error: index must be a positive integer or logical -- create movie --SOLVED

EDITED--> SOLVED

Hello, i have this matlab code and it gives me the error i have in title.

% solution of the scalar wave 1d equation
time=1000;
steps=1000;

a=input('Please enter the value of the ratio cdt/dx :');

%preallocate matrix u
u=zeros(steps,time);

%fill matrix u
for i=1:time-1
    for j=1:steps-1
        if (i==1)
            u(j,i)=0; EDIT--> u(j,i)=1;% initial condition

          else        EDIT --> j==1 becomes j==2       
              if (j==*2* && i>=*2* && i<=50) % 50 is the time step for the pulse
                  u(j,i)=50;
              else
                  %solution of wave equation
      u(j,i+1)=a*a*(u(j+1,i)-2*u(j,i)+u(j-1,i))+2*u(j,i)-u(j,i-1);-->here is the error
              end
          end
      end
  end
for k=1:steps          EDIT-->> if mod(i,100)==0

                             figure(i/100)
                              plot(u(:,i+1))
  plot(u(k,1:100))
  plot(u(k,1:200))
  plot(u(k,1:300))
  plot(u(k,1:400))
  plot(u(k,1:500))
  plot(u(k,1:600))
  plot(u(k,1:700))
  plot(u(k,1:800))
  plot(u(k,1:900))
  plot(u(k,1:999))
  end

Also ,how can i write more efficiently the last loop (with the plots) and how can i create a movie in this example? Thank you.

EDITED-->>SOLVED

Answer 1

[Sean de Wolski]

It's right here:

u(j-1,i))

j = 1; and thus j-1 = 0 which is not a valid index.

to plot more effectively

hold on

before the for-loop so the plots overlay.

[George]

Hello.Yes thats the problem.I am trying to change the i and j to start from 2,not from 1.But i am taking straight lines with plot,so sth i have wrong.I have done the code in c++ and there i have "for (int k=1;k<steps;k++){

data1<<k<<"\t"<<u[k][100]<<endl;

data2<<k<<"\t"<<u[k][200]<<endl;......" ,where u is float u[steps][time] and data are files . Isn't ok my approach?