基于行和列索引向量分配新的矩阵值 [英] assign new matrix values based on row and column index vectors

问题描述

MatLab的新功能（R2015a，Mac OS 10.10.5），并希望找到解决此索引问题的方法。

New to MatLab here (R2015a, Mac OS 10.10.5), and hoping to find a solution to this indexing problem.

我想基于一个行索引向量和一个列索引来更改大型二维矩阵的值。举个简单的例子，如果我有一个3 x 2的零矩阵：

I want to change the values of a large 2D matrix, based on one vector of row indices and one of column indices. For a very simple example, if I have a 3 x 2 matrix of zeros:

A = zeros(3, 2)

0 0
0 0
0 0

我想改变A（1,1）= 1，A（2,2）= 1，A（3,1）= 1，这样A现在是

I want to change A(1, 1) = 1, and A(2, 2) = 1, and A(3, 1) = 1, such that A is now

1 0
0 1
1 0

我想用向量来表示行和列索引：

And I want to do this using vectors to indicate the row and column indices:

rows = [1 2 3];
cols = [1 2 1];

有没有办法在没有循环的情况下执行此操作？请记住，这是一个需要在非常大的2D矩阵上工作的玩具示例。为了额外的功劳，我是否还可以添加一个向量来指示要插入的值，而不是将其固定为1？

Is there a way to do this without looping? Remember, this is a toy example that needs to work on a very large 2D matrix. For extra credit, can I also include a vector that indicates which value to insert, instead of fixing it at 1?

我的循环方法很简单，但很慢：

My looping approach is easy, but slow:

for i = 1:length(rows)

    A(rows(i), cols(i)) = 1;

end

推荐答案

sub2ind 可以在这里提供帮助，

sub2ind can help here,

A = zeros(3,2)
rows = [1 2 3];
cols = [1 2 1];

A(sub2ind(size(A),rows,cols))=1
A =

     1     0
     0     1
     1     0

带有'insert'的向量

with a vector to 'insert'

b = [1,2,3];
A(sub2ind(size(A),rows,cols))=b

A =

     1     0
     0     2
     3     0

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