# Get min from group(s) of data within an array

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Ludo Houben on 3 Nov 2022
Answered: Ludo Houben on 4 Nov 2022
Hi All
As a newbie I'm struggeling with the following.
I have created a subset of a data set into a 'value n' x 'value m' double matrix. It contains values for several days and I need a 'minimum' value per day of a specific column. So in example:
Source: 'n' x 'm' double Result: 'v' x 4 double
2022 | 02 | 01 | 1 | ... | ... | 6 | ...
2022 | 02 | 01 | 2 | ... | ... | 6 | ...
2022 | 02 | 01 | 3 | ... | ... | 5 | ...
2022 | 02 | 02 | 1 | ... | ... | 4 | ...
2022 | 02 | 02 | 2 | ... | ... | 3 | ... ==> 2022 | 02 | 01 | 5
2022 | 02 | 02 | 3 | ... | ... | 6 | ... 2022 | 02 | 02 | 3
2022 | 02 | 03 | 1 | ... | ... | 2 | ... 2022 | 02 | 03 | 2
2022 | 02 | 03 | 2 | ... | ... | 6 | ...
2022 | 02 | 03 | 3 | ... | ... | 3 | ...
For getting the minimum value I can use the 'min()' function, but how to create the 'grouping' of the data?
Thanks in advance for any answers.
Cheers
Ludo
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### Accepted Answer

Ludo Houben on 4 Nov 2022
Hi all
The final solution was combining both answers from Jan and Chris.
% Create table 'X'
X = [2022 02 01 1 6; ...
2022 02 01 2 6; ...
2022 02 01 3 5; ...
2022 02 02 1 4; ...
2022 02 02 2 3; ...
2022 02 02 3 6; ...
2022 02 03 1 2; ...
2022 02 03 2 6; ...
2022 02 03 3 3];
[DayInfo,~] = unique(X(:,1:3),'rows','stable'); % This creates an array 'DayInfo' containing all unique dates
temp = groupsummary(X(:,5),X(:,1:3),"min"); % This gets the minimum value of row 5 for each day
DayInfo = cat(2,DayInfo,temp); % This combines the 2 arrays
%% Combination of formula's temp + DayInfo
DayInfo = cat(2,DayInfo,groupsummary(X(:,5),X(:,1:3),"min")); % This combines the last 2 previous actions
So a big shout out to them for their help.
Regards
Ludo
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### More Answers (2)

Cris LaPierre on 3 Nov 2022
Use groupsummary. If you are really new to MATLAB, consider using the Compute by Group task inside a live script.
Since you haven't attached your data, here's a rough guess at what the code might be. This is written assuming your data is in a table.
dailyMin = groupsummary(tblNm,["Var1","Var2","Var3"],"min","Var7")
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Ludo Houben on 3 Nov 2022
Hi Chris
The solution is not completely the desired format, but it gives me a good direction to continue my quest with. Thank you for your reply.
Regards
Ludo

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Jan on 3 Nov 2022
Is the 4th row a counter for each day? Then this can be used:
(By the way, please post input data in a form, in which they can be used by copy&paste - this makes it much easier to use it in an answer)
X = [2022 02 01 1 6; ...
2022 02 01 2 6; ...
2022 02 01 3 5; ...
2022 02 02 1 4; ...
2022 02 02 2 3; ...
2022 02 02 3 6; ...
2022 02 03 1 2; ...
2022 02 03 2 6; ...
2022 02 03 3 3];
G1 = X(:, 4) == 1;
G = cumsum(double(G1));
minGValue = accumarray(G, X(:, 5), [], @min);
Result = [X(G1, 1:3), minGValue]
Result = 3×4
2022 2 1 5 2022 2 2 3 2022 2 3 2
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Ludo Houben on 4 Nov 2022
Hi Jan
Thanks for the thorrow explaination. This helps me in my further development :-)
Regards
Ludo

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