How to plot a repeated y value after some interval vaue of x?

I have (x,y) data but I want to repeat the y value after each multiple of 9 up to the last value of xn, i.e x= 100. I expect a triangular shape with touching at x =[1 9 18 29 etc ] . i tried with the following code but not effective , any one who can show my error . thanks

  y=[1 2 3 4 5 4 3 2 1]
   x=[1 2 3 4 5 6 7 8 9]
   xn=ones(1,100);
   j = 1;
  for i = 1:length(xn)
    if(j==10)
      j = 1;
    end 
      if i<=5
         y(i)=i;
      else 
        y(i)=y(5)-y(i-5);
      end
       j = j+1;
     plot(x(i), y(j));
  end

error

Index exceeds matrix dimensions.

Answer 1

[Walter Roberson]

    for i = 1:length(xn)
    ...
    plot(x(i), y(j));

for that plot to work, length(x) must be at least length(xn) . But length(xn) is 100, and length(x) is 9.

I do not see any use for xn except to provide length() for the for i loop. Why not just directly code for i = 1 : 100 ?

[seble mekonnen]

yes, right but I want to use xn for any arbitrary size, for instance for simulation time of xn. and to plot the (x,y)

[Walter Roberson]

Then perhaps you want

      plot(xn(i), y(j), '*');
      hold on;

However, all of your xn are 1, so it is not clear this is what you want.