Erlang学习三

桃扇骨 2022-08-13 15:57 135阅读 0赞

[http://www.erlang.org/doc/getting\_started/intro.html\#id60368][http_www.erlang.org_doc_getting_started_intro.html_id60368]

**变量：**

每个变量在它的作用域内只能给定一个值

1> M = 5.
 5
 2> M = 6.
 ** exception error: no match of right hand side value 6
 3> {X,Y} = {paris,{f,27}}.
 {paris,{f,27}}
 4> X.
 paris
 5> Y.
 {f,27}
 6>

**revert:**

-module(tut).
 -export([revert/1]).
 
 
 revert([]) ->
 	ok;
 
 revert([Head | Rest]) ->
 	revert(Rest,Head).
 
 
 revert([],TemLs) ->
 	TemLs;
 
 revert([Head | Rest],TemLs) ->
 	Tem = [Head | TemLs],
 	revert(Rest,Tem).
 
 
 
 25> tut:revert([1,2,3]).
 [3,2|1]
 26>

**把Head当做元素，导致Tem\[2|1\] **

-module(tut).
 -export([revert/1]).
 
 
 revert([]) ->
 	ok;
 
 revert(List) ->
 	revert(List,[]).
 
 
 revert([],TemLs) ->
 	TemLs;
 
 revert([Head | Rest],TemLs) ->
 	Tem = [Head | TemLs],
 	revert(Rest,Tem).
 
 
 27> c(tut).
 {ok,tut}
 28> tut:revert([1,2,3]).
 [3,2,1]
 29>

**list中\[ \] 和\[Head | Rest\]区别**

-module(tut).
 -export([revert/1]).
 
 
 
 revert([Head | Rest]) ->
 	1.
 
 
 
 4> c(tut).
 tut.erl:8: Warning: variable 'Head' is unused
 tut.erl:8: Warning: variable 'Rest' is unused
 {ok,tut}
 5> tut:revert([]).
 ** exception error: no function clause matching tut:revert([]) (tut.erl, line 8)
 6>

查找最大最小值：

-module(tut).
 -export([findmaxandmin/1]).
 
 
 findmaxandmin([Head | Rest]) ->
 	findmaxandmin(Rest,Head,Head).
 
 
 
 findmaxandmin([Head | Rest],Max,Min) ->
 	if Head > Max ->
 		TemMax = Head;
 	true ->
 		TemMax = Max
 	end,
 	if Head < Min ->
 		TemMin = Head;
 	true ->
 		TemMin = Min
 	end,
 	findmaxandmin(Rest,TemMax,TemMin);
 
 
 findmaxandmin([],Max,Min) ->
 	{Max,Min}.
 
 
 
 3> c(tut).
 {ok,tut}
 4> tut:findmaxandmin([1,2,3,2,1]).
 {3,1}
 5>

if and case:

end之前没有分号

-module(tut).
 -export([test/2]).
 
 test(A,B) ->
 	if
 		A == 1 ->
 			io:format("A == 1~n");
 		B == 2 ->
 			io:format("B == 2~n");
 		A == 2,B == 3 ->
 			io:format("A == 2,B == 3~n");
 		A == 4;B == 5 ->
 			io:format("A == 4;B == 5~n")
 	end.
 
 
 
 24> c(tut).
 {ok,tut}
 25> tut:test(2,2).
 B == 2
 ok
 26> tut:test(2,4).
 ** exception error: no true branch found when evaluating an if expression
 in function tut:test/2 (tut.erl, line 5)
 27> tut:test(2,3).
 A == 2,B == 3
 ok
 28> tut:test(4,5).
 A == 4;B == 5
 ok
 29>

-module(tut).
 -export([convert/1]).
 
 
 convert(Val) ->
 	case Val of
 		{cent,X} ->
 			{inch,X / 2.54};
 		{inch,Y} ->
 			{cent,Y * 2.54}
 	end.
 
 
 31> c(tut).
 {ok,tut}
 32> tut:convert(1).
 ** exception error: no case clause matching 1
 in function tut:convert/1 (tut.erl, line 6)
 33> tut:convert({cent,2}).
 {inch,0.7874015748031495}
 34> tut:convert({inch,2}).
 {cent,5.08}
 35>

built in functions (BIFs)

因某些原因把函数构造在Erlang虚拟机中，例如为了实现在Erlang中不可能的实现或不高效的功能。

BIFs可以直接用其函数名，但是，他们属于erlang模块，例如调用trunc(向下取整）等价于erlang：trunc.

36> trunc(5.5).537> round(5.5).638> length(\[a,b,2,3\]).439> float(5).5.040> is\_atom(H).\* 1: variable 'H' is unbound41> is\_atom(h).true42> is\_atom("h").false43> is\_atom(\{1,2\}).false44> is\_tuple(\{1,2\}).true45> is\_tuple(\[1,2\]).false46> atom\_to\_list(hello)."hello"47> list\_to\_atom("hello47> ").'hello\\n'48> list\_to\_atom("hello").hello49> integer\_to\_list(22)."22"50>

-module(tut).
    -export([monthdays/2]).
    
    
    monthdays(Year,Mon) ->
    	Leap = if
    				trunc(Year / 400) * 400 == Year ->
    					leap;
    				trunc(Year / 100) * 100 == Year ->
    					not_leap;
    				trunc(Year / 4) * 4 == Year ->
    					leap;
    				true ->
    					not_leap
    			end,
    
    	case Mon of
    		sep -> 30;
    		apr -> 30;
    		feb when Leap == leap -> 29;
    		feb -> 28
    	end.
    
    
    2> c(tut).
    {ok,tut}
    3> tut:monthdays(2004,feb).
    29
    4>

higher order functions(Funcs)

-module(tut).
 -export([foreach/2,map/2]).
 
 
 foreach(Fun,[First | Rest]) ->
 	Fun(First),
 	foreach(Fun,Rest);
 
 foreach(Fun,[]) ->
 	ok.
 
 
 map(Fun,[First | Rest]) ->
 	T= Fun(First),
 	Tem = [T | map(Fun,Rest)],
 	Tem;
 
 map(Fun,[]) ->
 	[].
 
 
 
 4> Xf = fun(X) -> X * 2end.
 #Fun<erl_eval.6.90072148>
 5> Xf(3).
 6
 6> c(tut).
 
 14> tut:map(Xf,[1,2,3]).
 [2,4,6]
 
 
 17> Fe = fun(X) -> io:format("~w",[X * 2])end.
 #Fun<erl_eval.6.90072148>
 18> tut:foreach(Fe,[1,2,3]).
 246ok
 21>

-module(tut).
 -export([convertlist/1]).
 
 
 convert({Name,{f,Temp}}) ->
 	{Name,{c,trunc((Temp - 32)*5 / 9)}};
 
 
 convert({Name,{c,Temp}}) ->
 	{Name,{c,Temp}}.
 
 
 convertlist(List) ->
 	Newls = lists:map(fun convert/1,List),
 	lists:sort(fun({_,{c,Temp1}},{_,{c,Temp2}}) ->
 					Temp1 < Temp2 end,Newls).
 
 
 21> c(tut).
 {ok,tut}
 22> tut:convertlist([{moscow,{c,-10}},{bj,{f,2}}]).
 [{bj,{c,-16}},{moscow,{c,-10}}]
 23>

[http_www.erlang.org_doc_getting_started_intro.html_id60368]: http://www.erlang.org/doc/getting_started/intro.html#id60368