Multiply each element of a vector with a matrix
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I want to multiply each element of a vector with a matrix such that I end up with a 3D matrix (or higher dimentions).
For example if A is a vector and B is a matrix I would write:
for indx=1:length(A)
result(:,:,indx)=A(indx).*B
end
Is for-loops the way to go here or are there any better solutions?
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채택된 답변
Sean de Wolski
2012년 3월 8일
BSXFUN!!!
B = [1 2; 3 4];
A = 1:5;
D = bsxfun(@times,B,reshape(A,1,1,numel(A)));
kron is notorious (at least in older versions) for being very slow.
And obligatory timings:
B = repmat([1 2; 3 4],10,10);
A = 1:50;
t1 = 0;
t2 = 0;
t3 = 0;
for ii = 1:100
tic
C = reshape(kron(A,B),[size(B),numel(A)]);
t1=t1+toc;
tic
D = bsxfun(@times,B,reshape(A,1,1,numel(A)));
t2=t2+toc;
tic
for indx = length(A):-1:1 % Backwards for pre-allocation
result(:, :, indx) = A(indx) .* B;
end
t3=t3+toc;
end
isequal(C,D,result)
[t1 t2 t3]
%{
ans =
1
ans =
0.0128 0.0069 0.0098
%}
댓글 수: 5
Sean de Wolski
2012년 3월 9일
for matrix multiplication, use a for-loop or James Tursa's mtimex on the FEX.
for element by element:
bsxfun(@times,magic(3),rand(3,3,10));
추가 답변 (3개)
James Tursa
2012년 3월 8일
Yet another method for completeness using an outer product formulation. May not be any faster than bsxfun et al:
result = reshape(B(:)*A(:).',[size(B) numel(A)]);
(This tip actually comes from Bruno via another thread)
Friedrich
2012년 3월 8일
Hi,
try kron and reshape:
B = [1 2; 3 4]
A = 1:5
reshape(kron(A,B),[size(B),numel(A)])
댓글 수: 6
Jan
2012년 3월 8일
I feel so blind without a Matlab installation. Is MESHGRID free of FOR loops now? I've read all the basic function in Matlab 4, 5.3, 6.5, 2008b, 2009a. Now I'm starting to get confused when I read them in 2011b again. It would be so nice to have a list of changes for each function...
Sean de Wolski
2012년 3월 8일
Looking in meshgrid in 2012a it is just a series of reshapes and ones() used for indexing.
Jan
2012년 3월 8일
Did you pre-allocate the output?
result = zeros([size(B), length(A)]);
for indx = 1:length(A)
result(:, :, indx) = A(indx) .* B;
end
Or:
for indx = length(A):-1:1 % Backwards for pre-allocation
result(:, :, indx) = A(indx) .* B;
end
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