Extract numbers from Matrix and create a matrix

An M-by-M matrix X is given. Without using loops, extract values from matrix X to create the following:

%Generate an MxM matrix X. For part iv), X must have an even number of rows %and columns.

M = randi([20 40], 1);

M = M + mod(M,2);

X = magic(M);

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Maybe I just don't understand the problem.. how do I create a matrix for problem A, the dimension of matrix A is not given?

This is what I got for A, Is there a better answer than

A = a(:,1:2:end);

for problem A?

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Any body knows how to do B and C, I need some hints

Answer 1

[Amit]

For problem A, you are looking for odd rows and column, so the indexes should go for 1 with an increment of 2.

Like

A = X(1:2:end,1:2:end); % end simply represents the end

For B: you are excluding the first and last, so the indexes should exclude those.

B = X(2:end-1,2:end-1);

For C, I'd say first write down the indexes you're looking for on a piece of paper and see if you see a pattern. Post, even if it is wrong, and I'll tell you how to do it.

Sometimes it is easy to write out on a a piece of paper the indexes because it help you visualize.

[Amit]

Hint: Look for Linear Indexing

[Johnny]

Share: http://blogs.mathworks.com/steve/2008/02/08/linear-indexing/

[Amit]

So you figure it out?

Answer 2

[Johnny]

My answer for C :

firstRow = diag(ans, 1)'

secondRow = diag(ans, -1)'

C = [firstRow; secondRow]

Although it works. Probably not the most efficient way to do this. Any suggestions?

[Amit]

Your solution is a very good solution. I am glad you figured out diag. Other way would be: Lets say its a 5X5 matrix

C = [X(2:6:end)' X(6:6:end)'] % This method uses Linear indexing

Good luck in your studies and I hope you learn significantly about MATLAB Programming :)