Suppose I have a matrix A
and I sort the rows of this matrix. How do I replicate the same ordering on a matrix B
(same size of course)?
E.g.
A = rand(3,4);
[val ind] = sort(A,2);
B = rand(3,4);
%// Reorder the elements of B according to the reordering of A
This is the best I've come up with
m = size(A,1);
B = B(bsxfun(@plus,(ind-1)*m,(1:m)'));
Out of curiosity, any alternatives?
Update: Jonas' excellent solution profiled on 2008a (XP):
0.048524 1.4632 1.4791 1.195 1.0662 1.108 1.0082 0.96335 0.93155 0.90532 0.88976
0.63202 1.3029 1.1112 1.0501 0.94703 0.92847 0.90411 0.8849 0.8667 0.92098 0.85569
It just goes to show that loops aren't anathema to MATLAB programmers anymore thanks to JITA (perhaps).
A somewhat clearer way to do this is to use a loop
A = rand(3,4);
B = rand(3,4);
[sortedA,ind] = sort(A,2);
for r = 1:size(A,1)
B(r,:) = B(r,ind(r,:));
end
Interestingly, the loop version is faster for small (<12 rows) and large (>~700 rows) square arrays (r2010a, OS X). The more columns there are relative to rows, the better the loop performs.
Here's the code I quickly hacked up for testing:
siz = 10:100:1010;
tt = zeros(100,2,length(siz));
for s = siz
for k = 1:100
A = rand(s,1*s);
B = rand(s,1*s);
[sortedA,ind] = sort(A,2);
tic;
for r = 1:size(A,1)
B(r,:) = B(r,ind(r,:));
end,tt(k,1,s==siz) = toc;
tic;
m = size(A,1);
B = B(bsxfun(@plus,(ind-1)*m,(1:m).'));
tt(k,2,s==siz) = toc;
end
end
m = squeeze(mean(tt,1));
m(1,:)./m(2,:)
For square arrays
ans =
0.7149 2.1508 1.2203 1.4684 1.2339 1.1855 1.0212 1.0201 0.8770 0.8584 0.8405
For twice as many columns as there are rows (same number of rows)
ans =
0.8431 1.2874 1.3550 1.1311 0.9979 0.9921 0.8263 0.7697 0.6856 0.7004 0.7314