For loop versus Matrix notation

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mutt
mutt 2013-1-29
Can this be simplified to use a matrix expression instead of a for loop?:
X=[1 5.4; 1 6.3; 2 4.8; 3 7.1];
dates=X(:,1);
amounts=X(:,2);
uniquedates=unique(dates);
totals=zeros(size(uniquedates));
for d = 1:size(uniquedates,1)
totals(d,1)=sum(amounts(dates==uniquedates(d,1)));
end
Y = [uniquedates totals];
  2 个评论
Matt Kindig
Matt Kindig 2013-1-29
Probably, but likely not in an easy-to-understand way. The fact that totals differs in size from dates complicates things.
Is there a reason you don't want to just use the for-loop?
mutt
mutt 2013-1-30
I want to find out if there is a matrix approach which can outperform the for loop when the inputs are non-trivial

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Oleg Komarov
Oleg Komarov 2013-1-30
You can use accumarray():
Y = [uniquedates accumarray(X(:,1),X(:,2))];
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Oleg Komarov
Oleg Komarov 2013-1-30
Map the dates to a set X in the positive N, i.e. use unique on the dates and then X(:,1) is the ia index from the call to unique.
mutt
mutt 2013-1-31
This approach has delivered a 99% time saving versus the for loop.

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