# How do I create a matrix surrounding the center element?

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J AI on 4 Nov 2019
Edited: Bhaskar R on 4 Nov 2019
I want to create a matrix (n by n, n being odd) that has its central element fixed, and its surrounding elements increasing/decreasing by some constant value. For example: where my center element is 0 and the surrounding elements are decrementing by 0.1. I am pretty much blank from where to start exactly. Your time and help is highly appreciated.

Shubham Gupta on 4 Nov 2019
Try:
n = 5;
A = zeros(n);
[r,c] = find(A==0);
sub = [r,c]-(n+1)/2;
for i = 1:(n-1)/2
ind = find((sub(:,1)==-i&sub(:,2)>=-i&sub(:,2)<=i)|(sub(:,1)==i&sub(:,2)>=-i&sub(:,2)<=i)|(sub(:,2)==-i&sub(:,1)>=-i&sub(:,1)<=i)|(sub(:,2)==i&sub(:,1)>=-i&sub(:,1)<=i));
A(ind) = -0.1*i;
end
Let me know if you have doubts !

#### 1 Comment

J AI on 4 Nov 2019
Thanks a lot!

Bhaskar R on 4 Nov 2019
Edited: Bhaskar R on 4 Nov 2019
% works for only odd numbers as your requirement
n = 5; % matrix size
r = (n-1)/2; % surrounding rows
x = zeros(n); % array initialization
c = r-1:-1:0;
% assigning values
for i = 1:r
x([1+c(i), end-c(i)], :) = -i/10;
x(:,[1+c(i), end-c(i)]) = -i/10;
end
x % final matrix