在MATLAB中向量化代码

Question

如何在MATLAB中矢量化这段代码？ 如果可能，我希望矩阵B是一个稀疏矩阵。

%% Y is a matrix l*n 
%% X is a matrix k*n 
B = []; 
for i=1:l 
    for j=1:n 
     temp1 = zeros(1,n*l); 
     temp1((i-1)*n+j) = -1; 
     temp2 = zeros(1,l*k); 
     temp2((i-1)*k+1:i*k) = (-Y(i,j)).*(X(:,j)'); 
     B = [B;[temp1,temp2]]; 
    end 
end 

我不知道如何矢量化这段代码，请大家帮忙！谢谢！

Answer 1

制作掩蔽和按元素相乘的量化计算中使用的bsxfun，这里有一个量化的方法 -

%// Create left part of the output that is basically an identity matrix 
parte1 = -eye(n*l); 

%// Setup right part of output 
parte2 = zeros(n*l,l*k); 

%// Mask to set elements from the calculations of (-Y(i,j)).*(X(:,j)') 
M = bsxfun(@eq,reshape(repmat(1:l,n,1),[],1),reshape(repmat(1:l,k,1),1,[])); 
%// OR concisely : M = kron(eye(l),ones(n,k))==1 

%// Perform vectorized calculations of (-Y(i,j)).*(X(:,j)') and set those 
%// into second part at masked places 
parte2(M) = -bsxfun(@times,permute(X,[2,1,3]),permute(Y,[2,3,1])); 

%// Finally concatenate those parts for final output 
out = [parte1,parte2];