Is there a better way to compute metrics on labeled array elements.

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Burke Rosen
Burke Rosen 2018-6-17
编辑： Burke Rosen 2018-6-18
For example, I have a 1d double array 'data' and a 1d cell array of strings called 'labels'. For each unique label I want the mean of the data. The best I have come up with is below. I don't believe this is fully vectorized. Is there a better way?
%%make sample dataset
n = 1000;
data = rand(n,1);
labels = char(randsample(97:122,n,true)');%[a-z]
%%get means for each label
[uniLab,~,labIdx] = unique(labels,'stable');% stable for speed
mu = arrayfun(@(x) mean(data(labIdx==x)),1:numel(uniLab));

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Walter Roberson
Walter Roberson 2018-6-17
  2 个评论
Burke Rosen
Burke Rosen 2018-6-18
编辑：Burke Rosen 2018-6-18
This yields a ~25% speed increase at n = 1e3 and ~5% at n = 1e5. (500 trials per algorithm, randomized order). Thank you.

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更多回答（1 个）

Burke Rosen
Burke Rosen 2018-6-17
Thank you for that tip @Walter.
After further review:
1. The way I wrote the sample data set, labels is actually a character array not a cell array, one has to cellstr it to yield that.
2. mu = grpstats(data,labels,'mean') is compact, easy to read, and maybe 1 or 2 percent faster that my formulation, if one adds the cellstr.
3. My solutions is 5x faster than grpstats if labels is a character rather than a cell array.
4. My guess is that unique operates much faster on character arrays than cell arrays and the runtime of the loop (or arrayfun) over the unique labels is negligible compared the unique itself.

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