Assigning a vector to multiple rows of a matrix

Hi all,

This is actually something I've been wondering for a while... Say I have a 1-by-4 vector A and a 5-by-4 matrix B. Now I want to assign the vector A to certain rows of B, but unfortunately you can't do something like

B([1,3,5],:)=A

since the dimensions will mismatch. What I ended up doing is

B([1,3,5],:)=repmat(A,3,1)

which works but seems really ugly (and probably slow as well since I used repmat). Is there a better way to do this?

Thanks!

Niko

Answer 1

[W. Owen Brimijoin]

You could try using indices instead of subscripts:

%pick the rows in B you want to your A values to go into:
r = [1 3 5];

%and the corresponding columns:
c = [1 2 3];

%now get your indices:
idx = sub2ind(size(B),r,c);

%and use these indices as follows:
B(idx) = A(c);

This assumes that you want your columns filled in the order that they are found in A, but you could just as easily make a different index into A to pick particular values from that vector. This works as long as you ask for the same number of values from A as you have slots in B, if you follow me.

B(idx) = A([2 3 4]);

[Dang Manh Truong]

I'm sorry but it did not work :(

Answer 2

[Jos (10584)]

Indeed, it looks a little ugly. However, using repmat is not that slow. Another, little uglier, but slightly faster and more straightforward solution is using a for-loop

A = 1:5
B = zeros(5,5) ;
for k = [1 3 5], B(k,:) = A ; end

[Omar Peza]

Thanks!

[Chenglei QIAO]

Thanks! but I'm wondering why in this case, the for loop is even faster than the built-in repmat function?

Answer 3

[Yao Li]

    b=ones(5,4);
    a=[1;2;3;4];

If you wanna assign a to the 3rd row of b,

   b=[b(1:2,:);a';b(4:5,:)]