Why am I getting the error "Array indices must be positive integers or logical values"?

I have the following code. I am unsure as to why I am recieving this error when the array indices is always positive as far as I can tell.

function[f] = ntdd(x,f)

n = length(f);

for i = 2:n
    for j = n:-1:1
        f(j) = ((f(j)-f(j-1))/(x(j)-x(j-(i+1))));
    end 
end

end

[madhan ravi]

whats x and f?

[Siddart Fredrick]

x and f are vectors.

Both x and f are of the same length.

[madhan ravi]

If you illustrate with an example and your desired output it would be clear what you are doing.

Answer 1

[Kevin Phung]

for i = 2:n
    for j = n:-1:1
        f(j) = ((f(j)-f(j-1))/(x(j)-x(j-(i+1))));
    end 
end

looks like when j is 1, you have a f(j-1) which gives you f(0)

[Siddart Fredrick]

Thanks. It works now.