matrix neighbours chaning with iterations

Imagine I have matrix A=ones(5). I want to change center of the matrix to 17, A(3,3)=17; In next iteration I want to change all points that are next to this 20 to 5. i will have:

1  1  1  1  1
1  5  5  5  1
1  5 17  5  1
1  5  5  5  1
1  1  1  1  1

In next step i want to change all ones to 10. And so on...

10 10 10 10  10
10  5  5  5  10
10  5 17  5  10
10  5  5  5  10
10 10 10 10  10

Any idea how to do this to large matrix? How to adress only the neighbours but not the points "inside"?

Answer 1

[Matt J]

[M,N]=size(A);
center=size(A)/2+.5;

   [I,J]=ndgrid((1:M)-center(1), (1:N)-center(2));

   A(:) = 20-5*round(max(abs(I),abs(J)))

[Jakub]

But what if i want to calculate the value of the "edge" by some formula? I need to adress the points..

[Jakub]

And i want to have there any random numbers not just 20,15,10,... sorry