how to select from identical rows based on their associated value in one column?
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Deepika Vatsa
on 3 Apr 2019
Commented: Star Strider
on 4 Apr 2019
I have a matrix of 2 columns and lots of rows. First column has numeric values that occurred twice and second column has probabilities associated with it.
for e.g., a = [1 , 0.6
2 , 0.9
1, 0.4
3 , 0.5
4 , 0.2
2 , 0.1
4 , 0.8
3 , 0.5]
I want choose rows which have elements with higher probability i.e.,
ans = [1 , 0.6
2 , 0.9
4 , 0.8]
I want ignore rows with same probability. How to do this in fastest manner possible?
2 Comments
Accepted Answer
Star Strider
on 3 Apr 2019
Another approach using accumarray:
a = [1 , 0.6
2 , 0.9
1, 0.4
3 , 0.5
4 , 0.2
2 , 0.1
4 , 0.8
3 , 0.5];
[Ua,~,ix] = unique(a(:,1)); % Unique Values & Indices
L = accumarray(ix, a(:,2), [], @(x)max(sum(x)~=2*x)); % Eliminate Duplicates (Returns Logical Vector)
R = accumarray(ix, a(:,2), [], @max); % Maximum Probabilities
Out = [Ua(L), R(L)] % Report Only Maxima For Non-Duplicated Probabilities
producing:
Out =
1 0.6
2 0.9
4 0.8
4 Comments
Star Strider
on 4 Apr 2019
As always, my pleasure.
I had to think about that.
This works:
a = [1 , 0 , 0.6
1 , 1 , 0.4
2 , 0 , 0.1
2 , 1 , 0.9];
[Ua,~,ix] = unique(a(:,1)); % Unique Values
L = accumarray(ix, a(:,3), [], @(x)max(sum(x)~=2*x)); % Eliminate Duplicates (Returns Logical Vector)
R = accumarray(ix, a(:,3), [], @max); % Maximum Probabilities
S = accumarray(ix, a(:,3), [], @(x) a(find(x==max(x)),2)); % Get ‘sign’
Out = [Ua(L) S(L), R(L)] % Report Only Maxima & Sign For Non-Duplicated Probabilities
producing:
Out =
1 0 0.6
2 1 0.9
More Answers (1)
Bob Thompson
on 3 Apr 2019
Edited: Bob Thompson
on 3 Apr 2019
There might be some fancy function I don't know, but it's certainly possible to do this with a loop. I cannot comment on the speed of this method.
a = [1 , 0.6
2 , 0.9
1, 0.4
3 , 0.5
4 , 0.2
2 , 0.1
4 , 0.8
3 , 0.5];
uni = unique(a(:,1));
b = [];
for i = 1:length(uni)
b(i,:) = a(a(:,2) == max(a(a(:,1)==uni(i),:)),:);
end
EDIT* Changed from uniques in second column to uniques in first column. I misread the question.
4 Comments
madhan ravi
on 3 Apr 2019
Edited: madhan ravi
on 3 Apr 2019
Bob,I am not able to run your code but according to what I understand from the question is that if the probabilities are identical OP wants to omit those rows and find the max of probabilities of each group (which belong to column 1). Haven’t tested with the timings though.
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