> Vsn; 
    _ -> "0" 
    end. 

取偶数 
EvenN = lists:filter(fun (N) -> N rem 2 == 0 end, lists:seq(1,100)) 


lists:foldl(fun(F, Last) -> F(Last) end, foo(), [fun whee/1, fun bar/1]) 


将URL中的空格换成+ 
UW = lists:map(fun($ )->$+;(C)->C end,Words) 

判断是否为空格 
is_nb_space(X) -> 
    lists:member(X, [$\s, $\t]). 

>Data = [{"apple", "red"}, {"banana", "yellow"}, {"pear", "white"}]. 
>lists:keymember("pear",1,Data). 
  true 
>lists:keydelete("banana",1,Data). 
  [{"apple","red"},{"pear","white"}] 
>lists:keyreplace("apple",1,Data,{"tomato", "red"}). 
  [{"tomato","red"},{"banana","yellow"},{"pear","white"}] 

> rd(user,{id,name}).   %% 用户记录 
> lists:keysearch("wang", #user.name, [#user{id=1,name="li"}, #user{id=2,name="wang"}]). 
> {value,#user{id = 2,name = "wang"}} 


.二进制操作 

-define(BYTE, 8/unsigned-big-integer). 
<<C:2/unit:?BYTE>> 意思就是取两个单位，每单位8bits。 

unit的默认值取决于type，如果type是integer或者float，则为1，binary为8。 
<<C:4/binary, _/binary>> 意思就是取4个单位，每单位8比特，总共4*8比特 


.位操作 
    O2 = ((C1 band 16#03) bsl 4) bor (C2 bsr 4). 

. 文件操作 
Destination = filename:join([code:root_dir(), "include", no_vsn( New_lib )]),            file:make_dir(Destination), 

lists:foreach( fun(File) -> 
    FileName = lists:last(string:tokens(File,"/\\")), 
    file:copy(File, filename:join([Destination,FileName])) 
    end, filelib:wildcard(filename:join([Source,"*"])) ). 

remove(Path, File) -> 
    Desc = filename:join([Path,File]), 
    case filelib:is_dir(Desc) of 
    true -> 
        case file:list_dir(Desc) of 
        {ok,Sub} -> lists:foreach(fun(S) -> remove(Desc,S) end,Sub); 
        {error,Reason} -> io:format("error: ~p~n",[Reason]) 
        end, 
        file:del_dir(Desc); 
    false -> 
        file:delete(Desc) 
    end. 

uncompress(Archive) -> 
    case file:read_file(Archive) of 
    {ok,Tgz} -> 
        Tar = zlib:gunzip(Tgz), 
        erl_tar:extract({binary,Tar},[{cwd,code:lib_dir()}]); 
    Error -> 
        Error 
    end. 


.日期,时间操作 
{{Y,M,D},{H,M,S}} = calendar:local_time(). 
calendar:day_of_the_week(2009,2,24). 


.字符串操作 

格式化字符串, 像C语言的sscanf() 
Cmd = io_lib:format(\"cd ~s/include; rm ~s; ln -s ../lib/~s/include ~s\", [code:root_dir(), no_vsn( New_lib ), New_lib, no_vsn( New_lib )]), 
os:cmd(Cmd); 

PN = [erlang:list_to_integer(N) || N <- string:tokens( vsn( Package_that_might_be_newer ), "." )], 

packages_from_html(Html,Packages) -> 
    case string:str(Html,"class=\"package\"") of 
    0 -> Packages; 
    Start -> 
        Sub = string:sub_string(Html,Start), 
        T1 = string:sub_string(Sub,string:chr(Sub,$>)+1), 
        PName = string:sub_string(T1,1,string:chr(T1,$<)-1), 
        T2 = string:sub_string(T1,string:str(T1,"<i>")+3), 
        Descr = string:sub_string(T2,1,string:str(T2,"</i>")-1), 
        packages_from_html(T2,[{PName,Descr}|Packages]) 
    end. 

. mnesia相关操作 

$erl -mnesia dir '"/home/sw2wolf/data"' -s mnesia start 

-record(hitNum, { 
    oid, 
    hitnum 
}). 

% 在指定节点创建schema用数据表 

install( Nodes ) when is_list( Nodes ) -> 
    mnesia:stop(), 
    mnesia:delete_schema( Nodes ), 
    catch ( mnesia:create_schema( Nodes ) ), 
    mnesia:start(), 
    case mnesia:create_table ( hitNum, [ 
        { disc_copies, Nodes }, 
        { type, set }, 
        { attributes, record_info( fields, hitNum) } 
    ] ) of 
        { atomic, ok } -> ok; 
        _Any -> 
            io:format( "create table error!~n") 
    end, 
    mnesia:stop(), 
    ok. 

%增加记录 
add_hitnum(HitNums) -> 
    F = fun() -> 
            lists:foreach( fun mnesia:write/1, HitNums) 
        end, 
    mnesia:transaction( F ). 

%查寻 
qry_hitnum() -> 
    Q = qlc:q( [X || X  <- mnesia:table(hitNum)] ), 
    F = fun() -> qlc:e( Q ) end, 
    { atomic, Val } = mnesia:transaction( F ), 
    lists:sort(Val).