What's the problem in my loop?

A=imread('cameraman.tif');
B=zeros(255,255);
for j=0:255
    for k=0:255
        B(j,k)=A(j,k+1)-A(j,k-1);
    end
end
figure,imshow(B);

error: Attempted to access A(0,1); index must be a positive integer or logical.

Error in tryfebin (line 6)
        B(j,k)=A(j,k+1)-A(j,k-1);

[Jan]

The error message is very meaningful: "index must be a positive integer or logical"

Answer 1

[David Sanchez]

Your code should go like this:

A=imread('cameraman.tif');
B=zeros( size(A) );
for j=1:size(A,1)
    for k=2:(size(A)-1)  %edge pixels of B matrix has to be treated differently
        B(j,k)=A(j,k+1)-A(j,k-1);  %you can not apply this algorithm to edge pixels
    end
end
figure,imshow(B);

Answer 2

[ES]

Matlab Matrices do not have indices starting at 0. There is nothing like A(0) in MATLAB (But it is so in C or Python though). This is a common issue faced by people migrating from C or Python to MATLAB.

Solution: Your for loop with j should run from 1 to 256.