MATLAB: How to produce a matrix that averages the top 5 values, for a corresponding year

averagematrix

I would like to produce an average of the top 5 values (on the 5th column, blue) for the corresponding year. I have attached my initial data as an attachment. My difficulty is that the matrix has multiple columns, so how do I choose that column (circled in blue), to calculate averages?

Best Answer

Let A be your data of dimensions n*4. A has n number of rows and four columns. YOu can extract fourth column from A using.

flows = A(:,4) ;   % This gives you fourth column 
years = A(:,1) ;   % This extracts the years from the data 
% GEt mean of top five elements 
iwant = mean(flows(1:5)) ;

If you have multiple years in the data, then you need to extract the respective year flows and get mean of top five values.

flows_1938 = flows(years==1938) ;    % This gives flows of the year 1938
iwant_1938 = mean(flows(1:5)) ;

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MATLAB: How to subtract mean daily values from hourly data

So, based on what you've done so far here is a spiffied up version:

%%generate sample data.
dateN = datenum('03-Sep-2014 15:26:10');
total15minsamps = 4*24*4; %4 days * 24 hours* 4 (15 min sec).
DATA = zeros(total15minsamps,7);
for ind = 1:total15minsamps %sample every 15 min.  
    data= 10+5*sin(2*pi/12*ind);    
    dateN = addtodate(dateN,15,'minute');
    Dvect = datevec(dateN);
    DATA(ind,:) = [Dvect data];
end
%%parse data
Dates = DATA(:,1:6);
Flow = DATA(:,end);
QuarterH_Time = datenum(DATA(:,1:end-1));
%%get hour's average flow
[unDates, HourSep, subs] = unique(Dates(:,1:4),'rows');
% Accumulate by hour
Hour_Data=[unDates accumarray(subs, Flow, [], @mean)];
Hour_AVG_flow=Hour_Data(:,5);
Hour_Time=datenum([Hour_Data(:,1:4) zeros(length(Hour_Data),2)]);
%%get day's average flow
[unDates, DaySep, subs] = unique(Dates(:,1:3),'rows');
% Accumulate by day
Day_Data=[unDates accumarray(subs, Flow, [], @mean)];
Day_AVG_flow=Day_Data(:,4);
Day_Time=datenum(Day_Data(:,1:3));
Day_Time = Day_Time + QuarterH_Time(1)-Day_Time(1); %offset so we don't start at the begining of the day
%%build subtraction matrix
DaySep = [DaySep;length(Flow)+1];
DeltaFlow = zeros(size(Flow));
for ind =1:length(DaySep)-1
    POI = DaySep(ind):DaySep(ind+1)-1;
    DeltaFlow(POI)=Flow(POI)-Day_AVG_flow(ind);
end
%%lets plot stuff
figure,plot(Hour_Time,Hour_AVG_flow,Day_Time,Day_AVG_flow,QuarterH_Time,DeltaFlow);
legend('Hourly Flow','Daily Flow','15 min data - day average','location','best');
datetick ('x', 'mm-dd-yy', 'keepticks')
xlabel('Time');
ylabel('Flow [cfs]');
title('Daily Flow');

Basically most of the stuff you can ignore because i needed to do some extra stuff to generate dummy data. But keep close attention to what i was doing with the DaySep array. Since we have two different length arrays that are based on the data i needed to use this value that is given by the unique() function to determine which indexes were deleted. With that i now know which 15 min data points correspond to which day.

MATLAB: How to do an average for certains values of a column

I have done it wit some random data generated. You can follow the same procedure for your data.

A = randi(100,1000,2) ;  % generate random data which repeats
 years = A(:,1) ;
 value = A(:,2) ;
 [c,a,b] = unique(years) ;
 iwant = zeros(length(c),2) ;
 for i = 1:length(c)
     idx = find(years==c(i)) ;
     % get mean 
     iwant(i,:) = [c(i) mean(value(idx))] ;
 end

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MATLAB: How to subtract mean daily values from hourly data

MATLAB: How to do an average for certains values of a column

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