Thread Subject:

loop with manipulation of matrix multiplication

Hi everyone,

I'm a new matlab user and new to programming and would like to request some help.

I have 2 matrices MM and BB:

        MM = [ 1 2 2 ;
                       5 3 6 ;
                       9 8 7;
                       8 8 4 ;
                       6 1 1 ;
                       5 0 5 ]
        
        BB = [ 31 32 44 ; 20 17 33; 19 44 22]
        
        
        
I would like to multiply in the following manner using a looping procedure :
        
   
 result_part1=[1 2 2; 8 8 4] * [ 31 32 44]'
   
 result_part2 = [ 5 3 6 ; 6 1 1] * [ 20 17 33]'
   
  result_part3 = [9 8 7; 5 0 5]* [19 44 22]'


        
 Then, I would like to output them in the following manner:
        
final_answer = [ 183 349 677; 680 170 205]

cheers
doug

doug:
Great, it looks like you did it. And you didn't even need to use
looping (which we usually try to avoid).
Now, what is your question?
Does it need to be generalized to some variable number of rows and/or
columns, or what?

My question is how can I do this procedure, i.e , skip every 2 rows in matrix MM for each of the first 3 rows sequentially and multiply each by its corresponding transposed row in BB.

doug

Also,my actual work has large matrices that is why I implied that I would I have to use loops.

thanks
doug

On Aug 11, 10:18 pm, "doug kopet" <onedo...@gmail.com> wrote:
> Also,my actual work has large matrices that is why I implied that I would I have to use loops.
>
> thanks
> doug
-----------------------------------------------------------------------
Well you can start with this:
for startingRow = 1:3
    MM(startingRow:3:end, :) % Display just for info.
    BB(startingRow, :)' % Display just for info.
    % Do the multiplication
    result_part = MM(startingRow:3:end, :) * BB(startingRow)'
end

But I'm not sure how you're getting "final_answer." What's the
algorithm for that? It's not simply summing the result_part's.

thanks my friend,

i got the answer in your code by jus making small change to the code u suggested.

--> result_part = MM(startingRow:3:end, :) * BB(startingRow,:)'

Can I ask you how to get all the result_part's to store into a single matrix after the looping has finished.

doug

dont bother,i got it with the indexing

"doug kopet" <onedougk@gmail.com> wrote in message <i3vdpl$dn0$1@fred.mathworks.com>...
> Hi everyone,
>
> I'm a new matlab user and new to programming and would like to request some help.
>
> I have 2 matrices MM and BB:
>
> MM = [ 1 2 2 ;
> 5 3 6 ;
> 9 8 7;
> 8 8 4 ;
> 6 1 1 ;
> 5 0 5 ]
>
> BB = [ 31 32 44 ; 20 17 33; 19 44 22]
>
>
>
> I would like to multiply in the following manner using a looping procedure :
>
>
> result_part1=[1 2 2; 8 8 4] * [ 31 32 44]'
>
> result_part2 = [ 5 3 6 ; 6 1 1] * [ 20 17 33]'
>
> result_part3 = [9 8 7; 5 0 5]* [19 44 22]'
>
>
>
> Then, I would like to output them in the following manner:
>
> final_answer = [ 183 349 677; 680 170 205]
>
> cheers
> doug
- - - - - - - - - - - -
  Try this:

 p = size(MM,1)/size(BB,1);
 final_answer = reshape(sum(MM.*repmat(BB,p,1),2),[],p).';

The two requirements for this generalization are that size(MM,2)=size(BB,2) and size(MM,1) be an integral multiple of size(BB,1).

Roger Stafford