列表操作
| 操作符
| 操作符可以用于取列表中的首元素:
1> [M1|T1] = [paris, london, rome]. [paris,london,rome] 2> M1. paris 3> T1. [london,rome]
| 操作符也可以用于在列表 首部添加元素 :
5> L1 = [madrid | T1]. [madrid,london,rome] 6> L1 . [madrid,london,rome]
使用 | 操作符 翻转列表中的元素 :
-module(reverse). -export([reverse/1]). reverse(List) -> reverse(List, []). reverse([Head | Rest], Reversed_List) -> reverse(Rest, [Head | Reversed_List]); reverse([], Reversed_List) -> Reversed_List.
测试如下：
3> reverse:reverse([1,2,3]).
[3,2,1]
初始时 Reversed_List 其为 []，待翻转的列表的首元素被取出来再添加到 Reversed_List 列表中：
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]
实例
下面回到城市与温度的话题上，但是这一次会使用更加结构化的方法
首先，将整个列表中的温度都使用摄氏度表示：
-module(temperature). -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([]) -> [].
含义如下：
format_temps(List_of_cities) -> convert_list_to_c(List_of_cities).
format_temps/1 调用 convert_list_to_c/1 函数。covert_list_to_c/1 函数移除 List_of_cities 的首元素，并将其转换为摄氏单位表示 （如果需要）。| 操作符用来将被转换后的元素添加到转换后的剩余列表中：
[Converted_City | convert_list_to_c(Rest)];
或者：
[City | convert_list_to_c(Rest)];
一直重复上述过程直到列表空为止。当列表为空时，则执行：
convert_list_to_c([]) ->
[].
测试下：
2> temperature:format_temps([{moscow, {c, -10}}, {cape_town, {f, 70}},
2> {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}}]
当列表被转换后，用新增的打印输出函数将其输出：
-module(temperature). -export([format_temps/1]). format_temps(List_of_cities) -> % convert_list_to_c(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.
4> temperature:format_temps([{moscow, {c, -10}}, {cape_town, {f, 70}},
4> {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
接下来，添加一个函数来搜索拥有最高温度与最低温度值的城市：
-module(temperature). -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, 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. 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]).
6> temperature:format_temps([{moscow, {c, -10}}, {cape_town, {f, 70}},
6> {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
现在的代码并不是最高效的方式，因为它遍历了四次列表 但是首先应当保证程序的清晰性和正确性，然后才是想办法提高程序的效率