Mouse Problem: Why do I keep getting the error message, "Array indices must be positive integers or logical values."?

I'm not sure why I keep getting the following error messages:

Array indices must be positive integers or logical values.

Error in hehemouse (line 13)

dx=x(mod(i+1,5))-x(i)/sqrt(((x(mod(i+1,5))-x(i))^2)+((y(mod(i+1,5))-y(i))^2));

My code is the following.

% Initialize the positions of the 5 mice 
 
x = [0, -sin(2*pi/5), -sin(4*pi/5), sin(4*pi/5), sin(2*pi/5)];
y = [1, cos(2*pi/5), -cos(pi/5), -cos(pi/5), cos(2*pi/5)];
 
% For each iteration, each mouse moves a distance d=0.01 towards neighbour ccw
% To find mouse's neighbour, just do (x+1)%5 = generate coordinates 
% Repeat iterations until mice are sufficiently close to each other
 
while ((sqrt((y(2)-y(1)^2))+(x(2)-x(1))^2)>0.01)
    for i=1:5
        fprintf('%lf %lf', x(i), y(i));
        
        dx=x(mod(i+1,5))-x(i)/sqrt(((x(mod(i+1,5))-x(i))^2)+((y(mod(i+1,5))-y(i))^2));
        dy=y(mod(i+1,5))-y(i)/sqrt(((x(mod(i+1,5))-x(i))^2)+((y(mod(i+1,5))-y(i))^2));
 
        x(i)=x(i)+0.01*dx;
        y(i)=y(i)+0.01*dy;
    end 
end 
       
% To graph, store sequence of vectors in matrices with each line representing coordinates at one of the time steps 
x = mat(i,:);
y = mat(:,i);

Answer 1

[Walter Roberson]

When i becomes 4 in the for i=1:5 loop, then mod(i+1,5) is mod(4+1,5) which is mod(5,5) which is 0. You then try to use that 0 to index x.

The correct neighbour is mod(i,5)+1

Answer 2

[Sakz]

The source of error is in : x(mod(i+1,5))

when i=4, then mod(5,5) = 0; then x(0) is causing the error.

Matlab vector index starts from 1.

[ Subscript indices must either be real positive integers or logicals.]