Defining a matrix using empty parenthesis

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Zena Assaad
Zena Assaad on 22 Oct 2014
Commented: Zena Assaad on 23 Oct 2014
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!

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Accepted Answer

Image Analyst
Image Analyst on 22 Oct 2014
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.
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Image Analyst
Image Analyst on 23 Oct 2014
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
Zena Assaad on 23 Oct 2014
Thank you I see the problem now!

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