## Find the maximum value in each group of a big matrix

### Yaser Khojah (view profile)

on 26 Apr 2019
Latest activity Commented on by Yaser Khojah

on 26 Apr 2019

### Jan (view profile)

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)];

Yaser Khojah

### Yaser Khojah (view profile)

on 26 Apr 2019
so I should have something the below and I want to know ther indices in the original from the original data
1 3.71378812760058
3 11.1735613881447
NaN -0.0684932810334807
Jan

### Jan (view profile)

on 26 Apr 2019
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

### Yaser Khojah (view profile)

on 26 Apr 2019
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

on 26 Apr 2019
Edited by Jan

### Jan (view profile)

on 26 Apr 2019

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.

Rik

### Rik (view profile)

on 26 Apr 2019
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

### Yaser Khojah (view profile)

on 26 Apr 2019
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))