How to do a group by in matlab
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Gimpy
le 26 Mai 2013
Réponse apportée : Lola Davidson
le 3 Juin 2024
Hi, I have the following data:
data=[10 1 2 3; 11 4 5 6;10 0 20 30; 11 4 5 6; 12 7 8 9; 17 40 50 60]
I want to look for:
lookfor=[10;11]
and get the following result:
anwser=[10 1 22 33; 11 8 10 12]
So it's a group by...
I'm looking for a dynamic anwser, data matrix and lookfor matrix will vary and be much more bigger.
thank you in advance for your precious anwsers.
2 commentaires
Azzi Abdelmalek
le 26 Mai 2013
Modifié(e) : Azzi Abdelmalek
le 26 Mai 2013
It's grouped by what? how did you get 1,22 and 33?
Réponse acceptée
Azzi Abdelmalek
le 26 Mai 2013
Modifié(e) : Azzi Abdelmalek
le 26 Mai 2013
data=[10 1 2 3; 11 4 5 6;10 0 20 30; 11 4 5 6; 12 7 8 9; 17 40 50 60]
lookfor=[10;11];
a=arrayfun(@(x) data(find(data(:,1)==x),:),lookfor,'un',0);
b=cell2mat(cellfun(@(x) [x(1) sum(x(:,2:end),1)],a,'un',0))
%or
data=[10 1 2 3; 11 4 5 6;10 0 20 30; 11 4 5 6; 12 7 8 9; 17 40 50 60]
lookfor=[10;11]
for k=1:numel(lookfor)
ii=data(ismember(data(:,1),lookfor(k)),:);
res(k,:)=[ii(1,1) sum(ii(:,2:end))];
end
res
9 commentaires
Azzi Abdelmalek
le 27 Mai 2013
Modifié(e) : Azzi Abdelmalek
le 27 Mai 2013
You should clear the variable res
clear res
% or better, pre-allocate
res=zeros(numel(lookfor),size(data,2))
Plus de réponses (2)
Andrei Bobrov
le 27 Mai 2013
[i1,i2] = ismember(data(:,1),lookfor);
d2 = data(i1,2:end);
[j1,j2] = ndgrid(i2(i1),1:size(d2,2));
anwser = [lookfor,accumarray([j1(:),j2(:)],d2(:))];
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Lola Davidson
le 3 Juin 2024
For those still stumbling on this, MATLAB now has several more functions to help with grouping workflows, including groupsummary and pivot.
For this problem, if you are expecting several different lookfor values on the same dataset, it may be faster to compute all the sums with groupsummary in one go:
[sums,grps] = groupsummary(data(:,2:end),data(:,1),"sum");
out = [grps sums]
On the other hand, if you only want to compute a small subset of the grouped sums per dataset, it may be quicker to filter down with ismember first, as others have mentioned.
idx = ismember(data(:,1),lookfor);
[sums,grps] = groupsummary(data(idx,2:end),data(idx,1),"sum");
out = [grps sums]
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