Manipulating matrix with 'for' loop

Create a matrix named A of 1s with 4 columns and 6 rows. Loop the matrix and assign each element a new value using the following rules: assign 2 on the main diagonal and -1 on the adjacent diagonals.

No idea how to solve this with 'for' loop, but this is what homework quesiton asks for. Any help is much appreciated.

[Guillaume]

How would you do this manually? Most likely, this would be the same method you'd use with a loop.

However, I do agree that if that's the exact wording of the assignment it's very poorly worded. For a start, 'loop the matrix' is not even proper english. Secondly, it doesn't explain what you should loop over, I assume you're expected to write a double for loop over the rows and columns, but you could also loop over the diagonals.

Of course, the proper way of doing this in matlab is without a loop, so the pedagogical aspect of that homework is questionable (in my opinion). Here's what I'd do:

for i = pi  %pointless loop that only does one iteration. Only here because the assignment asks for a loop
    A = toeplitz([2, -1, ones(1, 4)], [2, -1, ones(1, 2)]);  %can be achieved many different ways in just one line
end

This is likely to get you a fail however even if it does produce the correct result.

[Rik]

I would have given you bonus points for that loop. It's cheeky and shows understanding of Matlab. If I were to ask such a question I would always follow up with 'and how can you improve the performance (hint: array operations tend to be much faster)'.

Answer 1

[Koushik Vemula]

According to what I understand you want to change the values of 'main diagonal’ and ‘adjacent diagonals’ of the given matrix.

You can do it in the following manner :

A = ones(6, 4) 
for i = 1:6 
    for j=1:4 
        if i==j 
            A(i,j) = 2; 
        elseif abs(i-j) == 1 
                A(i,j) = -1; 
        end 
    end 
end 
disp(A)