How to multiply special elements

Hi,

I am a new with Matlab and I need some help. I need to multiply by (-1) all the elements that their row is even and their column is odd or their row is odd and their column is even. How can I do that?

Thanks you in advance

[James Tursa]

What have you done so far? Please post your current code and then we can comment on it and offer suggestions. There are a lot of ways to do this. Do you know how to write a for loop? Do you know how to determine if a row number is even or odd? Are you familiar with the function bsxfun?

[Image Analyst]

If this is a homework problem you should have Tagged it as homework below. If so, tag it and read this.

[Brian]

Our teacher told us that we can use the function ind2sub. I know that I can use mod but i dont know after to put a condition. A[i,j] if ([(i+j) mod 2]==0) A[i,j]=-A[i,j]

Answer 1

[Chad Greene]

Here's one way:

A = magic(5)
[cols,rows] = meshgrid(1:size(A,1),1:size(A,2));

A(mod(rows,2)==0&mod(cols,2)==1 | mod(cols,2)==0&mod(rows,2)==1)=-A(mod(rows,2)==0&mod(cols,2)==1 |mod(cols,2)==0&mod(rows,2)==1)

[James Tursa]

Homework is easy these days ...

Answer 2

[Image Analyst]

Simply use numel and linear indexing:

m=randi(255, 5, 6) % Create sample data.
indexesToInvert = 2 : 2 : numel(m);
m(indexesToInvert) = -m(indexesToInvert)

[Brian]

I think that if the size of the matrix is odd it's not working