Matlab 中“结束"的语义是什么? [英] What are the semantics of 'end' in Matlab?

问题描述

在 Matlab 中使用 end 关键字作为访问或扩展数组的快捷方式是很常见的，如

It's common to use the end keyword as a shortcut for accessing or extending an array in Matlab, as in

>> x = [1,2,3];
>> x(1:end-1)
ans =
    1   2
>> x(end+1) = 4
x =
    1   2   3   4

然而，我惊讶地发现以下也有效

However, I was surprised to find that the following also works

>> x(1:min(5, end))
ans =
    1   2   3   4

我认为 end 可能是一种特殊形式，例如 :，可以在索引操作中进行特殊处理，因此我创建了一个类来检测这一点

I thought that end might be a special form, like :, that can be special-cased in indexing operations, so I created a class to detect this

classdef IndexDisplayer
  methods
    function subsref(self, s)
      disp(s);
    end
  end
end

您可以在以下示例中看到 : 的特殊情况

You can see how : is special cased in the following example

>> a = IndexDisplayer;
>> a(1:3)
    type: '()'
    subs: {[1 2 3]}
>> a(:)
    type: '()'
    subs: {':'}

但是，当我用 end 索引时，我只看到

However, when I index with end I just see

>> a(end)
    type: '()'
    subs: {[1]}

这里的 end 被替换为 1.1 从哪里来?我的第一个猜测是索引表达式 x(end) 中的任何 end 都将被替换为对 length(x) 的调用，所以我尝试覆盖 length 以及

Here the end is replaced with a 1. Where does that 1 come from? My first guess was that any end inside an indexing expression x(end) would be replaced with a call to length(x) so I tried overriding length as well

classdef IndexDisplayer
  methods
    function subsref(self, s)
      disp(s);
    end
    function len = length(self)
      len = 10;
    end
  end
end

然而，这给了

>> a = IndexDisplayer;
>> length(a)
ans =
    10
>> a(end)
    type: '()'
    subs: {[1]}

所以这个理论是窗外的.谁能解释一下end的语义?

so that theory is out the window. Can anyone explain the semantics of end?

推荐答案

首先，我认为您的语法 x(1:min(5, end)) 完全有效.当我在 MathWorks 时，我记得有人指出了这一点，很多开发人员不得不花一些时间弄清楚发生了什么.我不确定他们是否真的同意这是否有问题.

Firstly, I think it's kind of a bug, or at least an unexpected feature, that your syntax x(1:min(5, end)) works at all. When I was at MathWorks, I remember someone pointing this out, and quite a few of the developers had to spend a while figuring out what was going on. I'm not sure if they ever really agreed whether it was a problem or not.

解释end的(预期)语义:end被实现为一个函数ind = end(obj, k, n).k 是包含end 的表达式的索引，n 是表达式中索引的总数.

To explain the (intended) semantics of end: end is implemented as a function ind = end(obj, k, n). k is the index of the expression containing end, and n is the total number of indices in the expression.

因此，例如，当您调用 a(1,end,1) 时，k 为 2，因为 end 在参数 2，并且 n 是 3，因为有 3 个参数.

So, for example, when you call a(1,end,1), k is 2, as the end is in argument 2, and n is 3 as there are 3 arguments.

ind 返回作为可以替换表达式中end 的索引.

ind is returned as the index that can replace end in the expression.

你可以为你自己的类重载end(就像你可以重载colon、size、subsref 等).

You can overload end for your own classes (in the same way as you can overload colon, size, subsref etc).

扩展您的示例:

classdef IndexDisplayer
  methods
    function ind = end(self,k,n)
        disp(k)
        disp(n)
        ind = builtin('end', self, k, n);
    end
  end
end

>> a = IndexDisplayer;
>> a(1,end,1)
 2
 3

请参阅此处了解更多信息.

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