有没有更好的方法来随机生成双随机矩阵? [英] Is there a better way to randomly generate a Doubly Stochastic Matrix?
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问题描述
这是通话n = 2*10^3; M = DStochMat02(n,ones(n)./n);
time calls line
1 function M = DStochMat02(n,c)
2 % Generate a random doubly stochastic matrix using
3 % Theorem (Birkhoff [1946], von Neumann [1953])
4 % Any doubly stochastic matrix M can be written as a convex combination
5 % of permutation matrices P1,...,Pk (i.e. M = c1*P1+...+ ck*Pk
6 % for nonnegative c1,...,ck with c1+...+ck = 1).
7 % Complexity: O(n^2)
8 % USE: M = DStochMat02(4,[1/2 1/8 1/8 1/4])
9 % Derek O'Connor, Oct 2006, Nov 2012. derekroconnor@eircom.net
0.02 1 10 M = zeros(n,n);
< 0.01 1 11 I = eye(n);
< 0.01 1 12 for k = 1:n
1.64 2000 13 pidx = GRPdur(n); % Random Permutation
107.72 2000 14 P = I(pidx,:); % Random P matrix
41.09 2000 15 M = M + c(k)*P;
< 0.01 2000 16 end
function p = GRPdur(n)
% -------------------------------------------------------------
% Generate a random permutation p(1:n) using Durstenfeld's
% Shuffle Algorithm, CACM, 1964.
% See Knuth, Section 3.4.2, TAOCP, Vol 2, 3rd Ed.
% Complexity: O(n)
% USE: p = GRPdur(10^7);
% Derek O'Connor, 8 Dec 2010. derekroconnor@eircom.net
% -------------------------------------------------------------
p = 1:n; % Start with Identity permutation
for k = n:-1:2
r = 1+floor(rand*k); % random integer between 1 and k
t = p(k);
p(k) = p(r); % Swap(p(r),p(k)).
p(r) = t;
end
return % GRPdur
推荐答案
如何将14
和15
行更改为以下行:
How about changing lines 14
and 15
to the following lines:
l = ( [ pidx ; 1:n ] - 1 ) * [1;n] + 1; % convert pairs (pidx,1:n) to linear indices
M(l) = M(l) + c(k);
由于P
非常稀疏,所以仅增加P
的非零值可能会更有效.
since P
is very sparse, maybe it would be more efficient to increment only the non-zeros of P
.
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