How to check adjacent data with logical indexing?
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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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