Calculate median with accumarray
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Hello,
I have an array with two columns. The rows are sorted by the values of the first column and then indexed by the use of histc. Now I want to calculate the median of every interval for the columns. Procedure for the first column is clear with accumarray but before calculating the median for the second column I have to sort every interval, right? How can I do this?
groupMedian = accumarray(ID,Measurement(:,2),[],@median);
采纳的回答
Walter Roberson
2018-10-4
编辑：Walter Roberson
2018-10-4
No, you do not need to sort each interval. Just make sure that each interval is given a distinct ID vector row.
[unique_col1, ~, ID] = unique(Measurement(:,1));
group_median = accumarray(ID, Measurement(:,2), [], @median);
output = [unique_col1, group_median];
No sorting by column 1 beforehand is needed (unless you happen to need that output for a different purpose.)
更多回答（2 个）
Bruno Luong
2018-10-4
1 个投票
Internally the median command "sort" input data and take a middle element of the sorted array (or average of two left and right for even length array). Some other technique exists, such as partial-sorting which has better complexity than SORT (I don't think MATLAB use it however the last time I check it)
You, as user don't have to sort it when calling median, that's how the function like this exists at the first hand.
franky
2018-10-4
0 个投票
1 个评论
Walter Roberson
2018-10-4
It is not required to sort the values before calling median()
data = randi(20, 1, 1000);
>> median(data)
ans =
11
>> median(data(randperm(length(data))))
ans =
11
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