# Skip subs/index values with accumarray

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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

Stephen Cobeldick on 7 Jun 2018
Edited: Stephen Cobeldick on 7 Jun 2018
Use unique's third output to get the indices:
[uni,~,idx] = unique(groupID(:));
groupMeans = accumarray(idx,A,[],@mean);
[uni,groupMeans]

leonidas86 on 7 Jun 2018
How can I do this?
Stephen Cobeldick on 7 Jun 2018
"How can I do this?"
The histc help explains how the second input defines the bin ranges. If you want to change the bin ranges to make wider bins then you need to change the vector that you used (to have a larger step size).
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.