MATLAB Matrix Sum using Nested For

Question

I am trying to find the sum of the following matrix in matlab [1 1 1 1; 1 2 1 2; 4 5 3 2; 1 3 2 4; 10 11 1 1; 90 9 2 1]

I am trying to do so using a nested for statement yet i keep getting errors. please help

Must use nested for

My code:

A = [1 1 1 1; 1 2 1 2; 4 5 3 2; 1 3 2 4; 10 11 1 1; 90 9 2 1];
for j=1:4,
  for i=1:6,
    sum = A(j,:)+A(j+1,:)+A(j+2,:)
  end
end

Answer 1

Why don't you just use sum()?

Answer 2

You will need to change your code from this:

A = [1 1 1 1; 1 2 1 2; 4 5 3 2; 1 3 2 4; 10 11 1 1; 90 9 2 1];
for j=1:4,
    for i=j:6,
        sum = A(j,:)+A(j+1,:)+A(j+2,:);
    end
end

to this:

A = [1 1 1 1; 1 2 1 2; 4 5 3 2; 1 3 2 4; 10 11 1 1; 90 9 2 1];
sum = 0;
for j=1:4,
    for i=1:6,
        sum = sum + A(j,i);
    end
end

Note various modifications:

1. Initialize sum=0. If you're using this in the interpreter, you'll be starting off with the previous result, guranteeing you don't get the right result.
2. Cumulate the values. If you assign to sum at each iteration, you'll throw away the result of other iterations.
3. There is no point in writing the outer loop if you're going to hardcode j+1, j+2, etc. in the inner loop.
4. Fix the inner loop so that it starts iterating at 1.
5. Suppress output in the inner loop by using a semicolon to get a clean result.

Answer 3

I will not post the corrected code, I'll instead add comments to the code you posted:

A = [1 1 1 1; 1 2 1 2; 4 5 3 2; 1 3 2 4; 10 11 1 1; 90 9 2 1];
% you are missing sum initialization here - you should first set sum to zero
for j=1:4, % there is no comma needed at the end
  for i=j:6,  % you want to iterate all the rows, from 1 to 6
    sum = A(j,:)+A(j+1,:)+A(j+2,:)  % you should be adding to the sum - i.e sum is sum + current field A(j, i)
  end
end