Find values in 3d matrix

Question

I would like to do the equivalent of

x = [1, 0, 3; 2, 3, 0; 0, 0, 3];
[yy, xx, vals] = find(x);

where I really need the vals variable. I need all three, but vals is important. Now consider the 3d case, and flip one, so it's more interesting.

x = repmat(x, [1, 1, 3]);
x(:, :, 2) = fliplr(x(:, :, 1));

I'd like to do the same as before. I found this in several places

[yy, xx, zz] = ind2sub(size(x), find(x));

but then I don't know how to extract vals properly... I also don't really care about zz , but I'm sure they somehow need to be used for indexing.

Any help would be appreciated.

Answer 1

find with one output argument, as you used in your last statement:

[yy, xx, zz] = ind2sub(size(x), find(x));

returns linear indices into the matrix. You can use these to index:

index = find(x);
vals = x(index);
[xx,yy,zz] = ind2sub(size(x), index);

Answer 2

I'm not sure I've understood what you want to achieve, nevertheless, considering your last matrix x

x = [1, 0, 3; 2, 3, 0; 0, 0, 3]
z = repmat(x, [1, 1, 3]);
x(:, :, 2) = fliplr(x(:, :, 1))

with

[yy, xx, vals] = find(x)

you have:

• yy the indices of the rows of the found elements
• xx the indices of the columns of the found elements

then you can use

lin_idx=sub2ind(size(x),yy,xx)

to get the linear indices of the values inside the matrix x

now you can use

[a,b,c]=ind2sub(size(x),lin_idx)

to get the 3D indices of the elements in the matrix

You can access the values using that indices:

for i=1:length(a)
   k(i)=x(a(i),b(i),c(i))
end

Now the array k contains the values (as per the array vals returned by find ).