Programming Erlang读书笔记3: Sequential Programming
module是Erlang代码的基本单元，我们写的所有function都存储在module里面，module存储在.erl文件里面module编译成.beam文件后方可运行
在Erlang_HOME建立一个名为.erlang的文件:
io:format("consulting .erlang in ~p~n", ).%% Edit to the directory where you store your codec:cd("C:/Program Files/erl5.6/work").io:format("Now in:~p~n", ).
这样启动Erlang自带的Windows程序时会自动切换到work目录
funs是匿名方法:
1> Z = fun(X) -> 2*X end.2> TempConvert = fun({c, C}) -> {f, 32 + C*9/5}; ({f, F}) -> {c, (F-32)*5/9} end.
返回funs的方法和可以接受funs作为参数的方法被称为higher-order方法
接受funs作为参数的方法:
1> L = .2> lists:map(Double, L).3> lists:filter(Even, L).
返回funs的方法:
1>Fruit = .2> MakeTest = fun(L) -> (fun(X) -> lists:member(X, L) end) end.#Fun<erl_eval.6.56006484>3> IsFruit = MakeTest(Fruit).#Fun<erl_eval.6.56006484>4> IsFruit(pear).true5> IsFruit(dog).false6> lists:fitler(IsFruit, ).
Erlang没有for循环，但我们可以自己定义一个:
for(Max, Max, F) -> ;for(I, Max, F) -> .1> lib_misc:for(1,10,fun(I) -> I end).
module用来声明本module名
export用来导出本module的公开方法
import用来导入其它module的公开方法
使用List Comprehension()来简化编程和增加可读性:
1> L = .2> lists:map(fun(X) -> 2*X end, L).3>.
用List Comprehension实现Quicksort:
qsort([]) -> [];qsort() -> qsort() ++ ++ qsort().
用List Comprehension实现Pythagorean Triplets:
pythag(N) -> [ {A,B,C} || A <- lists:seq(1,N), B <- lists:seq(1,N), C <- lists:seq(1,N), A+B+C =< N, A*A+B*B =:= C*C ].
算数表达式
Op Argument Type Priority+X Number 1-X Number 1X*Y Number 2X/Y Number 2bnot X Integer 2X div Y Integer 2X rem Y Integer 2X band Y Integer 2X+Y Number 3X-Y Number 3X bor Y Integer 3X bxor Y Integer 3X bsl N Integer 3X bsr N Integer 3
优先级为1的先执行，然后是2，and so on
可以用括号改变优先级，括号内的先执行
相同优先级的按从左到右执行
Guard是Erlang里的的一种表达式，关键字为when
max(X, Y) when X > Y -> X;max(X, Y) -> Y.
Guard Sequence是一个或多个Guard，用“;”隔开
G1;G2;...;Gn中至少有一个Guard为true的话则这个Guard Sequence为true
Guard是一个或多个Guard表达式，用“,”隔开
GuardExpr1,GuardExpr2,...,GuardExprN中所有Guard表达式都为true时为true
合法的Guard表达式:
1，The atom true
2，Other constants(terms and bound variables)，all evaluate to false
3，Calls to the guard predicates and to the BIFs
4，Term comparisons
5, Arithmetic expressions
6, Boolean expressions
7, Short-circuit boolean expressions
Guard predicates:
is_atom(X)is_binary(X)is_constant(X)is_float(X)is_function(X)is_function(X, N)is_integer(X)is_list(X)is_number(X)is_pid(X)is_port(X)is_reference(X)is_tuple(X)is_record(X, Tag)is_record(X, Tag, N)
Guard true用于在if表达式中catch all:
if Guard -> Expressions; Guard -> Expressions; ... true -> Expressionsend
Guard buit-in functions:
abs(X)element(N, X)float(X)hd(X)length(X)node()node(X)round(X)self()size(X)trunc(X)tl(X)
Record用来给Tuple的元素起名字:
-record(todo, {status=reminder, who=joe,text}).
Record的filed可以有默认值
Record定义可以在Erlang源代码里或者放在外部.hrl文件里并被Erlang源代码所引入
我们可以在Eshell里使用rr()方法来读取Record定义:
1> rr("records.hrl").
创建和修改Record:
2> X=#todo{}.#todo{status = reminder, who = joe, text = undefined}3> X1=#todo{status=urgent, text="Fix errata in book"}.#todo{status = urgent, who = joe, text = "Fix errata in book"}4> X2=X1#todo{status=done}.#todo{status = done, who = joe, text = "Fix errata in book"}5> #todo{who=W, text=Txt} = X2.#todo{status = done, who = joe, text = "Fix errata in book"}6> W.joe7> Txt."Fix errata in book"8> X2#todo.text."Fix errata in book"
Record是Tuple的伪装:
1> X.#todo{status = done, who = joe, text = "Fix errata in book"}2> rf(todo).ok3> X.{todo, done, joe, "Fix errata in book"}
case表达式:
case Expression of Pattern1 -> Expr_seq1; Pattern2 -> Expr_seq2; ...end
if表达式:
if Guard1 -> Expr_seq1; Guard2 -> Expr_seq2; ...end
将一个整数List按奇偶分成两个List:
odds_and_evens(L) ->odds_and_evens(L, [], []).odds_and_evens(, Odds, Evens) ->case (H rem 2) of 1 -> odds_and_evens(T, , Evens); 0 -> odds_and_evens(T, Odds, )end;odds_and_evens([], Odds, Evens) ->{lists:reverse(Odds), lists:reverse(Evens)}.
页:
[1]