Why do I get "Array indices must be positive integers or logical values" error?

Sorry if this question is basic and the code is written badly. I am just starting out.

Why do I get "Array indices must be positive integers or logical values" error? My code is trying to tell me in which parts of the k by k by k grid on the unit square (actually torus) intersect squares of length d, with centres given by the collection of vectors u (i.e a 3 x m matrix). I'd like to get an output which is a k x k x k cell whose non - empty entries are the squares that intersect said cell

Here is my code;

function [G] = grid(u,d,k)
m = length(u);
K = zeros(3,15,length(u)); 
X = transpose(unique([(dec2bin(0:2^3-1)-'0');-(dec2bin(0:2^3-1)-'0')],'rows')) ;
Gi = cell(k,k,k);
h = zeros(m,1);
Z = cell(1,m);
%all possible combinations of standard basis multiplied by either 1 or -1
    for i = 1:15
        for j = 1:m
            K(:,i,j) = floor(k.*mod((u(:,j) + (d./2).*X(:,i)),1));
        end
    end
    for l = 1:m
        Z{l} = [unique(K(:,:,l).','rows','stable')].';
    end
    for t = 1:m
        h(t) = size(Z{t},2);
         for n = 1:h(t)
             Gi{Z{t}(1,n),Z{t}(2,n),Z{t}(3,n)} = cat(2,Gi{Z{t}(1,n),Z{t}(2,n),Z{t}(3,n)},t);
         end
    end
    G = Gi;
end

Typically I set u = rand(3,10), k=10, d=0.01. The error I keep getting is "Index in position 1 is invalid. Array indices must be positive integers or logical values." I have tried emptying my workspace and checking my idices and still I can't seem to fix this. Furthermore, if I run line with the operator "cat" ,on its own, with a toy example it works.

Many Thanks for any info.

Answer 1

[Walter Roberson]

K(:,i,j) = floor(k.*mod((u(:,j) + (d./2).*X(:,i)),1));

something mod 1 can give a result of 0. k times 0 will be 0. So K can contain entries that are zero.

Z{l} = [unique(K(:,:,l).','rows','stable')].';

If K contains zeros then unique(K) will contain zeros.

Gi{Z{t}(1,n),Z{t}(2,n),Z{t}(3,n)} = cat(2,Gi{Z{t}(1,n),Z{t}(2,n),Z{t}(3,n)},t);

You then use those zeros as indices.

[Milos]

thanks so much! This clears a lot up now.