Thread Subject: How to vectorise

Hello,

I have a matrix A of size m x n. I want to
add a vector x1 of size 1 x n to every other row of A starting from
m=1. Then I want to add
a vector x2 (also of size 1 x n) to every other row of A starting
from m=2. I tried the following :

A(1:2:end,:) = A(1:2:end,:) + x1;
A(2:2:end,:) = A(2:2:end,:) + x2;

This does not work. So the alternative is a for loop :

for i = 1:2:m
    A(i,:) = A(i,:) + x1;
end;
for j=2:2:m
    A(j,:) = A(j,:) + x2;
end;

This goes way too slow if A is large. Is ther e a vecorized way of
doing the above ?

-sgrongst

sgrongst wrote:
>
>
> Hello,
>
> I have a matrix A of size m x n. I want to
> add a vector x1 of size 1 x n to every other row of A starting from
> m=1. Then I want to add
> a vector x2 (also of size 1 x n) to every other row of A starting
> from m=2. I tried the following :
>
> A(1:2:end,:) = A(1:2:end,:) + x1;
> A(2:2:end,:) = A(2:2:end,:) + x2;
>
> This does not work. So the alternative is a for loop :
>
> for i = 1:2:m
> A(i,:) = A(i,:) + x1;
> end;
> for j=2:2:m
> A(j,:) = A(j,:) + x2;
> end;
>
> This goes way too slow if A is large. Is ther e a vecorized way of
> doing the above ?
>
> -sgrongst

To add matrices they need to have the same size (exception: adding a
scalar). Didn't the error message say that? Thus, you need to "blow
up" the row vectors, x1 and x2, to take the same size as the matrix
in the expression. That can be done with REPMAT or in this case as in
the sample below, with is a bit fasater.

/per

function A = cssm( m, n )

    A = rand( m, n );
    x1 = rand( 1, n );
    x2 = rand( 1, n );

    ix1 = 1:2:m;
    ix2 = 2:2:m;

    A( ix1, : ) = A( ix1, : ) + x1( ones(1,length(ix1)), : );
    A( ix2, : ) = A( ix2, : ) + x2( ones(1,length(ix1)), : );

return

A(1:2:end,:)=A(1:2:end,:) + repmat(x1,size(A(1:2:end,:),1),1);
A(2:2:end,:)=A(2:2:end,:) + repmat(x2,size(A(2:2:end,:),1),1);

Rentian

"per isakson" <poi3@bim1.kth4.se> wrote in message
news:eef386d.0@webx.raydaftYaTP...
> sgrongst wrote:
>>
>>
>> Hello,
>>
>> I have a matrix A of size m x n. I want to
>> add a vector x1 of size 1 x n to every other row of A starting from
>> m=1. Then I want to add
>> a vector x2 (also of size 1 x n) to every other row of A starting
>> from m=2. I tried the following :
>>
>> A(1:2:end,:) = A(1:2:end,:) + x1;
>> A(2:2:end,:) = A(2:2:end,:) + x2;
>>
>> This does not work. So the alternative is a for loop :
>>
>> for i = 1:2:m
>> A(i,:) = A(i,:) + x1;
>> end;
>> for j=2:2:m
>> A(j,:) = A(j,:) + x2;
>> end;
>>
>> This goes way too slow if A is large. Is ther e a vecorized way of
>> doing the above ?
>>
>> -sgrongst
>
> To add matrices they need to have the same size (exception: adding a
> scalar). Didn't the error message say that? Thus, you need to "blow
> up" the row vectors, x1 and x2, to take the same size as the matrix
> in the expression. That can be done with REPMAT or in this case as in
> the sample below, with is a bit fasater.
>
> /per
>
> function A = cssm( m, n )
>
> A = rand( m, n );
> x1 = rand( 1, n );
> x2 = rand( 1, n );
>
> ix1 = 1:2:m;
> ix2 = 2:2:m;
>
> A( ix1, : ) = A( ix1, : ) + x1( ones(1,length(ix1)), : );
> A( ix2, : ) = A( ix2, : ) + x2( ones(1,length(ix1)), : );
>
> return