Erlang案例声明 [英] Erlang case statement

问题描述

我有以下Erlang代码，它正在给出警告如下，当我尝试编译，但这是有道理的。功能需要两个参数，但是我需要拍照匹配所有其他，而不是x，y或z。

I have the following Erlang code and it is giving the warning as follows, when i try to compile it, but that make sense. function need two arguments, but i need to patten match "everything else" rather x, y or z.

-module(crop).
-export([fall_velocity/2]).

fall_velocity(P, D) when D >= 0 ->
case P of
x -> math:sqrt(2 * 9.8 * D);
y -> math:sqrt(2 * 1.6 * D);
z -> math:sqrt(2 * 3.71 * D);
(_)-> io:format("no match:~p~n")
end.

crop.erl:9: Warning: wrong number of arguments in format call. 

我在io：格式之后尝试一个匿名变量，但仍然不高兴。 >

I was trying an anonymous variable after io:format, but still it is not happy.

推荐答案

以您使用的格式〜p。这意味着 - 打印值。所以你必须指定要打印的值。

In the format you use ~p. It means -- print value. So you must specify what value to print.

最后一行必须是

_ -> io:format("no match ~p~n",[P])

此外，io：格式returms'ok'。所以如果P不是x y或z，你的函数将返回'ok'而不是数值。我建议返回标记值以分离正确和错误的返回。当D> = 0时，

Besides, io:format returms 'ok'. So if P is not x y or z, your function will return 'ok' instead of numeric value. I would suggest to return tagged value to separate correct and error returns. Kind of

fall_velocity(P, D) when D >= 0 ->
case P of
x -> {ok,math:sqrt(2 * 9.8 * D)};
y -> {ok,math:sqrt(2 * 1.6 * D)};
z -> {ok,math:sqrt(2 * 3.71 * D)};
Otherwise-> io:format("no match:~p~n",[Otherwise]),
            {error, "coordinate is not x y or z"}
end.

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