What does this function do in matlab?

Say I have D = D(somevector);

and A = A(:,somevector);

Does the former make this a column? What is it doing if not? Does the latter make a matrix A in some arrangement?

Please provide a link to where I can read more about this. I have been web surfing for too long.

Cheers, Neo!

Answer 1

[James Tursa]

See the link on indexing:

http://www.mathworks.com/help/matlab/math/matrix-indexing.html

In particular, if you supply only one subscript as in your first example, then MATLAB will use what is known as "Linear Indexing" in the link, and give you a column vector as a result with the elements being composed of the linear indexed elements you specified with somevector.

In your second example, you supplied two indexes. The result will be a sub-matrix composed of all the rows (via the : supplied for the first index), and the columns supplied by your somevector.

[Walter Roberson]

The first syntax

D = D(somevector)

would typically be used for either selecting only a portion of D, or for re-arranging D. For example,

D = D([3 1 2])

would be the same as

D = [D(3), D(1), D(2)]

(except that it has some subtle issues about whether a row vector or column vector is produced.)

Occasionally, not often, you might see the same subscript repeated, like

D = D([1 1 1])

This is making copies of that element, like [D(1), D(1), D(1)], which would be like repmat(D(1),1,3) . Indeed, repmat() is programmed by repeated subscripts.

[Neo]

Thanks Walter. I would've accepted your answer but you left it as a comment!

-Cheers Neo

[Neo]

Thanks James!