Read elements on one side of matrix diagonal into a 1D array

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Jordan 2021-6-21

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https://ww2.mathworks.cn/matlabcentral/answers/861850-read-elements-on-one-side-of-matrix-diagonal-into-a-1d-array

评论： Scott MacKenzie 2021-6-22

I would like to read a 2D nxn matrix into a 1D array with only one column. I would like only values on one side of the matrix's diagonal to be put into the resulting array. For example, if there are rows A-D, and columns 1-4 the resulting 1D array would be:

A2

A3

A4

B3

B4

C4

Is there a straightforward way of achieving this? Thank you.

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Answer 1

Scott MacKenzie 2021-6-21

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https://ww2.mathworks.cn/matlabcentral/answers/861850-read-elements-on-one-side-of-matrix-diagonal-into-a-1d-array#answer_730055

编辑：Scott MacKenzie 2021-6-21

在 MATLAB Online 中打开

There might be a tighter solution, but I think this achieves what you are after:

a = magic(4)
b = a';
c = logical(triu(ones(4),1));
d = b(c')

Output:

a =
    16     2     3    13
     5    11    10     8
     9     7     6    12
     4    14    15     1
d =
     2
     3
    13
    10
     8
    12

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Jordan 2021-6-22

Is this solution also applicable to many different matrices stacked in the z-direction of a 3D array? Then converting the 3D array to a 2D array but doing so with this same process on each individual matrix composing the 3D array? Thank you so much!

Scott MacKenzie 2021-6-22

在 MATLAB Online 中打开

The solution is inherently 2D. But, of course, it would work fine in other contexts, such as on a flattened 3D matrix or on individual 2D layers along any dimension of an nD matrix. It will also work, a minor tweak, on rectangular matricies, for example

a = [0 1 2 3 4; 5 6 7 8 9; 4 3 2 1 0]
b = a';
c = logical(triu(ones(size(a)),1));
d = b(c')

Output:

a =
     0     1     2     3     4
     5     6     7     8     9
     4     3     2     1     0
d =
     1
     2
     3
     4
     7
     8
     9
     1
     0