How can i build a symmetric matrix from a vector ?

Question

sajjad 2015-8-21

1
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/235071-how-can-i-build-a-symmetric-matrix-from-a-vector

回答： Moe Money 2017-6-12

采纳的回答： Moe Money

assume i have a 1*n vector like x = [1 2 3 4 5 6], i want to reshape this vector to a symmetric matrix like

symx = [1 2 3; 2 4 5; 3 5 6];

or something like this:

symx = [1 4 6; 4 2 5; 6 5 3];

Thanks for your help

1 个评论
显示 -1更早的评论隐藏 -1更早的评论

John D'Errico 2015-8-21

Sorry, but unless you give the rules by which you have chosen to call that a "reshape" where you take 6 numbers and somehow magically decide to reshape them into a 9 element array, then nobody can help you. The mind reading toolbox is still in beta test.

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Moe Money 2017-6-12

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/235071-how-can-i-build-a-symmetric-matrix-from-a-vector#answer_270453

在 MATLAB Online 中打开

I used the functions SQUAREFORM and DIAG

Example: Imagine you want to create a symmetric 4x4 matrix. You need a vector with 10 elements.

>> n=4;

>> vec=[1:10]';

>> sqmat = diag(vec(1:n)) + squareform(vec(n+1:end))

sqmat =

   5     6     7
   2     8     9
   8     3    10
   9    10     4

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

Answer 2

Steven Lord 2015-8-21

4
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/235071-how-can-i-build-a-symmetric-matrix-from-a-vector#answer_189952

在 MATLAB Online 中打开

The SQUAREFORM function from Statistics and Machine Learning Toolbox gets close, but it puts zeros on the diagonal and fills in the upper and lower triangles, which is not quite what you're asking for but is close. In order to do what you're asking, you can use logical indexing.

 x = 1:6;
 y = zeros(3);
 upperTriangleIndices = triu(true(3));
 y(upperTriangleIndices) = x; % Fill in the upper piece
 y = y + y.' - diag(diag(y)) % Mirror the upper piece to the lower, which doubles the diagonal
                             % so subtract off the diagonal