如何在 MATLAB 中使用数组掩码缩小矩阵？

发布于 2024-08-30 09:43:23 字数 536 浏览 5 评论 0原文

这似乎是我的一个非常常见的问题：

data = [1 2 3; 4 5 6];
mask = [true false true];

mask = repmat(mask, 2, 1);

data(mask) ==> [1; 4; 3; 6]

我想要的是 [1 3; 4 6]。

是的，我可以将其重塑到正确的大小，但这似乎是错误的方法。有更好的办法吗？当数据实际上是矩形时，为什么data(mask)不返回矩阵？我知道在一般情况下可能不是，但在我的情况下，因为我的原始掩码是一个数组，所以它总是会是。

推论

感谢您的回答，我也想指出这也适用于返回数字索引的任何内容，例如 ismember、sort 或独特。

当您可以使用此概念一次完成时，我曾经从 sort 获取第二个返回值并将其手动应用于每一列。

原文

This seems to be a very common problem of mine:

data = [1 2 3; 4 5 6];
mask = [true false true];

mask = repmat(mask, 2, 1);

data(mask) ==> [1; 4; 3; 6]

What I wanted was [1 3; 4 6].

Yes I can just reshape it to the right size, but that seems the wrong way to do it. Is there a better way? Why doesn't data(mask) return a matrix when it is actually rectangular? I understand in the general case it may not be, but in my case since my original mask is an array it always will be.

Corollary

Thanks for the answer, I just also wanted to point out this also works with anything that returns a numeric index like ismember, sort, or unique.

I used to take the second return value from sort and apply it to every column manually when you can use this notion to do it one shot.

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孤星 2024-09-06 09:43:23

这将为您提供您想要的：

>> data = [1 2 3; 4 5 6];
>> mask = [true false true];
>> data(:,mask)

ans =

     1     3
     4     6

这是可行的，因为您可以简单地将逻辑索引 mask 应用于列，并使用 : 选择所有行。

即使使用二维逻辑数组作为输入，输出也将是索引值的列数组。这是因为不能保证索引元素可以组织成二维（即矩形）输出。考虑一下您的 2-D 掩码是否如下：

mask = [true false true; true false false];

这将索引 3 个值，这些值除了输出的行或列向量之外无法组织成任何内容。这是另一个示例：

mask = [true true true; true false false];

这将索引 4 个值，但 3 个值来自第一行，1 个值来自第二行。这些值应该如何形成矩形输出矩阵？由于通常对于任意二维索引矩阵没有明确的方法来执行此操作，因此返回索引值的列向量。

This will give you what you want:

>> data = [1 2 3; 4 5 6];
>> mask = [true false true];
>> data(:,mask)

ans =

     1     3
     4     6

This works because you can simply apply the logical index mask to the columns, selecting all the rows with :.

Even when a 2-D logical array is used for an input, the output will be a column array of indexed values. This is because there is no guarantee that the indexed elements can be organized into a 2-D (i.e. rectangular) output. Consider if your 2-D mask were the following:

mask = [true false true; true false false];

This would index 3 values, which can't be organized into anything but a row or column vector for the output. Here's another example:

mask = [true true true; true false false];

This would index 4 values, but 3 are from the first row and 1 is from the second row. How should these values be shaped into a rectangular output matrix? Since there's no clear way to do this in general for an arbitrary 2-D index matrix, a column vector of indexed values is returned.

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