Permuting elements of matrix with a polynomial (3x^3 + 6x^2 + 7x) mod 9

A = [1 2 3; 4 5 6; 7 8 9] %original position of matrix elements

[r c] = size(A); N = 9;

B = reshape(A,1,r*c); % B = [1 4 7 2 5 8 3 6 9]

I have obtained a permute vector after calculating (3x^3 + 6x^2 + 7x) mod 9 as:

per_vec = [7 8 3 1 2 6 4 5 9]; %permute vector

result = B(per_vec) %This gives me a row vector as [3 6 7 1 4 8 2 5 9]

Could you please help me understand what does B(per_vec) does?

Answer 1

[KSSV]

You have to read about matlab's matrix indexing first. https://in.mathworks.com/company/newsletters/articles/matrix-indexing-in-matlab.html

per_vec gives you the positions/ indices. When you use B(per_vec), it arranges the elements of B as given in per_vec. You can see it yourself by comparing B and B(per_vec)

[Neha W]

Thank you Sir. I will got through it.

[Neha W]

Sir, I got how elements got arranged acc to per_vec. Now to get original matrix A back.

dec=[3 6 7 1 4 8 2 5 9]; dec(per_vec)=dec; is implemented.

Why dec(per_vec)is written on the RHS side? Is there any logic behind it?