Defining a matrix using empty parenthesis

Hi all,

Just a small problem with notation. I have the following for loop:

    G = [];
    W = [];
    W(1) = 77000;
    for i = 1:29
    G = [G, SFC*Thrust1(i)*(disp(i)/V1(i))]
    W = [W(i+1), (W(i)-G(i))]
    end

I am getting the following error:

Index exceeds matrix dimensions.

Error in fueloptimal (line 139)
  W = [W(i+1), (W(i)-G(i))]

I need to define W(i+1) using the empty parenthesis before the 'for loop', i.e. W = [ ]. Have I done this correctly? Or is the issue coming from somewhere else?

Thanks in advance!

Answer 1

[Image Analyst]

The problem is not only W(i+1). It's W(i). Defining W = [], which is null, does not mean that there is a first element, element #1. There is no element W(1). Null means nothing, not even a first element that is null. So you can't append/prepend anything to it. You can append to W if it's null, but not to W(1) or any other specific index number because they're not there yet.

And of course on the ith iteration there is no (i+1)th element either, because you haven't gotten there yet. When i = 1, why do you expect that a W(2) exists already? It doesn't.

[Zena Assaad]

I'm sorry, I should have specified that I have predefined W(1) as a positive constant before applying the for loop.

[Image Analyst]

Oh, you're right - it is there. If you did that then there was no need to do W=[] just prior to it.

So the first time through your loop with i=1, you're doing W = [W(2), (W(1)-G(1))] but the problem is, on the first iteration there is no W2 yet!

[Zena Assaad]

Thank you I see the problem now!