How to check adjacent data with logical indexing?

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Daniel H 2021-9-9

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1449679-how-to-check-adjacent-data-with-logical-indexing

评论： Daniel H 2021-9-11

How can I select matrix entries which have a certain criteria and are adjacent to entries having another criteria? (with logical indexing!)

Suppose I have a matrix and I first use logical indexing to select all elements which are greater than a certain number, say 20:

A=magic(5)
A =
    17    24     1     8    15
    23     5     7    14    16
     4     6    13    20    22
    10    12    19    21     3
    11    18    25     2     9
idx1=A>20
idx1 =
  5×5 logical array
   0   1   0   0   0
   1   0   0   0   0
   0   0   0   0   1
   0   0   0   1   0
   0   0   1   0   0

Now I would like to select all entries which (1) are adjacent to those and (2) are greater than a certain (different) number, say 18.

I can select the left and right elements with:

K>> idx1_right = circshift(idx1,1,2)
idx1_right =
  5×5 logical array
   0   0   1   0   0
   0   1   0   0   0
   1   0   0   0   0
   0   0   0   0   1
   0   0   0   1   0
K>> idx1_left = circshift(idx1,-1,2)
idx1_left =
  5×5 logical array
   1   0   0   0   0
   0   0   0   0   1
   0   0   0   1   0
   0   0   1   0   0
   0   1   0   0   0

However, how can I combine these logical index matrices with another logical query?

Like

idx2 = A(idx1_left)>18 | A(idx1_right)>18;

?

This does not work because it just A(idx1_left) returns a vector with the matching elements and hence idx2 as a whole is a vector which is incompatible with the size of A.

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Answer 1

Chunru 2021-9-9

0
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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1449679-how-to-check-adjacent-data-with-logical-indexing#answer_783804

编辑：Chunru 2021-9-9

在 MATLAB Online 中打开

A=magic(5)
A = 5×5
  24     1     8    15
   5     7    14    16
   6    13    20    22
  12    19    21     3
  18    25     2     9
idx1=A>20
idx1 = 5×5 logical array
 1   0   0   0
 0   0   0   0
 0   0   0   1
 0   0   1   0
 0   1   0   0
idx1_right = circshift(idx1,1,2)
idx1_right = 5×5 logical array
 0   1   0   0
 1   0   0   0
 0   0   0   0
 0   0   0   1
 0   0   1   0
idx1_left = circshift(idx1,-1,2)
idx1_left = 5×5 logical array
 0   0   0   0
 0   0   0   1
 0   0   1   0
 0   1   0   0
 1   0   0   0
idx2 = idx1 & (A(idx1_left)>18 | A(idx1_right)>18)  % is this what you want?
idx2 = 5×5 logical array
 0   0   0   0
 0   0   0   0
 0   0   0   1
 0   0   1   0
 0   0   0   0