In an assignment A(I) = B, the number of elements in B and I must be the same.

I am working on this problem x(i+1)=rx(1-x) where the question says plot x as a function of r for 1<r<4.

Here is what I did:

r=1:0.01:4;

x=[];

x=0.35;

N=100;

for i=1:N;

x(i+1)=r*x(i)*(1-x(i));

end

but then this error message comes up. What's wrong with this code? How can I improve it?

Answer 1

[Roger Stafford]

Based on your description, presumably what you are requested to plot is the value of r along one axis and the corresponding value of x, starting with 0.35, after 100 repetitions of the given iteration along the other axis. To accomplish this you will need two for-loops, one nested inside the other. The single for-loop code you wrote does not work. It should be something like this:

 r = 1:.01:4;
 X = zeros(1,length(r));
 for k = 1:length(r)
   x = 0.35;
   for c = 1:100
     x = r(k)*x*(1-x);
   end
   X(k) = x; % You only need to store the last x value for each r(k)
 end
 plot(r,X,'y.')

Don't be discouraged by the strange appearance of the plot you get. There will be a certain degree of apparent chaos to it. That is inherent in the nature of the iteration formula.

EDIT:

Actually you don't have to use two nested for-loops. You can write it like this:

 r = 1:.01:4;
 x = repmat(0.35,1,length(r));
 for k = 1:100
   x = r.*x.*(1-x);
 end
 plot(r,x,'y.')

[Sarah]

Ok I get it now. Thank you so much.

Answer 2

[Walter Roberson]

x(i+1) can hold one element.

x(i) and (1-x(i)) are each one element, so x(i)*(1-x(i)) is one element.

r is 1:0.01:4 which is multiple elements. multiple elements multiplied by one element gives multiple elements. So the right hand side of the assignment has multiple elements and you are trying to save that into a single element.

Hint: you can make x into a two dimensional array and operate on rows at a time.