# How to find the indices of same values within a vector and bin them together

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MatlabUser17 on 24 Apr 2018
Commented: Star Strider on 25 Apr 2018
I have two vectors with the same length (N). For example:
A=[ 1 2 3 3 3 3 4 5]; B=[ 9 8 7 6 5 6 7 8];
What I need to do is to take an average over elements in vector 'B' that their corresponding indices in vector 'A' have same values. In this example, element 3,4,5,6 in vector A are all equal to '3' so I need to take an average over (7+6+5+6/4) in vector B. I need to do this over a large data and bin them together. I tried different ways and it didn't work. How can I do this?
Ameer Hamza on 24 Apr 2018

Please look at my answer below. Is this what you are trying to get?

Ameer Hamza on 24 Apr 2018
The following statement will return the list of all the averages according to the way you mentioned in your question.
meanVector = splitapply(@(x) mean(x), B', A')
since A have 5 unique elements, meanVector will also contain 5 elements corresponding to the mean value from B.
Ameer Hamza on 25 Apr 2018
You are welcome.

Star Strider on 25 Apr 2018
Edited: Star Strider on 25 Apr 2018
See if this works with your data:
A=[ 1 2 3 3 3 3 4 5];
B=[ 9 8 7 6 5 6 7 8];
[Au,~,ix] = unique(A(:),'stable'); % Unique Elements & Indices (Convert To Column Vector), Keep Original Order
Ar_mean = accumarray(ix, B, [], @mean); % Means Of Single & Repeated Values
Result = [Au, Ar_mean] % Elements Of ‘A’ In Column #1, Corresponding ‘mean’ Of ‘B’ In Column #2
AB_Tbl = table(Au, Ar_mean) % Table (Optional)
Result =
1 9
2 8
3 6
4 7
5 8
AB_Tbl =
Au Ar_mean
__ _______
1 9
2 8
3 6
4 7
5 8
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Star Strider on 25 Apr 2018
As always, my pleasure.