MATLAB Answers

function-style array indexing

40 views (last 30 days)
Ben Mitch
Ben Mitch on 13 Mar 2013
Edited: per isakson on 5 Aug 2016
Hi
This problem keeps coming up for me over the years, and there must be a standard solution, so perhaps someone knows it, I can't ever google it up.
Say I have an array A of any number of dimensions, and I want to apply an indexing operation to the last dimension. Now, if it's 2D, I just use:
B = A(:, ii);
If it's 3D, I use:
B = A(:, :, ii);
but what if it's n-D? Afaik there is no syntax in the language for this, so I'd have to use some function. A function "index_into()", for instance, to give:
B = index_into(A, n, ii);
At the moment, I always end up doing some horrible mash like:
X = permute(A, [ndims(A) ...]);
X = X(ii, :);
X = reshape(X, ...);
B = permute(X, ...);
Surely there must be a better way...? Any suggestions?
Cheers

  0 Comments

Sign in to comment.

Accepted Answer

Walter Roberson
Walter Roberson on 13 Mar 2013
idxcell = repmat({':'}, 1, ndims(A));
idxcell{n} = ii;
A(idxcell{:})

  2 Comments

Daniel Shub
Daniel Shub on 13 Mar 2013
This is the exact recommendation in Loren's blog.
Ben Mitch
Ben Mitch on 13 Mar 2013
Nice, thanks. I think I had seen that index-by-comma-separated-list before, but it hadn't taken root in my mind. Wonder, though, if a syntax would be possible that made this a bit more intuitive. Just curious...
A(..., ii)?

Sign in to comment.

More Answers (0)

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by