为什么在不正确的上下文中使用此函数句柄? [英] Why is this function handle being used in the incorrect context?

问题描述

我想了解如何将函数传递给 varfun，我想这适用于 arrayfun、cellfun 等.

I am trying to understand how to pass functions to varfun, which I suppose applies to arrayfun, cellfun etc.

阅读帮助文件，第一个参数应该是:

Reading the helpfile, the first argument should be:

函数，指定为函数句柄.您可以在文件中定义该函数，也可以将其定义为匿名函数.如果 func 对应多个函数文件(即如果 func 表示一组重载函数)，MATLAB 会根据输入参数的类别确定调用哪个函数.

Function, specified as a function handle. You can define the function in a file or as an anonymous function. If func corresponds to more than one function file (that is, if func represents a set of overloaded functions), MATLAB determines which function to call based on the class of the input arguments.

所以我尝试使用以下数据:

So I try it with the following data:

sampleId = [1 1 1 3 3 3]';
entity = [1 2 3 1 4 5]';
dataTable = table(sampleId, entity)

是的:

varfun(@mean, dataTable)

ans = 

    mean_sampleId    mean_entity
    _____________    ___________

    2                2.6667     

现在，我匿名定义自己的函数时出现问题，例如:

Now, my problem occurs when I define my own function anonymously, for example:

mymean = @(x){sum(x)/length(x)};

然后抛出一个错误:

varfun(@mymean, dataTable)
Error: "mymean" was previously used as a variable, conflicting with its use here as the name of a function or command.
See "How MATLAB Recognizes Command Syntax" in the MATLAB documentation for details.

然而，如果我不使用 at 符号，我会得到:

Yet, if I do not use the at symbol, I get:

varfun(mymean, dataTable)

ans = 

    Fun_sampleId    Fun_entity
    ____________    __________

    [2]             [2.6667]  

我觉得我一定是在错误的上下文中使用了函数句柄 @.任何人都可以启发我吗?(请注意，如注释中所述，ans 的显示很奇怪，因为 mymean 返回一个元胞数组.这是一个无意的错误.

I feel like I must be using the function handle @ in the wrong context. Can anyone enlighten me? (Remark, as noted in the comments the display of ans is strange because mymean returns a cell array. This was an unintentional error).

推荐答案

在第一个代码片段中，mean 是一个 (命名)函数，而 @mean 是一个 函数句柄到该函数.你可以等效地使用

In the first code snippet, mean is a (named) function, and @mean is a function handle to that function. You could equivalently use

f = @mean;
varfun(f, dataTable)

在第二种情况下，当你定义

In the second case, when you define

mymean = @(x){sum(x)/length(x)};

@(x){sum(x)/length(x)} 部分是 匿名函数，变量 mymean 又是一个 函数句柄那个(匿名)函数.所以你需要使用 varfun(mymean, dataTable)，而不是 varfun(@mymean, dataTable).

the @(x){sum(x)/length(x)}part is an anonymous function, and the variable mymean is again a function handle to that (anonymous) function. So you need to use varfun(mymean, dataTable), not varfun(@mymean, dataTable).

因此，正在使用 @ 符号 以两种不同的方式，尽管在​​这两种情况下它都会产生一个函数句柄:

So, the @ sign is being used in two different ways, although in both cases it produces a function handle:

• 案例 1:从命名函数创建函数句柄.命名函数是在其自己的文件中定义的函数.
• 案例 2:作为匿名函数定义的一部分.匿名函数是直接定义的，而不是在单独的文件中.该定义构造了一个匿名函数并自动返回该函数的句柄.

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