Find the average point with the four points surround it, using For-loop and function

Hi guys,

I had trouble on getting the average point with the four points surround it. Matlab said there is a error on a(i,n) = 0.2*(a(i-1,n) + a(i+1,n) + a(i,n+1) + a(i,n-1) + a(i,n)) and I can't get rid of it. Can anyone help me up with that? And also, for setting the four boundaries of the array, a, to zero, I use the zeros() function, I think I might also get wrong with that. I made some comment on it and hope it helps.

function averahepoint(k,n)
% k is the number of time steps
% n is the size of a square n x n matrix 

a = peaks(n); % make an arbitrary square matrix
a = zeros(k); % Set the four boundaries of the array, a, to zero. 
% It means every element in the array where the first index equals 1 or n, 
% or the second index equals 1 or n must be set equal to zero.

%  A loop that performs statements the number of times you entered as an input to your
% function.

for i = 1:k
    a(i,n) = 0.2*(a(i-1,n) + a(i+1,n) + a(i,n+1) + a(i,n-1) + a(i,n))
    % average each point in array a excluding the points on the boundaries with the four points
    % that surround it
    % For example a(23,60) = 0.2*(a(22,60) + a(24,60) + a(23,61) + a(23,69) + a(23,60))
    disp(i)

      if mod(i,10)==0  % Every 10 iterations ,make a filled contour plot of the array
          disp('Hello')
      end
  end

end

[Adam]

n is the size of your a array so trying to index into it with n + 1 will clearly be outside the range of the array. I'm not really sure what you are aiming to do with that statement.

a = zeros(k)

will just totally overwrite the matrix you created on the line above it though too so if k < n that would be another reason for the failure.

[david Chan]

I tried to set the four boundaries of the array, a, to zero. Like, the first index equals 1 or n, or the second index equals 1 or n must be set equal to zero

Answer 1

[Image Analyst]

Try using two loops (one over rows and one over columns), and have a separate matrix for the output called averagedMatrix, which you will then return:

function averagedMatrix = averagepoint(k,n)
% k is the number of time steps
% n is the size of a square n x n matrix 

a = peaks(n); % make an arbitrary square matrix
averagedMatrix = zeros(k); % Set the four boundaries of the array, a, to zero. 
% It means every element in the array where the first index equals 1 or n, 
% or the second index equals 1 or n must be set equal to zero.
%  A loop that performs statements the number of times you entered as an input to your function.

[rows, columns] = size(a);
for col = 2 : columns - 1
    for row = 2 : rows - 1
        averagedMatrix(row, col) = 0.2 * (a(row-1, col-1) + ......

[david Chan]

thanks!