Find the maximum value in each group of a big matrix

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Yaser Khojah 2019-4-26

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https://ww2.mathworks.cn/matlabcentral/answers/458763-find-the-maximum-value-in-each-group-of-a-big-matrix

评论： Yaser Khojah 2019-4-26

I have a big matrix and I'm looking for an easy way to group them by a certain way then find the maximum in each group. I have created this code but I got stuck. Anyway to help please. I need to know the indexes of all the maximum points to extract more information from the big matrix.

data          = randn(10,1)*10;
edges         = 0:5:35';
Y             = discretize(data,edges);
[B,I]         = sort(Y);
SortedMatrix  = [B data(I)];

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Jan 2019-4-26

编辑：Jan 2019-4-26

So is sortedMatrix your actual input data? Or do you want to process data, which is set to a random array for a demonstration?

It would be easier, if you avoid to create the NaNs, e.g. by adding a final edge Inf. Then splitapply(@max, data(I), B) would solve the problem.

Yaser Khojah 2019-4-26

编辑：Yaser Khojah 2019-4-26

在 MATLAB Online 中打开

Data.mat

I have attached my data (Data) and I will explain what I'm trying to do on this file.

edges         = 0:5:35';
% i want to split column 18 based on edges 
Y             = discretize(Data(:,18),edges);
% this is see the new data sorted 
[B,I]         = sort(Y);
SortedMatrix  = [B Data(I,:)];
% How can i find the max in every edges. It might be there is more than a max but I want to know how they are related to the original data so I need their indexs in the original data

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Answer 1

Jan 2019-4-26

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https://ww2.mathworks.cn/matlabcentral/answers/458763-find-the-maximum-value-in-each-group-of-a-big-matrix#answer_372462

编辑：Jan 2019-4-26

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data   = randn(10,1)*10;
edges  = [-Inf, 0:5:35, Inf];
Y      = discretize(data, edges);
Result = accumarray(Y, data, [], @max)

I'm confused that this does not work reliably with splitapply:

Result = splitapply(@max, data, Y)

It fails with an error message, if an interval is empty:

For N groups, every integer between 1 and N must occur at least
once in the vector of group numbers.

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Rik 2019-4-26

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I was thinking way too complicated here. This was the start of my idea:

nanflag=false;
if any(isnan(B))
    replaceNaN=max(B)+1;
    B(isnan(B))=replaceNaN;
    nanflag=true;
end
b=accumarray(B,Y,[],@max,NaN);

After which removal of empty lines (and putting back the NaN) to get to the desired output should be easy.

Yaser Khojah 2019-4-26

在 MATLAB Online 中打开

MAT_ALL.mat

Dear Jan and Rik. Thank you for your help. I'm sorry if i made you both get confused

So, here what I want to do from what I asked earlier. I have this big file (MAT_All). I would like to create this thick line that shows the maximum at every x-axis. So, that is why I was looking for maximum for every interval but not sure if i will get this from what I asked earlier. Any idea?

scatter(MaT_All(:,18), MaT_All(:,17))

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