Thread Subject:

An even nastier vector operation!

Hi Everyone,

In extension to a problem posted yesterday (Subject: Complicated vector operation!), I am having trouble with a perhaps even nastier version!

Suppose I have a horizontal vector, "A". For example:

A = [2 3 5 5 1 4]

Suppose also that I have a vertical vector of integers:

number_in_sample = [1 3 5]'

Now I wish to create a vertical vector, called "averages", which consists of averages of numbers drawn from A, where the number of elements drawn (without replacement), is defined by the vector number_in_sample.

So, the number of elements in "averages", is equal to the number of elements in "number_in_sample".

Can this be done without the need to loop?

I appreciate the help.

Regards,

Ulrik.

"Ulrik Nash" <uwn@sam.sdu.dk> wrote in message <hntltp$r3r$1@fred.mathworks.com>...
> Hi Everyone,
>
> In extension to a problem posted yesterday (Subject: Complicated vector operation!), I am having trouble with a perhaps even nastier version!
>
> Suppose I have a horizontal vector, "A". For example:
>
> A = [2 3 5 5 1 4]
>
> Suppose also that I have a vertical vector of integers:
>
> number_in_sample = [1 3 5]'
>
> Now I wish to create a vertical vector, called "averages", which consists of averages of numbers drawn from A, where the number of elements drawn (without replacement), is defined by the vector number_in_sample.
>
> So, the number of elements in "averages", is equal to the number of elements in "number_in_sample".
>
> Can this be done without the need to loop?
>
> I appreciate the help.
>
> Regards,
>
> Ulrik.

of course, no problem...

us

So you have the number of elements to average together, but what are
their indices? Let's say you are looking at number_in_sample(2) - it
says to average 3 elements of A. But which three?!?

ImageAnalyst <imageanalyst@mailinator.com> wrote in message <fa49b8fe-3d84-4bb8-a304-c39dad6b49a7@i25g2000yqm.googlegroups.com>...
> So you have the number of elements to average together, but what are
> their indices? Let's say you are looking at number_in_sample(2) - it
> says to average 3 elements of A. But which three?!?

assumption: random draws...

us

If I understand correctly:

C = cumsum(A);
C = C(B)./(B.')

So to clarify, in the example I have given, the first number in the "averages" vector would consist of:

Element 1) The sum of 1 randomly drawn number from A divided by 1
Element 2) The sum of 3 randomly drawn number from A divided by 3
Element 3) The sum of 5 randomly drawn number from A divided by 5

Note that each of the random numbers drawn to form a given element are drawn without repleacement, BUT there IS replacement once a given element has been computed and it is time for the next element is calculated.

I do hope this helps! :-)

Regards,

Ulrik.

O.k, try again. I am not convinced this will be faster than a loop approach.



% Data
A = [2 3 5 5 1 4]
B = [1 3 5]'

% Engine
L = length(B);
[S,S] = sort(rand(L,length(A)),2);
C = cumsum(A(S),2);
C = C((1:L)+(B.'-1)*L)./(B.')