using find on each column of a matrix independently
60 Ansichten (letzte 30 Tage)
Ältere Kommentare anzeigen
Hello, I would like to use the find function on each row independently in a vectorized way.
for example, I want to find the first point that is above a threshold defined by which column I am in, let this be thresh(j) where j is the column.
let M be an m by n matrix, then for loop way of doing this is as follows:
firstoverthresh = zeros(1,n);
for j = 1:n
firstoverthresh(j) = find(M(:,j)>thresh(j),1);
end
this should return the n sized vector (since there are n columns) of values that I am trying to find.
Thank you :)
0 Kommentare
Antworten (3)
Image Analyst
am 13 Jul. 2015
There's nothing wrong with that way. It's fine: it works, it's intuitive, and it's fast. I don't know of any non-looping way that will be faster.
1 Kommentar
Image Analyst
am 14 Jul. 2015
To compare Matt's solution and your "for loop" solution (with a constant threshold of 0.5:
n = 10000
M = rand(n,n);
tic
firstoverthresh = zeros(1,n);
for j = 1:n
firstoverthresh(j) = find(M(:,j)>.5,1);
end
toc
tic
map=bsxfun(@gt,M,0.5);
[~, firstoverthresh]=max(map,[],1);
toc
For a large matrix, a hundred million elements or 10,000 by 10,000, the for loop is faster.
Elapsed time is 0.125944 seconds.
Elapsed time is 0.151905 seconds.
while for a smaller array, like n = 100, Matt's bsxfun solution is faster.
Elapsed time is 0.000433 seconds.
Elapsed time is 0.000138 seconds.
though either one is so fast (less than a millisecond) you wouldn't notice the difference.
Matt J
am 13 Jul. 2015
map=bsxfun(@gt,M,thresh(:).');
[~, firstoverthresh]=max(map,[],1);
0 Kommentare
Siehe auch
Kategorien
Mehr zu Logical finden Sie in Help Center und File Exchange
Produkte
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!