Optimize the FOR loop
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I am learning to optimize the following nested FOR loop, any comments are appreciated,
x=[537 558 583 606 631 655 666 700 722 799 823 847];
y=[48 216 384 552 720 888 1056];
z = zeros(1,numel(x));
for j = 1:numel(x)
for i = 1:numel(y)
if(x(j) <= y(i) )
z(j) = i;
break;
end
end
end
%ans
%z = [4 5 5 5 5 5 5 5 6 6 6 6]
回答（2 个）
Sean de Wolski
2012-6-28
编辑：Sean de Wolski
2012-6-28
If you can guarantee that the is at least one occurence of x(:) <y (:) then this will work:
x=[537 558 583 606 631 655 666 700 722 799 823 847];
y=[48 216 384 552 720 888 1056];
[~,z] = max(bsxfun(@le,x,y'),[],1)
4 个评论
Ryan
2012-6-28
I have never used arrayfun or cellfun or bsxfun before and am trying to understand them and have two questions about this solution:
- Why do you need to transpose y?
- Is the ~ used to not waste space saving the max values because all you needed was the indices?
Sean de Wolski
2012-6-28
Hi Ryan,
For your first question,
bsxfun: please see my answer from a long time ago here: http://www.mathworks.com/matlabcentral/answers/16257#answer_22019
cellfun: Apply some function each cell. Can be useful sometimes for simple calculations, e.g: How many elements in each cell?
cellfun(@numel,C);
For any complicated function, just us e a for-loop, it'll likely be faster.
arrayfun: personal opinion is that it is confusing and slow and should be avoided.
Second question: exactly!
Tom
2012-6-28
I just compared arrayfun and bsxfun, the latter is a whole order of magnitude faster.
Ryan
2012-6-28
Thank you Sean and Tom!
Tom
2012-6-28
It seems what you're trying to is find the first instance of each value of x being less than each value of y. You can do this in using arrayfun:
arrayfun(@(n) find(n<y,1),x)
the first argument is an anonymous function. For each value in x, the find function is used to find the first instance of that x value being less than the y vector.
1 个评论
Tom
2012-6-28
Seeing what Sean said, this way also only works if there is an occurrence of for all of them- if there isn't then 'UniformOutput' has to be set to false, which means the output will be a cell array.
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