For loop and matrix indexing in matlab ques

Question

I have defined a matrix (initial_matrix) equal to 0 by using a for loop as:

I = 5;   % e.g number of nodes
for i =1:I
    initial_matrix = [0];    // an initial matrix will be generated for each node
end

Now, for each node i, I will consider all other nodes but not the node i itself and subtract each of them from 1 and take their product:

for node 1:

result = (1 - initial_matrix of node 2) * (1 - initial_matrix of node 3) * ...
    (1 - initial_matrix of node 4) * (1 - initial_matrix of node 5)

for node 2:

result = (1 - initial_matrix of node 1) * (1 - initial_matrix of node 3) * ...
    (1 - initial_matrix of node 4) * (1 - initial_matrix of node 5)

for node 3:

result = (1 - initial_matrix of node 1) * (1 - initial_matrix of node 2) * ...
    (1 - initial_matrix of node 4) * (1 - initial_matrix of node 5)

and so..for the remaining 2 nodes!

Can any one tell me or give me hints on how this can be achieved? Thanks!

This is very difficult to follow. Can you provide an example? — Dan, Sep 11 '13 at 08:58
@Dan. The above example was for the case when node 1 is considered. Now if I consider node 2: then i should get: result = (1 - initial_matrix of node 1) * (1 - initial_matrix of node 3) * ... (1 - initial_matrix of node 4) * (1 - initial_matrix of node 5) ....similarly for node 3 etc — Mokujin, Sep 11 '13 at 09:21
This is not an example. If the answers below aren't what you're looking for, you should ADD TO YOUR QUESTION (i.e. not a comment) a numerical small worked example — Dan, Sep 11 '13 at 09:30
Do you really want the matrix product, or the elementwise product? — Dennis Jaheruddin, Sep 11 '13 at 09:42

Eitan T · Accepted Answer · 2013-09-11T10:06:43.323

1

For each product (per node) you need to have all initial matrices in advance, so you should modify your initial loop to something along these lines:

initial_matrix = cell(I, 1);
for i = 1:I
    initial_matrix{i} = blah blah... %// Generated for each node
end

Then you can add another nested loop that does something like the following:

result = cell(I, 1);
for k = 1:I

    %// Compute product
    result{k} = 1;
    for n = setdiff(1:I, k)
        result{k} = result{k} * (1 - initial_matrix{n});
    end
end

edited Sep 11 '13 at 10:06

answered Sep 11 '13 at 08:58

Eitan T

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@ Eitan T, but this wont run for node 2 and so on..coz for node 1 i will consider 2,3,4 and 5, then for node 2: 1,3,4 and 5, for node 3: 1,2,4 and 5....and so on.... – Mokujin Sep 11 '13 at 09:09
So for each node you want to consider all nodes but itself? Please take the time to add this missing information to the question. – Eitan T Sep 11 '13 at 09:13
yes, i want to consider all the nodes expect the node in question! – Mokujin Sep 11 '13 at 09:16
Especially when you set it to zero, you will want to do: `prod(1-initial, 3)` -- This applied to your previous version, I can look at the udated one later. – Dennis Jaheruddin Sep 11 '13 at 09:33
@DennisJaheruddin You can't use `prod` and `cumprod` because they do element-wise multiplication, not matrix multiplication... – Eitan T Sep 11 '13 at 09:37
1

@EitanT: Ok. I am checking the code – Mokujin Sep 11 '13 at 09:42
@user2762192 Please use the update code that employs cell arrays :) – Eitan T Sep 11 '13 at 09:49
@EitanT, i get the following error: Cell contents reference from a non-cell array object. Error in colll (line 14) result{k} = eye(size(result{k})); – Mokujin Sep 11 '13 at 10:04
@user2762192 You're right, forgot to updated that. It's fixed now. – Eitan T Sep 11 '13 at 10:07

Dennis Jaheruddin · Answer 2 · 2013-09-11T09:40:59.760

I think these are the steps you need to take:

Assuming all matrices are of the same size and that you want to do elementwise multiplication:

Concatenate all of them into one big matrix initialMatrices, as desribed here.
Apply cumprod on 1-initialMatrices
If you used cumprod Extract the matrices that you need from this result

Some things to pay attention to:

Maybe preprocess the first initial matrix by doing one minus it, or remove it.
Make sure you build the total matrix in the right 'direction'. Check help cumprod for what the right direction is.

Yunus · Answer 3 · 2013-09-11T09:43:14.997

0

Currently, you are not creating an initial matrix for each node. If you want to have a separate one for each, I would suggest you to define a structure. So you will have:

I = 5;
for i =1:I
    initial_matrix.(sprintf('Node%d',i)) = [0];    % an initial matrix will be generated for each node
end

Then you can maybe do something like the following to do your operation (if I understood it right):

    for i =1:I
        numnode=[1:I]
        numnode(numnode==i)=[]
        for ind = 1:numel(numnode)-1
            NewMatrix.(sprintf('Node%d',i)) = (1- initial_matrix.sprintf('Node%d',numnode(ind))))*(1- initial_matrix.(sprintf('Node%d',numnode(ind+1))))
            end
    end

You can verify if this is correct by using a rand(1) statement instead of [0] while defining the initial_matrix.

I hope I understood you correctly

edited Sep 11 '13 at 09:43

answered Sep 11 '13 at 09:28

Yunus

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But where's the part that does multiplication? – Eitan T Sep 11 '13 at 09:41
@Eitan just realized my mistake and modified it. Should be multiplying now. – Yunus Sep 11 '13 at 09:43
One more thing: while it certainly works, it seems to me that using structs and dynamic referencing is an unnecessary over-complication. Using cell arrays here definitely would be much cleaner. – Eitan T Sep 11 '13 at 09:51

For loop and matrix indexing in matlab ques

3 Answers3