How to vectorize the find function?
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This is my code:
for i=1:size(Z,3)
indmax(i)=find(abs(Z(1,:,i))>z90(1,1,i),1);
indmin(i)=find(abs(Z(1,:,i))>z10(1,1,i),1);
end
trise(:,1)=(t(indmax)-t(indmin));
The variable Z is normally 3 or 6 rows, many columns and 5-10 pages. Something like:
size(Z)= 6 1920 8
The variables z10 and z90 have the same number of rows and pages as Z and 1 column. Something like:
size(z10)=size(z90)=6 1 8
I would actually need to repeat this code 6 more times (1 for each row).
Is there a way to vectorize this?
采纳的回答
Sean de Wolski
2011-5-13
%small sample because you only looped over first row
Z = rand(1,5,8)*2.5;
z90 = rand(1,1,8);
z10 = rand(1,1,8);
%Your way
for i=1:size(Z,3)
indmax(i)=find(abs(Z(1,:,i))>z90(1,1,i),1);
indmin(i)=find(abs(Z(1,:,i))>z10(1,1,i),1);
end
%Vectozed way:
[jk, indmx] = max(bsxfun(@gt,abs(Z),z90),[],2);
[jk, indmn] = max(bsxfun(@gt,abs(Z),z10),[],2);
indmx = squeeze(indmx);
indmn = squeeze(indmin);
isequal(indmin,indmn)
isequal(indmax,indmx.') %transpose rowvec
This method is extended to the full size, though just looping over the six rows is probably just as easy/fast. I.e. loop 1:6, extract trise on each iteration.
6 个评论
Matt Fig
2011-5-13
And the (you knew this was coming, right?) obligatory timings...
function [] = timeitagain
Z = rand(6,1920,8)*2.5;
z90 = rand(6,1,8);
z10 = rand(6,1,8);
% Proper for loop...
tic
for jj = size(Z,1):-1:1
for ii=size(Z,3):-1:1
R = abs(Z(jj,:,ii));
indmax(jj,ii)=find(R>z90(jj,1,ii),1);
indmin(jj,ii)=find(R>z10(jj,1,ii),1);
end
end
toc
%Vectozed way:
tic
[jk, indmx] = max(bsxfun(@gt,abs(Z),z90),[],2);
[jk, indmn] = max(bsxfun(@gt,abs(Z),z10),[],2);
indmx = squeeze(indmx);
indmn = squeeze(indmin);
toc
isequal(indmin,indmn)
isequal(indmax,indmx) %transpose rowvec
Now running from the command line:
Elapsed time is 0.001451 seconds.
Elapsed time is 0.012311 seconds.
Sean de Wolski
2011-5-13
At first your results made me sad. But vectorizing consistently wins by more 30% on my system! (I squeezed the ABS out of both of your and my bsx).
function [t1 t2] = timeitagain
Z = rand(6,1920,8)*2.5;
z90 = rand(6,1,8);
z10 = rand(6,1,8);
% Proper for loop...
tic
Z2 = abs(Z);
for jj = size(Z,1):-1:1
for ii=size(Z,3):-1:1
R = Z2(jj,:,ii);
indmax(jj,ii)=find(R>z90(jj,1,ii),1);
indmin(jj,ii)=find(R>z10(jj,1,ii),1);
end
end
t1 = toc;
%Vectozed way:
tic
Z3 = abs(Z);
[~, indmx] = max(bsxfun(@gt,Z3,z90),[],2);
[~, indmn] = max(bsxfun(@gt,Z3,z10),[],2);
indmx = squeeze(indmx);
indmn = squeeze(indmn);
t2 = toc;
if ~isequal(indmin,indmn) || ~isequal(indmax,indmx)
error('Sean you failed vectorizing!');
end
Called with:
t1 = 0;
t2 = 0;
for ii = 1:10
[t1c t2c] = timeitagain;
t1 = t1+t1c;
t2 = t2+t2c;
end
t1/t2
%{
ans =
1.3154
ans =
1.3181
ans =
1.3993
ans =
1.3189
%}
Sean de Wolski
2011-5-13
Ps. How's your new laptop?
Matt Fig
2011-5-13
Figures, it must be improvements in newer versions. I am still using 2007b.
My new laptop was delayed a day. "Out for delivery" today though!!
(P.S. Lenovo w520 fully loaded...)
Sean de Wolski
2011-5-13
^^^^Jealous.
Bernard Küsel
2011-5-16
更多回答（1 个）
Doug Hull
2011-5-13
0 个投票
Why vectorize?
Just add another for loop for the rows.
4 个评论
Bernard Küsel
2011-5-13
Matt Fig
2011-5-13
You will find that pre-allocating your arrays will speed your code up tremendously if your data is that large.
Doug Hull
2011-5-13
Your question is then about speeding up the code, not about vectorizing?
Bernard Küsel
2011-5-16
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2011-5-13
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