How can I find the max and min amongst a set of grouping variables?

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Prateek Samuel Daniels 2015-2-3

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评论： David Young 2015-2-3

I have a 150x2 matrix in which the first column contains numbers that can be considered 'grouping variables' and the second column contains values associated with those grouping variables. So a small 6x2 version would look like this

I'd like to find the maximum of the values associated with each grouping variable. Similarly the minimum amongst the values associated with each grouping variable. So the above example would yield an max of 43(for variable 1) and min of 23 (for variable 1). Similarly, max of 100 (for variable 2) and min of 22 (for variable 2). Does anybody know of a similar function or method to achieve this in Matlab?

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Answer 1

Michael Haderlein 2015-2-3

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Since you didn't write if every number will occur in the first column, I allow for "missing" numbers here (it doesn't make a big difference anyway):

a=1+fix(0:.23:8)' ;
b=randi(50,length(a),1);
M=[a b]
nums=unique(M(:,1));
mmin=zeros(max(nums),1);
mmax=zeros(max(nums),1);
for cnt=1:max(nums)
  mmin(nums(cnt))=min(M(M(:,1)==nums(cnt),2));
  mmax(nums(cnt))=max(M(M(:,1)==nums(cnt),2));
end

If every number will occur, nums(cnt)==cnt, so the expressions inside the loop can be simplified a bit.

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Prateek Samuel Daniels 2015-2-3

Thank u so much Michael. Never in my wildest dreams did I think that people would reply so fast and come up with accurate answers.

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Answer 2

David Young 2015-2-3

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% test data
data = [1 23; 1 29; 1 43; 2 38; 2 22; 2 100];
% compute minima and maxima
mins = accumarray(data(:,1), data(:,2), [], @min);
maxs = accumarray(data(:,1), data(:,2), [], @max);

Then mins(k) and maxs(k) hold the min and max for grouping variable k.

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Prateek Samuel Daniels 2015-2-3

编辑：Prateek Samuel Daniels 2015-2-3

Thank u David. I've tried going thru the documentation on accumarray and I couldn't understand why and how it was used. But when I saw your response, things were pretty clear, except for the [] (the 3rd argument). Why do we need to put that. would it be possible for you to recommend me any matlab advanced/ intermediate book which covers these functions and its usage. Many thanks !!!

David Young 2015-2-3

The third argument specifies the size of the output array, but if you're happy to let it be determined by the values in the first argument you can just let it default by putting [].

I'm afraid I don't know which books cover relatively recent functions like accumarray.

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