Asked by Saad
on 23 May 2013

Dear All,

I need some help please. I have two columns in a matrix B. B=[A, X]; X is a column vector that contains different daily observations. A is a column vector that contains the number of the month (for example for January it has "1" for february "2" etc till December where it has "12").

Column X has daily data for January, Feb till december. Thus Column A has numbers that change according to the month number. For instance, for january, Column A will have thirty one "1" corresponding to 31 days in january and twenty nine "2" corresponding to 29 days in February until the end of the year then i will start again with january with "1" and february with "2" etc till december .

What I would like to do is to do the gemetric mean of the values of X for similar values of A. Basically, it is like if Iam doing a geometric mean month by month untill the end of my data series. How can I do that please? I am stuck in this. ANy help is much appreciated. Best

S

Answer by Saad
on 23 May 2013

Or maybe simpler I could just pick the first and the last day of the month and do a geometric mean of all values in between. Any ideas on how I can do that? Thank you very much

Answer by Eli Duenisch
on 23 May 2013

Edited by Azzi Abdelmalek
on 23 May 2013

Use logical indexing to obtain indices belonging to the same month and calculate the geometric mean for each month:

if true for i=1:12, res(i)=geomean(X(A==i)); end end

If you have no statistics toolbox installed you also need to write a function geomean() that calculates the geometric mean.

Answer by Andrei Bobrov
on 23 May 2013

Edited by Andrei Bobrov
on 25 May 2013

out = accumarray(B(:,2),B(:,1),[],@geomean);

ADD

if the second column of your data is the same as:

b = arrayfun(@(x)x*ones(randi(7),1),repmat((1:12)',3,1),'un',0); b = cat(1,b{:});

and all your array (B):

B = [randi(7,numel(b),1), b];

solution:

out = accumarray(B(:,2:3),B(:,1),[],@geomean);

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