I have to use {} in my for loop, but now I can't multiply...

I have to develop random numbers where n=1-100. So, I did this:

x=1:100;
for n=1:length(x);
    y{n}=rand(n)
end

After that, I have to use only the last 50 matrices (50-100),

y2=y(50:100)

to put into this equation:

xn=r*(y2)*(1-(y2))

But it doesn't work, I'm guessing because of the {}... but I can't get rid of that.

[Andrew Newell]

Some curious constraints! Note that y(50:100) actually has 51 matrices.

[Andrew Newell]

An even more fundamental problem is that rand(n) produces an n x n matrix, so you're trying to add and multiply matrices of different sizes. Are you sure you really want that, instead of (say) rand(1)?

[James Tursa]

Can you post the actual wording of the assignment you were given so that we know how to advise you?

[Image Analyst]

What is r? And do you want matrix multiplication or element-by-element multiplication?

And you can't do

xn=r*(y2)*(1-(y2))

with cell arrays, which is what y2 is. You can only do it with regular numerical arrays.

[Rik]

It looks like your actual code should be something like this:

y=rand(1,100);%create a column vector of 100 length
y2=y(51:end);%copy the second half
xn=r.*(y2).*(1-y2);

But as James noted, this really depends a lot on the actual assignment/goal.

[cestcandice]

Sure, thank you for the help:

(Using the equation X=r(xn)(1-xn))

Start with x1=rand(). Calculate xn for n=1...100. Plot the sequence xn for n=50....100 for r=2.99,3.05,3.54, and 3.6.

[Image Analyst]

You can't calculate xn using that equation. You can calculate X from xn, but not the other way around. Then, what sequence? Simply plot xn, like in a line plot or bar plot? Or plot X vs. xn? This question is unanswerable as it is now stated.

[Andrew Newell]

Are you sure that is the correct statement of the problem, and not something like x(n+1) = r*x(n)*(1-x(n))?

[cestcandice]

I thought calculate xn meant like how x1=rand(), now xn needs to be found... So xn=rand(1:100)

[cestcandice]

Here's a screenshot... if it helps.

[James Tursa]

Thanks ... the screenshot is a HUGE help! That will clear things up.

[Andrew Newell]

It confirms my guess: it's x(n+1) = r*x(n)*(1-x(n)), with the starting value of x(1)=rand(). You definitely do not want to use a cell array for this.

[Andrew Newell]

Since it is a homework problem, I suggest you take a crack at answering the correct problem and then get back to us.

[cestcandice]

Copy that. Thanks!

[cestcandice]

So, if Xn isn't a cell array, what is it then??? I thought n meant any number between 1 and 100??

[Stephen23]

@Candice Stauffer: you do not need to guess how to use MATLAB, because MATLAB has documentation which explains how it works, and how to use it. I just tried several internet search engines with the term "MATLAB rand", and they all returned the same first result: the MATLAB documentation for the function rand. You can do this too! Then you would know what the input to rand does.

Also you need to learn to stop relying on what you wish/believe/imagine/want/assure/... that your code is doing, and start to actually look at what the code is really doing. Code does not care what is in your head: it will do exactly what it does, regardless. You job is to understand it, and check it. This means do not write code without knowing what it is doing: write one line at-a-time: read the help for the functions you need (try the examples too), test that each line really does what you need it to do, check its output by hand. Only then should you move on to the next line.

https://www.mathworks.com/matlabcentral/answers/228557-experts-of-matlab-how-did-you-learn-any-advice-for-beginner-intermediate-users

[Jan]

@Stephen: +1. Exactly.

@Candice: Please consider Stephen's advice. You find 16 comments here already, althought the problem is not hard. Something is going wrong here. Take Stephen's suggestion as motivation to learn a deterministic programming style.

Answer 1

[James Tursa]

Treat the x"sub n" in the screenshot as x(n) in MATLAB. I.e., the n'th element of a vector called "x".

So x"sub 1" in the screenshot would be x(1). So that first expression for x(1) would be:

x(1) = rand();

Then the recursion relationship in MATLAB is:

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

So you need to put this together in a loop:

r = something;  % <-- you need to fill in this value
x = zeros(1,100);  % <-- pre-allocate the result
x(1) = rand();  % <-- the 1st element
for n=1:99  % <-- end the loop at 99 so that 99+1=100 is the final element
    % put the recursion relationship here
end

Then you will be in a position to do the plotting. The elements 50 through 100 are simply x(50:100).

In your jpg, what is the lower part of the plot? I.e., what is the horizontal axis?

[cestcandice]

Thank you! And,Yes. it is the horizontal axis. And this still isn't working.

It's saying that In an assignment A(:)=B, the same.

r = [2.99,3.05,3.54,3.6];  
x = zeros(1,100); 
x(1) = rand(); 
for n=1:99
   x(n+1) = r*x(n)*(1 - x(n)); 
end

[cestcandice]

I think it must be because r only has 4 values... but that's all that I was given.

[Rik]

The line of code inside the for-loop is a matrix multiplication in your case. That is not what you want. If you want to calculate multiple values of r at once, you will have to change this to a matrix, but it is way easier to just use another loop.

[James Tursa]

"... Yes. it is the horizontal axis. ..."

Yes, but can you post a screen shot of that so we can see what variable exactly is on the horizontal axis?

[cestcandice]

Sure!

[Jan]

@Candice: "It's saying that In an assignment A(:)=B, the same." Do not post rephrased or abbreviated error message in the forum. Only a complete copy of the message is meaningful. In addition it tells you exactly where and what the problem is: The right hand side is a [1 x 4] vector, but on the left side of the = you have the scalar variable x(n+1). This cannot work and it is your turn to find out, what you want to do instead.

Do not provide r as a vector:

rv = [2.99,3.05,3.54,3.6];
for ir = 1:numel(rv)
  r = rv(ir);
  ...
end

Now you perform the calculations in a loop for the 4 different values of r separately.