How to speed up simple sorting

2017 2 8
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更新时间：2017 2 9
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Hello, I got two variables (Units and time points) that needs to me sorted with respect to each other. Meaning, I want to sort all the time points to its specific Unit. With large numbers of Units and many time points this sorting takes forever. Is there a way to speed this up?
What I got so far works, but it is rather slow:
%Prepare example RAW data
Unit=int32(randi(60,5000000,1)); %60 Units in an order, that matches the timepoints
timepoints=randi(600,5000000,1); %5*10^6 Timepoints (from a range of 600ms) in an order, that corresponds to the according unit
%Prepare the variable where the data should be sorted
for i=1:60
Unit_sort{i,1}=i;
Unit_sort{i,2}=[];
end
%Sort "time points" to the according "Unit"
for i=1:length(timepoints)
index = find([Unit_sort{:,1}] == Unit(i));
Unit_sort{index,2}=[Unit_sort{index,2} timepoints(i)];
end

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Jan
Jan 2017-2-8
Typo: "Units(i)" must be "Unit(i)". But after fixing this, the code fails, because find([Unit_sort{:,1}] == Units(i)) must be empty from the beginning.
Please post some code, which performs, what you want to achieve.

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Jan
Jan 2017-2-8
编辑：Jan 2017-2-8

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2 general rules:
  • Pre-allocate the output. The iterative growing of arrays is a Don't!
  • Prefer to run the loop over the smaller array.
uUnit = unique(Unit);
Unit_sort = cell(1, length(uUnit));
for k = 1:length(uUnit)
Unit_sort{k} = timepoints(Unit == uUnit(k));
end
Perhaps this does what you want - I'm not sure because your posted code does not run and I cannot guess, what the wanted result is. But if it runs, read the docs of splitapply and accumarray.

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Felix Ludwig
Felix Ludwig 2017-2-9
Thank you for your help, I corrected the code I provided - now it works (In the old slow way).
I will try to apply your suggestions now and see, if it speeds up the procedure ...
Felix Ludwig
Felix Ludwig 2017-2-9
ok, I tried your code and indeed its waaaay faster. Thank you! Note to myself: Always run the loop over the smaller array!

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Steven Lord
Steven Lord 2017-2-8

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I think what you're trying to do is sortrows.
% Sample data
A = randi(10, [20 2]);
% Sort by column 1, use column 2 to break ties
B = sortrows(A);
% Sort by column 2, use column 1 to break ties
C = sortrows(A, [2 1]);
% Show all three side-by-side
T = table(A, B, C)

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Felix Ludwig
Felix Ludwig 2017-2-9
Thank you for your input, It's not exactly what I meant. I want to have for example 60 unique Units and sort a high number of timepoints to the unit they belong to. I updated the provided code above. Now it works - in the original slow way. But I will try to apply the sortrow procedure to my problem.

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