Array indices must be positive integers or logical values

I am using the below code. When it gets to the line "left(j) = u - U(i-j+1)", it shows error "Array indices must be positive integers or logical values." How should I solve this issue? Any help would be appreciated!

clear all;
clc;
p=2
U = [0 0 0 1 2 3 3 3]
u=1
i=4
N = zeros(1,p+1)
left = zeros(1,p)
right = zeros(1,p)
N(1) = 1
for j=1:p
    left(j)  = u - U(i-j+1)
    right(j) = U(i+j) - u
    saved = 0
    for r = 0:j-1
        temp = N(r+1)/(right(r+1) + left(j-r))
        N(r+1) = saved+right(r+1)*temp
        saved = left(j-r)*temp
    end
    N(j+1) = saved
end

[Geoff Hayes]

Hossein - the above code seems to run fine for me...though I suppose if you chose different p and/or i then I can see how i-j+1 may be zero or negative. Since p is 2 and i is 4 then your only indices are 4 and 3 which are valid...but perhaps you want to make use of all the elements of U?

[CS]

Thanks for quick response! I am trying to use the below functions to drive the basisfunction of a bspline. The functions are:

For finding the i, which then will be used in the basisfuns(p,i,u,U)

function [i] = findspan(p,U,u)

%return index i for knotspan, w.r.t. value u
%p degree, U knotvector, u value
%Piegl, Les and Tiller, Wayne. The NURBS book (2nd ed.)
%page 68

n = size(U,2) - (p+1);
if u == U(n+1)
    i = n;
    return
end

low = p;
high = n+1;
mid = ceil((low + high) / 2);
while u < U(mid) || u >= U(mid+1)
    if u < U(mid)
        high = mid;
    else
        low = mid;
    end
    mid = ceil((low + high) /2);
end

i = mid;

end

The basisfuns(p,i,u,U) is as

function [N] = basisfuns(p,i,u,U)

%returns functionsvalues of basisfunctions in point u
%p degree, i index,u value, U knotvector
%Piegl, Les and Tiller, Wayne. The NURBS book (2nd ed.)
%page 70

N = zeros(1,p+1);
left = zeros(1,p);
right = zeros(1,p);
N(1) = 1;

for j=1:p
    left(j)  = u - U(i-j+1);
    right(j) = U(i+j) - u;
    saved = 0;
    for r = 0:j-1
        temp = N(r+1)/(right(r+1) + left(j-r));
        N(r+1) = saved+right(r+1)*temp;
        saved = left(j-r)*temp;
    end
    N(j+1) = saved;
end

As can be seen, the values for u, i, and U will change accordingly.

My questions:

1) In the above two functions, the value of u should be given as an input. How abuot if I want to have the value of u as

u=0:5

?

2) Also, the formula for basisfuns is not recursive. How can I make it recursive to be like the following formula?

Your help is appreciated!