how to calculating monthly average?

hi all I just want to be sure about this small monthly averaging code

 I=1;
 for i=1: length(x)/30; 
    y(i)=mean(x(I:I+29));
    L(i)=min(x(I:I+29));
    U(i)=max(x(I:I+29));
    I=I+29;
 end

where x is data matrix (365 day)

thank you

[Star Strider]

We cannot reassure you, unless all months have 30 days on your planet!

We would have to see your ‘data matrix’, preferably as a .mat file.

[Lilya]

the attached mat file is the data

[Star Strider]

Well, some of the data, but not what we need to do what you want with it.

Without knowing the times (at least months-days-years, in some format) at which these are collected, it’s not possible to separate them by month.

[Lilya]

Thank you for help

Answer 1

[Image Analyst]

Try this:

dailyValues = rand(1,365);  % Sample data

daysInMonths = [31,28,31,30,31,30,31,31,30,31,30,31]
monthEnds = [0, cumsum(daysInMonths)]
for m = 1 : length(monthEnds)-1
  firstDay = monthEnds(m)+1;
  lastDay = monthEnds(m+1);
  fprintf('For month #%d, the first day = %d and the last day = %d\n',...
    m, firstDay, lastDay);
  monthlyAverage(m) = mean(dailyValues(firstDay:lastDay));
end
% Echo to command window.
monthlyAverage

Of course you need to take leap years into account, which I didn't do.

[Lilya]

could you please help me (regarding the previous code) to extract the data matrix for each month separatly?

[Image Analyst]

That would just be

thisMonth = dailyValues(firstDay:lastDay);

[Lilya]

Thank you Analyst. but it doest extract it month by month

[Image Analyst]

What exactly do you want? You just specify the starting index (day) and the ending index (day) and extract them from your complete array of the whole year. Again, what do you want and why can't this approach get you the values of a single month?

[Lilya]

I only get one matrix (1*31), I want 12 matrices one by one.

[Image Analyst]

JanuaryDailyAmounts = dailyValues(1:31);
FebruaryDailyAmounts = dailyValues(32:59);
MarchDailyAmounts = dailyValues(60:90);

and so on for months ending at rows 120 151 181 212 243 273 304 334 365.

You will then have 12 matrices.

[Lilya]

I appreciate your help. Thank you.

Answer 2

[Steven Lord]

Find the month number (either directly from the file or using some of the tools associated with datetime objects then use the month numbers as the subscripts in a call to accumarray, specifying @mean as the accumulation function.

numRows = 30;
X = [randi(12, numRows, 1), randi(100, numRows, 1)] % Column 1 is month, column 2 is data
Y = accumarray(X(:, 1), X(:, 2), [], @mean)

[Lilya]

I appreciate your help