Using loops in matrix

Please help. give any tips or suggestions that you can

I am trying to create a script that is 3x3 matrix of random integers between 1 and 4. But, I need this matrix to produces the output matrix B which is also a 3x3 matrix. Calculates the elements of matrix B such that: the values of B along the diagonal are equal to the respective elements in A and the values in each element of B not along the diagonal are equal to the sum of the values in the respective row and column of A.

I have the following for A, but am confused on how to do the loop to create B.

A= randi (4,3,3);

[James Tursa]

Do you know how to write for-loops? Do you know how to index into a variable? Do you know the syntax for a row or a column of a matrix?

[Grace S]

I know the basic concept of these things, I'm just struggling to know how to put them all together

[James Tursa]

An outline to get you started for the B part using for-loops:

for i=1:3
    for j=1:3
        if( i==j )
            B(i,j) = _____; % on diagonal
        else
            B(i,j) = _____; % off diagonal
        end
    end
end

You need to fill in the blanks above. Figure out what the expression on the right hand side should be given that the row index is i and the column index is j. You might want to use more than one line of code to get this expression.

[Grace S]

Does this make the script generalizable, such that the input matrix A can be a square matrix of any size? If not, how do I achieve this?

[James Tursa]

To generalize it for a 2D matrix of any size:

[M,N] = size(A); % get the row and column size of A
for i=1:M % run the row loop up to M
    for j=1:N % run the column loop up to N
        :
      etc.

[Grace S]

So I tried working on this on my own without using your help just to see if I could try to understand it a little better. I came up with this.

A = [3 2 4;1 4 1;1 2 4];

B = zeros(3);

for i = 1:size(A,1)

for j = 1:size(A,2)

if i ==j % diagonal elements

B(i,j) = A(i,j);

else % off diagonal elements

B(i,j) = sum(A(i,:))+sum(A(:,j)) - A(i,j);

end

end

end

If this is wrong or I should change anything please let me know. Also could you explain the part about making your script generalized again because I still can't quite figure that part out. I copied and pasted from my assignment what is asked of us from that and put it below. Thank you for all your help!!

Problem 1 Stretch Goal (5 points) Make your script generalizable such that the input matrix A can be a square matrix of any size, not just a 3x3, and the script will work as desired without errors. To receive credit for these 5 points, create a new variable N as the first line of code; where N defines the size of matrix A (the default from above being that N = 3). First, change the code so that A is created using N, then change your code that determines B and generalize it to work for any N.

Answer 1

[James Tursa]

So your latest post is a really good effort! A couple of things you are missing from the instructions:

"create a new variable N as the first line of code"

So you first line of code should be:

N = 3; % or whatever value you want, or maybe use the input( ) function here

"N defines the size of matrix A"

"A is created using N"

So you need to create A using what you had in your original post but using N instead of 3, namely

A = randi(4,3,3); % change this to use N instead of hard coding 3

"change your code that determines B and generalize it to work for any N"

So change this line to generalize it:

B = zeros(3); % change this to use N instead of hard coding 3

The entire instructions about generalizing the code are basically to use N instead of 3. And also since you are using size(A,1) and size(A,2) that is generalizing also (although because of how you created A you could have used N here instead of size(A,1) and size(A,2) ... either one is correct).

[Grace S]

Okay that makes a lot of sense. I'll make those changes. Thank you so much for all your help, I really appreciate it.

Answer 2

[David Hill]

A=randi(4,3);
[b,c]=meshgrid(sum(A),sum(A,2));
B=(b+c-A).*(~eye(3))+diag(diag(A));

[James Tursa]

Seriously? This is a beginning programming homework assignment ...