Can someone help meto fix it. Error: File: Untitled.m Line: 42 Column: 16 Unexpected MATLAB expression. Thanks a lots

% This program implements an explicit scheme to solve the Burgers?Huxley
% eqution, with the addition of shifting the profile. The code ... outputs the
% travelling wave profile, with its corresponding speeds with respect to
% variation in time.
clear all; clc;
% Spacial increment.
dx = 0.1;
% Time increment. 
dt = (0.95)*(1/2)*dx^2;
% Initial spacial domain.
x = [-20:dx:20];
% Time frame for simulation.
T = 600;
% Initial conditions.
u = 1/2*(1-tanh(x)) ;
% Parameters k,m and n.
pk = 6; pm = 6; pn = 6;
% Defining initail states of paramters that will be used in the program.
iter = 1;
cumshift = 0;
xoffset = 0;
xpos = 0;
tm = 0;
% Implementing a shift every 300th iteration.
for j = 0:dt:T
% Applying a numerical shift every .
if (mod(iter,300)==0)
figure(1)
% Returns indices of the vector "u" that agree with the condition.
ind = find(u>0.1 & u<0.9);
% Returns the x value that corresponds to the u value at 0.5.
xc = interp1(u(ind),x(ind),0.5);
% Determining the number of steps xc is away from 0.
N = floor(xc/dx);
% Creating shift variable.
xshift = N*dx;
% Cumulated shift.
cumshift = cumshift + xshift;
% Rounding error.
xoffset = xc - xshift;
% Creating shifted u.
u = [u(N+1:end)zeros (1,N)];
% Shifting x domain.
newx = x+cumshift;
% Plotting shifting wave solution.
plot(newx,u,'b');
axis([newx(1) newx(end) -0.1 1.1])
pause(0.01)
% Adding the total shift to a vector.
xpos = [xpos cumshift+xoffset];
% Adding the accumulated time frames between shifts.
tm = [tm j];
end
% Explicit scheme.
u(2:end-1) = u(2:end-1) + ... (dt/(dx)ˆ2).*(u(3:end)?2.*u(2:end?1)+u(1:end?2)) ...
- (dt/(2*dx)).*u(2:end-1).^pk.*(u(3:end) - u(1:end-2)) ... 
+ dt.*u(2:end-1).^pm.*(1-u(2:end-1).^pn);
% Boundary conditions.
u(1) = 1;
u(length(x)) = 0;
iter = iter+1;
end
% Defining parameter values.
sum = 0;
k = 0;
speed(1) = 0;
% Computing the speeds with respect to time.
for j = 2:length(xpos)
    g = (xpos(j)-xpos(j-1))/(tm(j)-tm(j-1));
    sum = sum + g;
k = k+1;
speed(j) = sum/k;
end
figure (4)
% Plotting a Speed vs Time graph.
plot(tm,speed)
fprintf('final speed = %1.9f\n', speed(end));

Answer 1

[Walter Roberson]

u = [u(N+1:end) zeros(1,N)];

[Jin Hao Yen]

Error in Untitled (line 42)
u = [u(N+1:end) zeros(1,N)];

Subscript indices must either be real positive integers or logicals.But also cannot run :(

[Walter Roberson]

% Determining the number of steps xc is away from 0.
N = floor(xc/dx);

You are determining the number of steps from 0 but your x does not start at 0. Your N happens to come out as -1. When you use u(N+1:end) that becomes u(-1+1:end) which is u(0:end) but 0 is not a valid index.

Remember your x starts from -20.

[Jin Hao Yen]

How should I fix this problem ?

[Walter Roberson]

N = floor((xc-x(1))/dx)

[Jin Hao Yen]

Nonfinite endpoints or increment for colon operator in index.

Error in Untitled (line 42)

u = [u(N+1:end) zeros(1,N)];

It comes out this error.

[Walter Roberson]

I suspect that your interp1 failed and returned nan again. I showed you in your other question that using dx of 0.1 is too big for your calculations. 0.01 seemed to be okay but possibly 0.025 would be enough.

[Jin Hao Yen]

Nonfinite endpoints or increment for colon operator in index.

Error in Untitled (line 42)

u = [u(N+1:end) zeros(1,N)];

Sorry Walter, still cannot proceed

[Walter Roberson]

You can get around some of the problem by using

xc = interp1(u(ind),x(ind),0.5,'cubic','extrap');

However, I have to wonder whether it is appropriate to be using x there and x(1) in the calculation of N. You have been constructing newx and it seems to me that your u values correspond to newx values, not to x values.

... However if you make that change, you still run into problems. At iteration 2700, the find() returns no values. All of the u values except u(1) are less than 1E-8 by that iteration.