Matlab 中的 N 维数组索引：在中间找到数组

Question

I am working with N-dimensional array and have a problem with the array indexing. I have a task to find an (N-1)-dimensional array in the middle N-dimensional array.

Let me explain in detail with 3D array. A is a 3-dimensional array that has split into groups. In each group, there are b - number of 2-dimensional arrays in the group. I have simulated it as:

b=5;
A=rand(2,2,20);
groups = reshape(A, size(A,1), size(A,2),b, []);

groups is 4-dimensional array, the 4-th dimension is a number of groups ( here it 4).

To find a middle in each group I have added the following loop:

for ii=1:size(groups,4)  % Loop over all groups/slices
     middle(:,:,ii) = groups(:,:,(w-1)/2+1,ii);  % 1 2 3 4 5 : the middle is 3
end

middle is 3-dimensional array that collects middle array in each group.

As you see in my example I have used b=5 ( odd number). My problem is with even number b .

I have tried to implement it as ( rewrite the loop above);

l=rem(w,2);
 for ii=1:size(groups,4)  % Loop over all groups/slices
     if l==1
         middle(:,:,ii) = groups(:,:,(w-1)/2+1,ii);  
     else
         middle(:,:,ii) = groups(:,:,(w-1)/2,ii);  
     end
 end

But it doesn't work. Matlab gives me an error in the line l=rem(w,2); Could you suggest to me how I can fix it? Is there another way to implement it?

Answer 1

You should use floor of ceil to round the index to whichever element you want:

middle_index = floor((w-1)/2+1);

Here, the middle of 4 is 2, using ceil you'd pick index 3.

Next, you can extract the arrays in a single indexing operation:

middle = groups(:,:,middle_index,:);

Finally, use squeeze or reshape to get rid of the 3rd index:

middle = squeeze(middle);