Adding values to 3d matrix

Dear all,

I have an emtpy 16x16x16 matrix, which i'm trying to update on the following locations:

x = [1,16,16]
y = [1,8,16]
z = [2,4,16]

The values are:

values =[1, 2, 1]

I have tried to do this as such:

matrix(x,y,z) = values

However, I get the following error:

Assignment has fewer non-singleton rhs dimensions than non-singleton subscripts

What am I missing here?

Any help would be greatly appreciated

Cheers

Answer 1

[Image Analyst]

That's not the way indexing works. It will go through every possible permutation of x, y, and z and then will expect to set it to a SINGLE value. Essentially you are specifying a rectangular block in the 3D matrix - a chunk out of the larger full 3D block. See this small demo:

m=rand(6,6,6)
m(1:3,2:4,3:5) = 0

Look where it put the zeros - not in just 3 elements, but in a whole block. So each element (in imaging we call it a voxel) should have one value, but you're trying to set three values equal to that value, not 1. If you don't want all permutations, use a for loop to take just one element from values and set it to the same index from x, y, and z, like this:

for k = 1 : length(x)
  matrix(x(k), y(k), z(k)) = values(k);
end

[B]

Thanks for your reply,

I was hoping I could do it without a for loop, but it seems there is no way around it.

[Image Analyst]

Don't worry about it. For only 3 iterations, it's not really a consideration - it'll be faster than a rocket powered cheetah on steroids. But if it were tens of thousands or millions of iterations, then you might try Roger's method - test it both ways to see which is faster.

Answer 2

[Roger Stafford]

There is a way around for-loops using 'sub2ind'.

 M = zeros(16,16,16);
 M(sub2ind(size(M),x,y,z)) = values;

Roger Stafford