使用向量作为矩阵的索引 [英] Use a vector as an index to a matrix

问题描述

我正在编写一个MATLAB函数来将数据读出到一个n维数组（可变维度大小）。我需要能够访问Matrix中的特定点（例如，写入或读取它），但我不知道要指定多少索引。

I'm writing a MATLAB function to read out data into an n-dimensional array (variable dimension size). I need to be able to access a specific point in the Matrix (to write to it or read it, for example), but I don't know ahead of time how many indexes to specify.

目前我有一个 current_point 向量，我迭代指定每个索引，并且 max_points vector，指定数组的大小。所以，例如，如果我想要一个尺寸为1000×15×3， max_points = [1000 15 3] 和<$ c $的三维数组c> current_point 从 [1,1,1] 迭代到 [1000,15,3] （ [1,1,1] - > [1000,1,1] - > [1,2,1] - > [1000,2,1] - > ...）。我希望能够做的是将 current_point 作为矩阵的索引，如下所示：

Currently I have a current_point vector which I iterate through to specify each index, and a max_points vector which specifies the size of the array. So, if for example I wanted a 3-dimensional array of size 1000-by-15-by-3, max_points = [1000 15 3], and current_point iterates from [1, 1, 1] to [1000, 15, 3] ([1, 1, 1] -> [1000, 1, 1] -> [1, 2, 1] -> [1000, 2, 1] ->...). What I'd like to be able to do is feed current_point as an index to the matrix like so:

output_matrix(current_point) = val

但显然类似 output_matrix（[1 2 3]）= val 将只设置 outputmatrix（1：3）= 30 。我不能只使用虚拟变量，因为有时矩阵需要3个索引，其他时间4，其他时间2等，所以我需要一个可变长度的矢量。有一种使用一个向量作为点中的索引以简单的方式？

But apparently something like output_matrix([1 2 3]) = val will just set outputmatrix(1:3) = 30. I can't just use dummy variables because sometimes the matrix will need 3 indexes, other times 4, other times 2, etc, so a vector of variable length is really what I need here. Is there a simple way to use a vector as the points in an index?

推荐答案

使用功能 sub2ind 来创建线性索引是此问题的典型解决方案，如显示在此密切相关的问题中。您也可以自己计算线性索引，而不是调用 sub2ind 。

Using the function sub2ind to create a linear index is the typical solution to this problem, as shown in this closely-related question. You could also compute a linear index yourself instead of calling sub2ind.

但是，您的案例可能比我链接的其他问题更简单。如果你只是用你的 current_point 向量索引单个点（也就是说它只是你的n维下标的n元素向量）矩阵），然后您可以使用一个简单的解决方案，您可以使用函数 current_point 转换为下标的单元格数组/help/matlab/ref/num2cell.html 相对= nofollow noreferrer> num2cell 并使用它来创建以逗号分隔的索引列表。例如：

However, your case may be simpler than those in the other questions I linked to. If you're only ever indexing a single point with your current_point vector (i.e. it's just an n-element vector of subscripts into your n-dimensional matrix), then you can use a simple solution where you convert current_point to a cell array of subscripts using the function num2cell and use it to create a comma-separated list of indices. For example:

current_point = [1 2 3 ...];        % A 1-by-n array of subscripts
subCell = num2cell(current_point);  % A 1-by-n cell array of subscripts
output_matrix(subCell{:}) = val;    % Update the matrix point

操作 subCell {：} 创建相当于输入 subCell {1}，subCell {2}，... ，这相当于输入 current_point（1 ），current_point（2），... 。

The operation subCell{:} creates the equivalent of typing subCell{1}, subCell{2}, ..., which is the equivalent of typing current_point(1), current_point(2), ....

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