Erlang入门教程 - 11. List进阶

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记住”|”运算符可以用于获取列表的头元素：

47> [M1|T1] = [paris, london, rome].
[paris,london,rome]
48> M1.
paris
49> T1.
[london,rome]

“|”运算符也可以用于向列表头添加一个元素：

50> L1 = [madrid | T1].
[madrid,london,rome]
51> L1.
[madrid,london,rome]

现在举个例子说明它是如何工作的——反转列表：

-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]

考虑一下 Reversed_List 是如何构造出来的。它最开始是空列表”[]”，然后列表头元素被取走并添加到 Reversed_List 里，如下所示：

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]

lists模块包含了很多操纵列表的函数，比如反转列表。所以在写一个列表操纵的函数之前最好先检查一下是否它早已存在。

现在让我们回到城市温度的例子，但使用更结构化的方法。首先让我们把列表转化成只包含摄氏度：

-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([]) ->
    [].

测试这个函数：

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}}]

解释：

format_temps(List_of_cities) ->
    convert_list_to_c(List_of_cities).

在这里 format_temps/1 调用 convert_list_to_c/1 。而 convert_list_to_c/1 取列表 List_of_cities 头元素，如果是摄氏度就什么也不做如果是华氏度还需要转化为摄氏度。 “|”运算符用于把转化后的元素添加到转换后的列表中，像这样：

[Converted_City | convert_list_to_c(Rest)];

或者：

[City | convert_list_to_c(Rest)];

这个操作将会持续到到达列表尾部，即列表为空。

convert_list_to_c([]) ->
    [].

当列表完成转换后，添加一个输出的函数：

-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

还必须添加一个函数寻找城市最高气温最低气温。下面的程序不是最佳解决方案，因为它遍历了列表四次。但最好是先力求清晰和正确，并且只有在需要时才改善程序效率。

-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([]) ->
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}}) ->
        Temp > Max_Temp ->
            Max_City = {Name, {c, Temp}};           % Change
        true -> 
            Max_City = {Max_Name, {c, Max_Temp}} % Unchanged
    end,
         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