如何在MATLAB中使用2-D掩模索引3-D矩阵？ [英] How can I index a 3-D matrix with a 2-D mask in MATLAB?

问题描述

假设我有D，一个X-by-Y-by-Z数据矩阵。我也有M，一个X-by-Y掩蔽矩阵。我的目标是当M中的（Xi，Yi）为假时将D中的元素（Xi，Yi，:)设置为NaN。

Suppose I have D, an X-by-Y-by-Z data matrix. I also have M, an X-by-Y "masking" matrix. My goal is to set the elements (Xi,Yi,:) in D to NaN when (Xi,Yi) in M is false.

有什么办法可以避免在循环中这样做？我尝试使用 ind2sub ，但失败了：

Is there any way to avoid doing this in a loop? I tried using ind2sub, but that fails:

M = logical(round(rand(3,3))); % mask
D = randn(3,3,2); % data

% try getting x,y pairs of elements to be masked
[x,y] = ind2sub(size(M),find(M == 0));
D_masked = D;
D_masked(x,y,:) = NaN; % does not work!

% do it the old-fashioned way
D_masked = D;
for iX = 1:size(M,1)
    for iY = 1:size(M,2)
        if ~M(iX,iY), D_masked(iX,iY,:) = NaN; end
    end
end

我怀疑我在这里遗漏了一些明显的东西。 （：

I suspect I'm missing something obvious here. (:

推荐答案

您可以通过复制逻辑掩码来实现此目的 M 使用 REPMAT 跨越第三维度，以便它是与 D 相同的大小。然后，索引离开：

You can do this by replicating your logical mask M across the third dimension using REPMAT so that it is the same size as D. Then, index away:

D_masked = D;
D_masked(repmat(~M,[1 1 size(D,3)])) = NaN;

如果不希望复制掩码矩阵，还有另一种选择。你可以先找到一组线性索引，其中 M 等于0，然后复制设置 size（D，3）次，然后将每组索引移动 numel（M）的倍数，这样它在第三维中为 D 的不同部分编制索引。我将在这里使用 BSXFUN ：

If replicating the mask matrix is undesirable, there is another alternative. You can first find a set of linear indices for where M equals 0, then replicate that set size(D,3) times, then shift each set of indices by a multiple of numel(M) so it indexes a different part of D in the third dimension. I'll illustrate this here using BSXFUN:

D_masked = D;
index = bsxfun(@plus,find(~M),(0:(size(D,3)-1)).*numel(M));
D_masked(index) = NaN;

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