How to make a loop run multiple times for different values of a variable.

Thanks for taking the time to read this, I am new to matlab. This is the script I'm working on:

    N=50

    alpha=2.8

    x1=0.2;

    for i=1:N

        x(i+1)= alpha*x1*(1-x1);

        x1=x(i+1);

    end

    x1

I would like this script to run for numerous values of alpha and return the answers, so I can plug them into a function and graph them. Specifically I would like to have 120-1200 different values of alpha from 2.8 to 4.0. Right now I just want to know how to make the script run for multiple different values of alpha and I can try and figure out how to do the other part on my own. I tried to put alpha =linspace(2.8,4), but it gives me this error

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

Error in lyanupov2 (line 5) x(i+1)= alpha*x1*(1-x1);

So I figure I'm just thinking of it wrong. I imagine this is a very simple problem for someone who knows Matlab even remotely well.

Answer 1

[Youssef Khmou]

hi, Tom

As you want to get 1200 differents values of alpha then you have to create an N by 1200 matrix :

     N=50
     alpha=linspace(2.8,4,1200); 
     x=zeros(N,length(alpha));

    x1=0.2;

    for j=1:1200
        for i=1:N-1
          x(i+1,j)= alpha(j)*x1*(1-x1);

          x1=x(i+1,j);

    end
end

Then you can plot only 10 graphs to see how it acts :

 plot(x(:,1:10)) % plot 10 graphs for specific 10 alpha values

Answer 2

[Jan]

N = 50;
alpha = linspace(2.8, 4, 100);
x1 = 0.2;
x  = zeros(N + 1, numel(alpha));  % Pre-allocate!!!
for i = 1:N
  x(i+1, :) = alpha .* x1 .* (1 - x1);
  x1        = x(i+1, :);
end

Here ".*" is the elementwise multiplication, instead of the matrix multiplication "*".