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From: alice on 28 Apr 2010 01:10
I have two equations which are dependent on eachother in a loop . All
these variables are matrices.
I'd like to keep track of previous values so i can have a running sum
throughout the execution of the loop. Any suggestions? thanks.


for i=0:90
p=[1,2,3:6,4,2]; some intial value
p0=[5,3,6:5,3,2]

%i need the sum of all previous p for this step
w=p-psum?
p=p+w
end

From: Darren Rowland on 28 Apr 2010 01:30
Try rearranging some of the code, so perhaps

p0 = [5 3 6; 5 3 2];
psum = p0;

p = [1 2 3; 6 4 2];

for i = 1:90
w = p - psum;
psum = psum + p;
p = p + w;
end

Darren
From: alice on 28 Apr 2010 01:55
On Apr 28, 1:30 am, "Darren Rowland"
<darrenjremovethisrowl...(a)hotmail.com> wrote:
> Try rearranging some of the code, so perhaps
>
> p0 = [5 3 6; 5 3 2];
> psum = p0;
>
> p = [1 2 3; 6 4 2];
>
> for i = 1:90
>     w = p - psum;
>     psum = psum + p;
>     p = p + w;
> end
>
> Darren

Thanks for the above comments. This is basically what my code is doing
now. I guess I'm not sure if psum is a sum of all previous ps of just
the current and previous p. I'll go through the number to make sure.

A
From: alice on 28 Apr 2010 21:46
I think I need to preallocate and array which has dimensions which
will change inside with loop so I can keep track of all the previous
ps as they are created. So I would define the array before the loop,
sort of like p = ones(X,Y, Z), Z will increase inside the loop based
on i so I end up with i amoung of X by Y arrays. Not sure if I need
two separate loops or not. Any advise on this general sort of method
would be welcome. Thanks.

From: Darren Rowland on 29 Apr 2010 02:49
Alice,

Looking at your equations, it doesn't matter if p is given as a 2x3 array or as a 6x1 vector. Just to keep the indexing easier, I would have p in the form of a vector then the history of p can be recorded in a matrix of dimension 6-by-i, i.e.

pMat = zeros(6,i); % preallocate storage for p vectors

then within the loop you will have
pMat(:,i+1) = pMat(:,i) + w;

Hth
Darren
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