matrix indexing and manipulation

Hi

(Newbie question)

I have a matrix which is Z=131x47 If I do ZA=Z(1:end); am I right in thinking the elements are rearragned columnwise? Say for example I am interested in the element ZA(1,3000), how do I know what the position of this value is in the original matrix Z?

Thanks

Answer 1

[Yoav Livneh]

You can use ind2sub.

[rowIndex, colIndex] = ind2sub(size(Z),Index);

I think this should work.

Answer 2

[Image Analyst]

You could use numel() to tell how many elements are in your Z matrix, then check if 3000 <= numel(Z).

Answer 3

[Andreas Goser]

First, you are correct that this code would arrange elements columnwise. You may have a look at the RESHAPE command for a more general way of reshaping a matrix. Using REHSHAPE or your code however, does not preserve the size of the orginal matrix. You need to store that to be able to invert the reshape.

You may also be interested to know that matrices in MATLAB can be accessed as vectors

X=rand(10,7);
X(5,4)=1 % Setting to one to spot it better
X(35) % element 4*7+5

Maybe there is a way to write your code without creating an additional vector ZA,

[dia-2011]

Thanks. I understand I can go back to the original matrix Z by using reshape(ZA,131,47);

But how can I find the row and column index of for example element 3000?

[Andreas Goser]

OK, the answer above WRT IND2SUB seems to meet your needs. My additional point was that you may be able to avoid the "conversion".