How to find rank within group

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Garima Sharma
Garima Sharma 2017-6-26
评论： Simon 2023-8-7
I have a dataset with 100 different groups, and between 8-11 entries in each group. I want to rank them, within a group, by the value of a variable y. How would I do this?
Concretely, if I have 2 groups with 2 people each such that: y=[1; 3; 6; 5] group_id=[1;1;2;2]
then I want a third vector: rank=[1;2;2;1]

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采纳的回答

Andrei Bobrov
Andrei Bobrov 2017-6-27
编辑：Andrei Bobrov 2017-6-27
[~,a] = cellfun(@sort,accumarray(group_id,y,[],@(x){x}),'un',0);
rank = cell2mat(a);
or within for..end:
y=[1; 3; 6; 5];
group_id=[1;1;2;2];
rank = group_id;
a = unique(group_id);
for ii = 1:numel(a)
t = group_id == a(ii);
[~,rank(t)] = sort(y(t));
end
  2 个评论
Simon
Simon 2023-8-7
The result created by the cellfun-based method is different from the for-loop based method when group_id is not in sorted order. The within-group ranking is the same in both methods. But rank = cell2mat(a) put all 1st group's within-group ranks together, followed by the second group's.
I have came across similar problem and in a subsequent analysis I need to put rank as a new column side-by-side next to group_id. The cellfun-based method would give me erroneous result.

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更多回答（1 个）

Jess Lovering
Jess Lovering 2017-6-26
Do you mean that you want to sort the data by a specific array? You can do this using the sort function. It will also give you the indices of the new order so you can use that to apply to the other arrays, example:
y=[1; 3; 6; 5]
group_id=[1;1;2;2]
rank=[1;2;2;1]
[sorted_y,I]= sort(y)
sorted_group_id = group_id(I)
sorted_rank = rank(I)
  1 个评论
Garima Sharma
Garima Sharma 2017-6-27
no, what I want to do is rank y=[1; 3; 6; 5] within its respective group_id=[1;1;2;2].
So, y(1,1) is rank 1 in group 1; y(2,1) is rank 2 in group 1. y(3,1) is rank 2 in group 2, and y(4,1) is rank 1 in group 2. I want to only input vectors y and group, and get vector rank as the output.

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