How to check adjacent data with logical indexing?

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.

Answer 1

[Chunru]

A=magic(5)

A = 5×5

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

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

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

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   0   0   0   1
   0   0   0   1   0
   0   0   0   0   0

[Daniel H]

Oh yes, so simple. Thank you!