What does this array operation do?

I have two 2D square arrays of the same size, array1 and array 2. What does this operation do?

x = array1(array2)

Answer 1

[Image Analyst]

If array2 is a logical (binary) array then it takes those elements from array1 that are 1 in array2.

For example

array1 = magic(5)

array1 = 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

array2 = false(5);
array2(2, 3) = true;
array2(3, 1) = true

array2 = 5×5 logical array
   0   0   0   0   0
   0   0   1   0   0
   1   0   0   0   0
   0   0   0   0   0
   0   0   0   0   0

x = array1(array2) % This will be a vector, not a 2-D array.

x = 2×1

4 7

So see how it took the (3,1) and (2,3) element and strung them together into a vector?

On the other hand if the array2 is an integer array, it will use those as linear indexes into the array 1.

The linear indexes are in column major order (down rows first, them move across columns):

1 6 11 16 21

2 7 12 17 22

3 8 13 18 23

4 9 14 19 24

5 10 15 20 25

So if array 2 was

array2 = [2, 7; 17,22]

array2 = 2×2

2 7 17 22

x = array1(array2)

x = 2×2

23 5 14 16

It will take those elements from row 2 that had those linear indexes.

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[Eric]

Thank you. That answers my question!

Answer 2

[KSSV]

1. If array2 is double and have fractions it will throw error.
2. If array2 is logical i.e. it has 0 and 1's. It will give elements of array1 where array2 is 1.
3. If array2 has all integers, it will give the respective elements. As array2 acts like indecing.

[Eric]

Thank you. That answers my question.