Using loops in matrix

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Grace S
Grace S 2020-11-16
评论： Grace S 2020-11-17
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);
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James Tursa
James Tursa 2020-11-16
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
Grace S 2020-11-17
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.

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回答（2 个）

James Tursa
James Tursa 2020-11-17
编辑：James Tursa 2020-11-17
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).
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Grace S
Grace S 2020-11-17
Okay that makes a lot of sense. I'll make those changes. Thank you so much for all your help, I really appreciate it.

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David Hill
David Hill 2020-11-16
A=randi(4,3);
[b,c]=meshgrid(sum(A),sum(A,2));
B=(b+c-A).*(~eye(3))+diag(diag(A));
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James Tursa
James Tursa 2020-11-16
编辑：James Tursa 2020-11-16
Seriously? This is a beginning programming homework assignment ...

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