A singleton dimension contains only 1 index in that dimension, e.g., A(3:5,17,2:3) has the 2nd dimension as a singleton since the size of this array would be 3×1x2.

–Loren

MATLAB doesn’t currently support the syntax you’d like to use. You must do the variable assignment and then index into the result.

–Loren

The part of code I can’t get working is ‘randperm(end)(1:3)’. I want the first three random elements from matrix A. I could get the same result with the code:
A = [21 23 24 25 26];
idx = randperm(length(A));
B = A( idx(1:3) );
but I would prefer not to use the additional variable ‘idx’.

Thanks in advance,
Pieter Laurens

Glad you were able to get your code working. Perhaps it would be worth posting the new version here for people to compare.

I fixed your code by reinserting the > and < . However, it doesn't look identical to the code you posted to the newsgroup (an if-end chunk missing). FYI, the software doesn't remove >,< if they are inside the html pre tags.

–Loren

incidentally, the code i posted worked fine, i just thought it was ugly and knew you would know of a better way. it does have comments that describe its specification and i think they are pretty simple, plus i posted a reference to the matlab documentation that explains the algorithm. i did not know that pre tags were necessary to prevent the blog software from mangling code (i guess less than and greater than signs are dangerous on the web) — i merely pasted in text from my m-file editor. anyway, the code pasted fine into that matlab forum at:
http://newsreader.mathworks.com/WebX?14@896.8e9dbxh5WSs@.ef47e04

it looks like this blog software removes anything between a less than or equal sign and the next greater than sign (even if that is several lines later) from your readers’ posts. just fyi.

thanks again,
-erik

Your code has no comments or examples. I plan to pass on reading it to try to understand it. I put some html markers surrounding the code ( < pre > and < /pre > but without the spaces) to make the code you placed there more readable in case someone else wants to tackle it. Plus it appears to be invalid code the way you typed it in, e.g., any(inds(:)maxes(:)) and some mlint warnings that could get cleaned up to help a reader and the code. Finally, you are allowed to index into an array with more indices than the value returned by ndims provided all the extra indices are 1.

You may also benefit from reading one of the more recent posts on using the : operator, though I don’t know if it is actually applicable here: http://blogs.mathworks.com/loren/?p=65

–Loren

function out=lookup(a,inds)
%a is an n-d array
%inds is an m x n matrix
%out is an 1 x m vector containing the entries in a
%specified by the coordinates in each row of inds

if length(size(a))==2 && any(size(a)==1)
    out=a(inds);
else
    maxes=repmat(size(a),size(inds,1),1);
    if size(inds,2)~=length(size(a))
        error('wrong num cols in inds')
    end
    if any(inds(:)>maxes(:))
        ers=find(inds>maxes)
        inds(ers)
        maxes(1,:)
        size(a)
        error('got inds > size(a)')
    end

    factors=ones(size(inds,1),1);
    offsets=zeros(size(inds,1),1);
    sz=size(a);
    for i=1:length(sz)-1
        factors(:,i+1)=prod(sz(1:i));
        offsets(:,i+1)=-1;
    end
    out=a(sum([(inds+offsets).*factors]'));
end

I recommend you look at the comments by Erik and me in this blog article. You’ll see examples where cellfun and arrayfun can do what you need.

–Loren