index exceeds matrix dimensions

Hello, I've tried to do a reflecting random walk simulator 1d. I've did it first without the barrier and it graphs perfectly. However, when I tried adding reflecting barrier to it, it kept on producing the "index exceeds matrix dimension". Do you know what is causing the error? Thank you.

% This program will compute and plot the paths of a set of many
% random walkers, which are confined by a pair of barriers at +B and -B.
% Assume all walkers start at z = 0
nwalks = 10; 
nsteps = 100;
figure;
hold on;
for j = 1:nwalks
  x = randn(nsteps,1);
  z = zeros(nsteps,1);
    for k = 1:nsteps
        % Reflecting barrier
        B = 3;   
        if z(k+1) > B
            z(k+1) = B - abs(B - z(k+1));
        elseif z(k+1) < (-B)
            z(k+1) = (-B) + abs((-B) - z(k+1));
        end
        z(k+1) = z(k) + x(k);
    end

% Plot
plot(z);
xlabel('Time (step number)'), ylabel(' Distance');
hold off;
end

Answer 1

[Image Analyst]

z has nsteps elements. k goes from 1 to nsteps. But in the loop you are trying to assign z(k+1), in other words, z(nsteps + 1). Try k = 1:(nsteps-1)