multiplying each non zero element of a sparse matrix by a certain number

23 views (last 30 days)
Hello all,
I have a sparse matrix S with few non-zero elements in its row and columns. I have a vector P with the same number of elements as the total non-zero elements of S. I would like to multiply each non-zero element of S by a value in P. For example, let
S = [0 0 1 1;0 1 1 0;1 1 0 0; 1 0 0 1]
P = [0.7421 0.3751 1.9079 1.4141 1.4411 2.0796 2.4199 1.1002]
I would like to get
W = [0 0 0.7421 0.3751;0 1.9079 1.4141 0;1.4411 2.0796 0 0; 2.4199 0 0 1.1002]
Can somebody please help me with a fast way of doing this task? Thank you.

Accepted Answer

Star Strider
Star Strider on 25 Nov 2015
This works — and NO LOOPS:
S = [0 0 1 1;0 1 1 0;1 1 0 0; 1 0 0 1] ;
P = [0.7421 0.3751 1.9079 1.4141 1.4411 2.0796 2.4199 1.1002];
Snz = find(S == 1);
Wv = S(Snz).*P';
W = zeros(1,numel(S));
W(Snz) = Wv;
W = reshape(W, size(S))'
W =
0 0 0.7421 0.3751
0 1.9079 1.4141 0
1.4411 2.0796 0 0
2.4199 0 0 1.1002

More Answers (2)

James Tursa
James Tursa on 25 Nov 2015
Edited: James Tursa on 25 Nov 2015
W = S;
G = logical(S);
W(G) = W(G) .* P(:);
Note: This preserves the sparse attribute of S. I.e., if S is sparse, then W will be sparse. If S is full, then W will be full.
  3 Comments
James Tursa
James Tursa on 27 Nov 2015
Ah ... I didn't notice that you wanted the multiplication by rows, not columns. So some transposes will be required for this method. E.g.,
W = S';
G = logical(W);
W(G) = W(G) .* P(:);
W = W';

Sign in to comment.


Stalin Samuel
Stalin Samuel on 25 Nov 2015
S = [0 0 1 1;0 1 1 0;1 1 0 0; 1 0 0 1] ;
 P =  [0.7421 0.3751 1.9079 1.4141 1.4411 2.0796 2.4199 1.1002] ;
[r c] = size(S);
W = zeros(r,c);
n = 1;
for r1 = 1:r
    for c1 = 1:c
        if S(r1,c1)~=0
            W(r1,c1) =  S(r1,c1)*P(n) ;
            n = n+1;
        end
    end
end

Categories

Find more on Sparse Matrices in Help Center and File Exchange

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!