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How to do a group by in matlab

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Gimpy
Gimpy 2013-5-26
回答： Lola Davidson 2024-6-3
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 个评论
Azzi Abdelmalek
Azzi Abdelmalek 2013-5-26
编辑：Azzi Abdelmalek 2013-5-26
It's grouped by what? how did you get 1,22 and 33?
Gimpy
Gimpy 2013-5-26
I'm looking for the data associate with 10 and 11 lookfor=[10;11]
The element associate with the look for a group sum except the data I'm looking for:
lookfor=[10;11]
1 2 3 + 0 20 30; this is for the query on value 10
for 11
4 5 6; + 4 5 6 final result:
lookfor=[10;17]
anwser=[10 1 22 33; 11 8 10 12]

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采纳的回答

Azzi Abdelmalek
Azzi Abdelmalek 2013-5-26
编辑：Azzi Abdelmalek 2013-5-26
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 个评论
显示 7更早的评论
Azzi Abdelmalek
Azzi Abdelmalek 2013-5-27
编辑：Azzi Abdelmalek 2013-5-27
You should clear the variable res
clear res
% or better, pre-allocate
res=zeros(numel(lookfor),size(data,2))
Gimpy
Gimpy 2013-5-27
wonderful thanks a lot ! merci

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更多回答（2 个）

Andrei Bobrov
Andrei Bobrov 2013-5-27
[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(:))];
Lola Davidson
Lola Davidson 2024-6-3
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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