matlab和数组中的索引问题

Question

Here is a simplified example

x = reshape(1:2*4*3,2,4,3); % 3d array
i1 = [1 1 2]; % index in dim 1
i2 = [2 1 3]; % index in dim 2

I want to extract x(i1(ii),i2(ii),:) , namely x(1,2,:); x(1,1,:); x(2,3,:) x(1,2,:); x(1,1,:); x(2,3,:) x(1,2,:); x(1,1,:); x(2,3,:) , which is of size [3,3].

But x(i1,i2,:) gives a 3d array of size [3,3,3].

So I used a stupid loop

y = nan(length(i1),3);
for ii = 1 : 3
    y(ii,:) = squeeze(x(i1(ii),i2(ii),:));
end

which gives

  3 11 19 1 9 17 6 14 22 

I believe this must not be the fancy way.

Answer 1

You can reshape x to a [8 * 3] matrix and convert subscripts to linear indexes:

x = reshape(x,[],3);
y = x(sub2ind([2 4],i1,i2),:)