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Skip subs/index values with accumarray

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leonidas86
leonidas86 on 7 Jun 2018
Commented: Walter Roberson on 7 Jun 2018
I created an index (groupID) for a vector A. The vector is sorted from the smallest value to the biggest one. The function and the groupID look like:
[~,groupID]=histc(A,0:5:100);
groupID = [1 1 3 3 3 5 5];
In the next step I want to calculate the average for every groupID. I did this as follows:
groupMeans = accumarray(groupID,A,[],@mean);
My problem is that accumarray fills in the output array for every skipped groupID (because e.g. 2 and 4 are missing in the index) a zero. I dont want these zeros in my output array. Is there a solution for this? My Matlab version is R2012b

Accepted Answer

Stephen
Stephen on 7 Jun 2018
Edited: Stephen on 7 Jun 2018
Use unique's third output to get the indices:
[uni,~,idx] = unique(groupID(:));
groupMeans = accumarray(idx,A,[],@mean);
[uni,groupMeans]
  5 Comments
Walter Roberson
Walter Roberson on 7 Jun 2018
In the original posting you used
histc(A,0:5:100);
This would create 21 bins, [0 5 10 15 20 ... 100]. To increase the width of your bins in this case you would use an increment larger than 5, such as 0:8:100 .
Note: more robust would be to use linspace(0, 100, N) where N is the number of bins you wanted.
hist() and histcounts() permit you to pass the number of bins directly.

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