How can I find the max and min amongst a set of grouping variables?
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Prateek Samuel Daniels
on 3 Feb 2015
Commented: David Young
on 3 Feb 2015
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
1 23
1 29
1 43
2 38
2 22
2 100
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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Accepted Answer
Michael Haderlein
on 3 Feb 2015
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.
More Answers (1)
David Young
on 3 Feb 2015
% 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.
3 Comments
David Young
on 3 Feb 2015
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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