Erlang入门备忘录(1):单行编程
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把“Sequential Programming”翻译成“单行编程”,与“Concurrent Programming”并行编程形成对应比较,比翻译成“顺序编程”要好。对应“并发编程”的是“单发编程”。
当然,这样咬文嚼字有些矫情。
2.1 The Erlang Shell
To shutdown the Erlang system and the Erlang shell type Control-C.
Another way to shutdown the Erlang system is by entering halt().
2.2 Modules and Functions
-module(tut1).
-export([fac/1]).
fac(1) ->
1;
fac(N) ->
N * fac(N - 1).
Compile the file
5> c(tut1).
{ok,tut1}
And now calculate the factorial of 4.
6> tut1:fac(4).
24
2.3 Atoms
-module(tut2).
-export([convert/2]).
convert(M, inch) ->
M / 2.54;
convert(N, centimeter) ->
N * 2.54.
Compile and test:
9> c(tut2).
{ok,tut2}
10> tut2:convert(3, inch).
1.1811023622047243
11> tut2:convert(7, centimeter).
17.78
2.4 Tuples
-module(tut3).
-export([convert_length/1]).
convert_length({centimeter, X}) ->
{inch, X / 2.54};
convert_length({inch, Y}) ->
{centimeter, Y * 2.54}.
14> c(tut3).
{ok,tut3}
15> tut3:convert_length({inch, 5}).
{centimeter,12.7}
16> tut3:convert_length(tut3:convert_length({inch, 5})).
{inch,5.0}
2.5 Lists
17> [First |TheRest] = [1,2,3,4,5].
[1,2,3,4,5]
18> First.
1
19> TheRest.
[2,3,4,5]
20> [E1, E2 | R] = [1,2,3,4,5,6,7].
[1,2,3,4,5,6,7]
21> E1.
1
22> E2.
2
23> R.
[3,4,5,6,7]
24> [A, B | C] = [1, 2].
[1,2]
25> A.
1
26> B.
2
27> C.
[]
-module(tut4).
-export([list_length/1]).
list_length([]) ->
0;
list_length([First | Rest]) ->
1 + list_length(Rest).
28> c(tut4).
{ok,tut4}
29> tut4:list_length([1,2,3,4,5,6,7]).
7
list_length([]) ->
0;
list_length([First | Rest]) ->
1 + list_length(Rest).
30> [97,98,99].
"abc"
2.6 Standard Modules and Manual Pages
% erl -man io
ERLANG MODULE DEFINITION io(3)
MODULE
io - Standard I/O Server Interface Functions
DESCRIPTION
This module provides an interface to standard Erlang IO
servers. The output functions all return ok if they are suc-
...
http://www.erlang.org/doc/r9b/doc/index.html
2.7 Writing Output to a Terminal
31> io:format("hello world~n", []).
hello world
ok
32> io:format("this outputs one Erlang term: ~w~n", [hello]).
this outputs one Erlang term: hello
ok
33> io:format("this outputs two Erlang terms: ~w~w~n", [hello, world]).
this outputs two Erlang terms: helloworld
ok
34> io:format("this outputs two Erlang terms: ~w ~w~n", [hello, world]).
this outputs two Erlang terms: hello world
ok
2.8 A Larger Example
%% This module is in file tut5.erl
-module(tut5).
-export([format_temps/1]).
%% Only this function is exported
format_temps([])-> % No output for an empty list
ok;
format_temps([City | Rest]) ->
print_temp(convert_to_celsius(City)),
format_temps(Rest).
convert_to_celsius({Name, {c, Temp}}) -> % No conversion needed
{Name, {c, Temp}};
convert_to_celsius({Name, {f, Temp}}) -> % Do the conversion
{Name, {c, (Temp - 32) * 5 / 9}}.
print_temp({Name, {c, Temp}}) ->
io:format("~-15w ~w c~n", [Name, Temp]).
35> c(tut5).
{ok,tut5}
36> tut5:format_temps([{moscow, {c, -10}}, {cape_town, {f, 70}},
{stockholm, {c, -4}}, {paris, {f, 28}}, {london, {f, 36}}]).
moscow -10 c
cape_town 21.11111111111111 c
stockholm -4 c
paris -2.2222222222222223 c
london 2.2222222222222223 c
ok
2.9 Matching, Guards and Scope of Variables
-module(tut6).
-export([list_max/1]).
list_max([Head|Rest]) ->
list_max(Rest, Head).
list_max([], Res) ->
Res;
list_max([Head|Rest], Result_so_far) when Head > Result_so_far ->
list_max(Rest, Head);
list_max([Head|Rest], Result_so_far) ->
list_max(Rest, Result_so_far).
37> c(tut6).
{ok,tut6}
38> tut6:list_max([1,2,3,4,5,7,4,3,2,1]).
7
39> M = 5.
5
40> M = 6.
** exception error: no match of right hand side value 6
41> M = M + 1.
** exception error: no match of right hand side value 6
42> N = M + 1.
6
43> {X, Y} = {paris, {f, 28}}.
{paris,{f,28}}
44> X.
paris
45> Y.
{f,28}
46> {X, Y} = {london, {f, 36}}.
** exception error: no match of right hand side value {london,{f,36}}
list_max([Head|Rest], Result_so_far) when Head > Result_so_far ->
New_result_far = Head,
list_max(Rest, New_result_far);
2.10 More About Lists
47> [M1|T1] = [paris, london, rome].
[paris,london,rome]
48> M1.
paris
49> T1.
[london,rome]
50> L1 = [madrid | T1].
[madrid,london,rome]
51> L1.
[madrid,london,rome]
-module(tut8).
-export([reverse/1]).
reverse(List) ->
reverse(List, []).
reverse([Head | Rest], Reversed_List) ->
reverse(Rest, [Head | Reversed_List]);
reverse([], Reversed_List) ->
Reversed_List.
52> c(tut8).
{ok,tut8}
53> tut8:reverse([1,2,3]).
[3,2,1]
reverse([1|2,3], []) =>
reverse([2,3], [1|[]])
reverse([2|3], [1]) =>
reverse([3], [2|[1])
reverse([3|[]], [2,1]) =>
reverse([], [3|[2,1]])
reverse([], [3,2,1]) =>
[3,2,1]
-module(tut7).
-export([format_temps/1]).
format_temps(List_of_cities) ->
convert_list_to_c(List_of_cities).
convert_list_to_c([{Name, {f, F}} | Rest]) ->
Converted_City = {Name, {c, (F -32)* 5 / 9}},
[Converted_City | convert_list_to_c(Rest)];
convert_list_to_c([City | Rest]) ->
[City | convert_list_to_c(Rest)];
convert_list_to_c([]) ->
[].
54> c(tut7).
{ok, tut7}.
55> tut7:format_temps([{moscow, {c, -10}}, {cape_town, {f, 70}},
{stockholm, {c, -4}}, {paris, {f, 28}}, {london, {f, 36}}]).
[{moscow,{c,-10}},
{cape_town,{c,21.11111111111111}},
{stockholm,{c,-4}},
{paris,{c,-2.2222222222222223}},
{london,{c,2.2222222222222223}}]
format_temps(List_of_cities) ->
convert_list_to_c(List_of_cities).
convert_list_to_c([]) ->
[].
-module(tut7).
-export([format_temps/1]).
format_temps(List_of_cities) ->
Converted_List = convert_list_to_c(List_of_cities),
print_temp(Converted_List).
convert_list_to_c([{Name, {f, F}} | Rest]) ->
Converted_City = {Name, {c, (F -32)* 5 / 9}},
[Converted_City | convert_list_to_c(Rest)];
convert_list_to_c([City | Rest]) ->
[City | convert_list_to_c(Rest)];
convert_list_to_c([]) ->
[].
print_temp([{Name, {c, Temp}} | Rest]) ->
io:format("~-15w ~w c~n", [Name, Temp]),
print_temp(Rest);
print_temp([]) ->
ok.
56> c(tut7).
{ok,tut7}
57> tut7:format_temps([{moscow, {c, -10}}, {cape_town, {f, 70}},
{stockholm, {c, -4}}, {paris, {f, 28}}, {london, {f, 36}}]).
moscow -10 c
cape_town 21.11111111111111 c
stockholm -4 c
paris -2.2222222222222223 c
london 2.2222222222222223 c
ok
-module(tut7).
-export([format_temps/1]).
format_temps(List_of_cities) ->
Converted_List = convert_list_to_c(List_of_cities),
print_temp(Converted_List),
{Max_city, Min_city} = find_max_and_min(Converted_List),
print_max_and_min(Max_city, Min_city).
convert_list_to_c([{Name, {f, Temp}} | Rest]) ->
Converted_City = {Name, {c, (Temp -32)* 5 / 9}},
[Converted_City | convert_list_to_c(Rest)];
convert_list_to_c([City | Rest]) ->
[City | convert_list_to_c(Rest)];
convert_list_to_c([]) ->
[].
print_temp([{Name, {c, Temp}} | Rest]) ->
io:format("~-15w ~w c~n", [Name, Temp]),
print_temp(Rest);
print_temp([]) ->
ok.
find_max_and_min([City | Rest]) ->
find_max_and_min(Rest, City, City).
find_max_and_min([{Name, {c, Temp}} | Rest],
{Max_Name, {c, Max_Temp}},
{Min_Name, {c, Min_Temp}}) ->
if
Temp > Max_Temp ->
Max_City = {Name, {c, Temp}}; % Change
true ->
Max_City = {Max_Name, {c, Max_Temp}} % Unchanged
end,
if
Temp < Min_Temp ->
Min_City = {Name, {c, Temp}}; % Change
true ->
Min_City = {Min_Name, {c, Min_Temp}} % Unchanged
end,
find_max_and_min(Rest, Max_City, Min_City);
find_max_and_min([], Max_City, Min_City) ->
{Max_City, Min_City}.
print_max_and_min({Max_name, {c, Max_temp}}, {Min_name, {c, Min_temp}}) ->
io:format("Max temperature was ~w c in ~w~n", [Max_temp, Max_name]),
io:format("Min temperature was ~w c in ~w~n", [Min_temp, Min_name]).
58> c(tut7).
{ok, tut7}
59> tut7:format_temps([{moscow, {c, -10}}, {cape_town, {f, 70}},
{stockholm, {c, -4}}, {paris, {f, 28}}, {london, {f, 36}}]).
moscow -10 c
cape_town 21.11111111111111 c
stockholm -4 c
paris -2.2222222222222223 c
london 2.2222222222222223 c
Max temperature was 21.11111111111111 c in cape_town
Min temperature was -10 c in moscow
ok
2.11 If and Case
if
Condition 1 ->
Action 1;
Condition 2 ->
Action 2;
Condition 3 ->
Action 3;
Condition 4 ->
Action 4
end
-module(tut9).
-export([test_if/2]).
test_if(A, B) ->
if
A == 5 ->
io:format("A == 5~n", []),
a_equals_5;
B == 6 ->
io:format("B == 6~n", []),
b_equals_6;
A == 2, B == 3 -> %i.e. A equals 2 and B equals 3
io:format("A == 2, B == 3~n", []),
a_equals_2_b_equals_3;
A == 1 ; B == 7 -> %i.e. A equals 1 or B equals 7
io:format("A == 1 ; B == 7~n", []),
a_equals_1_or_b_equals_7
end.
60> c(tut9).
{ok,tut9}
61> tut9:test_if(5,33).
A == 5
a_equals_5
62> tut9:test_if(33,6).
B == 6
b_equals_6
63> tut9:test_if(2, 3).
A == 2, B == 3
a_equals_2_b_equals_3
64> tut9:test_if(1, 33).
A == 1 ; B == 7
a_equals_1_or_b_equals_7
65> tut9:test_if(33, 7).
A == 1 ; B == 7
a_equals_1_or_b_equals_7
66> tut9:test_if(33, 33).
** exception error: no true branch found when evaluating an if expression
in function tut9:test_if/2
convert_length({centimeter, X}) ->
{inch, X / 2.54};
convert_length({inch, Y}) ->
{centimeter, Y * 2.54}.
-module(tut10).
-export([convert_length/1]).
convert_length(Length) ->
case Length of
{centimeter, X} ->
{inch, X / 2.54};
{inch, Y} ->
{centimeter, Y * 2.54}
end.
67> c(tut10).
{ok,tut10}
68> tut10:convert_length({inch, 6}).
{centimeter,15.24}
69> tut10:convert_length({centimeter, 2.5}).
{inch,0.984251968503937}
-module(tut11).
-export([month_length/2]).
month_length(Year, Month) ->
%% All years divisible by 400 are leap
%% Years divisible by 100 are not leap (except the 400 rule above)
%% Years divisible by 4 are leap (except the 100 rule above)
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 Month of
sep -> 30;
apr -> 30;
jun -> 30;
nov -> 30;
feb when Leap == leap -> 29;
feb -> 28;
jan -> 31;
mar -> 31;
may -> 31;
jul -> 31;
aug -> 31;
oct -> 31;
dec -> 31
end.
70> c(tut11).
{ok,tut11}
71> tut11:month_length(2004, feb).
29
72> tut11:month_length(2003, feb).
28
73> tut11:month_length(1947, aug).
31
2.12 Built In Functions (BIFs)
74> 2004 rem 400.
4
Year rem 400 == 0 ->
leap;
75> trunc(5.6).
5
76> round(5.6).
6
77> length([a,b,c,d]).
4
78> float(5).
5.0
79> is_atom(hello).
true
80> is_atom("hello").
false
81> is_tuple({paris, {c, 30}}).
true
82> is_tuple([paris, {c, 30}]).
false
83> atom_to_list(hello).
"hello"
84> list_to_atom("goodbye").
goodbye
85> integer_to_list(22).
"22"
2.13 Higher Order Functions (Funs)
86> Xf = fun(X) -> X * 2 end.
#Fun<erl_eval.5.123085357>
87> Xf(5).
10
foreach(Fun, [First|Rest]) ->
Fun(First),
foreach(Fun, Rest);
foreach(Fun, []) ->
ok.
map(Fun, [First|Rest]) ->
[Fun(First)|map(Fun,Rest)];
map(Fun, []) ->
[].
88> Add_3 = fun(X) -> X + 3 end.
#Fun<erl_eval.5.123085357>
89> lists:map(Add_3, [1,2,3]).
[4,5,6]
90> Print_City = fun({City, {X, Temp}}) -> io:format("~-15w ~w ~w~n",
[City, X, Temp]) end.
#Fun<erl_eval.5.123085357>
91> lists:foreach(Print_City, [{moscow, {c, -10}}, {cape_town, {f, 70}},
{stockholm, {c, -4}}, {paris, {f, 28}}, {london, {f, 36}}]).
moscow c -10
cape_town f 70
stockholm c -4
paris f 28
london f 36
ok
-module(tut13).
-export([convert_list_to_c/1]).
convert_to_c({Name, {f, Temp}}) ->
{Name, {c, trunc((Temp - 32) * 5 / 9)}};
convert_to_c({Name, {c, Temp}}) ->
{Name, {c, Temp}}.
convert_list_to_c(List) ->
lists:map(fun convert_to_c/1, List).
92> tut13:convert_list_to_c([{moscow, {c, -10}}, {cape_town, {f, 70}},
{stockholm, {c, -4}}, {paris, {f, 28}}, {london, {f, 36}}]).
[{moscow,{c,-10}},
{cape_town,{c,21}},
{stockholm,{c,-4}},
{paris,{c,-2}},
{london,{c,2}}]
lists:map(fun convert_to_c/1, List)
-module(tut13).
-export([convert_list_to_c/1]).
convert_to_c({Name, {f, Temp}}) ->
{Name, {c, trunc((Temp - 32) * 5 / 9)}};
convert_to_c({Name, {c, Temp}}) ->
{Name, {c, Temp}}.
convert_list_to_c(List) ->
New_list = lists:map(fun convert_to_c/1, List),
lists:sort(fun({_, {c, Temp1}}, {_, {c, Temp2}}) ->
Temp1 < Temp2 end, New_list).
93> c(tut13).
{ok,tut13}
94> tut13:convert_list_to_c([{moscow, {c, -10}}, {cape_town, {f, 70}},
{stockholm, {c, -4}}, {paris, {f, 28}}, {london, {f, 36}}]).
[{moscow,{c,-10}},
{stockholm,{c,-4}},
{paris,{c,-2}},
{london,{c,2}},
{cape_town,{c,21}}]
- Erlang入门备忘录(1):单行编程
- Erlang入门备忘录(2):并行编程
- Erlang入门备忘录(3):系统可靠性
- Erlang入门备忘录(4):记录和宏
- Erlang入门(二)— 并发编程
- Erlang编程快速入门
- Erlang入门(三)—— 分布式编程
- Erlang 语言入门-1
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