在矩阵的每一行中找到前N个非零元素 [英] Find the first N non-zero elements in each row of a matrix

问题描述

我在MATLAB中有一个零的矩阵，我想在每一行中得到另一个带有第一个N非零元素的矩阵.例如，假设N = 3，矩阵为

I have a matrix in MATLAB with zeroes and I would like to get another matrix with the first N non-zero elements in each row. Let's say for example N = 3, and the matrix is

A = [ 0 0 2 0 6 7 9;
      3 2 4 7 0 0 6;
      0 1 0 3 4 8 6;
      1 2 0 0 0 1 3]

我希望结果是:

B = [2 6 7;
     3 2 4;
     1 3 4;
     1 2 1]

我有一个巨大的矩阵，所以我想不加循环地进行操作，您能帮我吗?非常感谢！

I have a huge matrix so I would like to do it without a loop, could you please help me? Thanks a lot!

推荐答案

因为MATLAB根据存储矩阵按列优先顺序，我首先转置A，将非零气泡冒出，然后选择前N行，然后转置回去:

Since MATLAB stores a matrix according to column-major order, I first transpose A, bubble up the non-zeros, and pick the first N lines, and transpose back:

N = 3;
A = [ 0 0 2 0 6 7 9;
      3 2 4 7 0 0 6;
      0 1 0 3 4 8 6;
      1 2 0 0 0 1 3];

转置并预分配输出B

At = A';
B = zeros(size(At));

At =
     0     3     0     1
     0     2     1     2
     2     4     0     0
     0     7     3     0
     6     0     4     0
     7     0     8     1
     9     6     6     3

索引零

idx = At == 0;

idx =
     1     0     1     0
     1     0     0     0
     0     0     1     1
     1     0     0     1
     0     1     0     1
     0     1     0     0
     0     0     0     0

冒泡非零

B(~sort(idx)) = At(~idx);

B =
     2     3     1     1
     6     2     3     2
     7     4     4     1
     9     7     8     3
     0     6     6     0
     0     0     0     0
     0     0     0     0

选择前N行并向后转置

B(1:N,:)'

您可以按行优先的顺序进行冒泡，但是您需要使用find检索行和列下标，并在那里进行一些排序和选择.它变得更加乏味且可读性较低.

You can do the bubbling in row-major order, but you would need to retrieve the row and column subscripts with find, and do some sorting and picking there. It becomes more tedious and less readable.

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