Help with element manipulation of matrix

So the question I have been asked to do is:

Write a function called digit_compare.m which takes as input two 28 × 28 matrices, and returns the sum of the squares of the differences between the corresponding elements.

Here is what I have done so far. What did I do wrong?

function sum=digit_compare(X,Z)
X=zeros(28);
Z=zeros(28);
n=0;
sum=0;
for i=[1:28]
    for j=[1:28]
        n=((X(i,j))-(Z(i,j))).^2;
        sum=sum+n;
    end
end
end

Answer 1

[Walter Roberson]

You did not take into account the possibility that the inputs might be unsigned integers. When you subtract a larger unsigned integer from a smaller unsigned integer, the result is 0 rather than a negative number.

Also, you overwrite all user-provided inputs with zeros.