calculate the accumulation of a variable over time within a loop?

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Michael 2014-9-17

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编辑： dpb 2014-9-17

Hello,

I have evaporation as daily values (m). I have calculated the weighted fieldmean, which gives me a vector with length = no. of days. However, I would like to display these values as monthly sums.

How can I do this in a loop?

For example:

newsum(1) = nansum(data(1:30)) and so forth...

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dpb 2014-9-17

Do you have a corresponding date/day of month vector or does the series simply begin at a known date? And, of course, year to know if leap year or not...

Michael 2014-9-17

it starts in 1993 1st January and ends 2010 31st December, i'm not sure how to do it other than every 30 days..

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Sean de Wolski 2014-9-17

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Once you get the month data, the code will be almost identical to this answer:

http://www.mathworks.com/matlabcentral/answers/155115#answer_151972

Instead of @std, you'll want to pass in @sum

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Michael 2014-9-17

I have read the documents but I can't seem to find a way of increasing the interval to 3 or 6 months, is there a way to do this?

Michael 2014-9-17

I have realised it makes more sense to calculate 3-month sums..I know how to do this the long way but wondered if I can modify this function

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dpb 2014-9-17

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编辑：dpb 2014-9-17

在 MATLAB Online 中打开

... to calculate 3-month sums...

There it's simpler to use another Matlab "trick" of recognizing and utilizing internal storage order and Matlab's handling of arrays. Generate the vector of monthly sums as above then to group by threes reshape and sum over the reshaped array and recast back...

mnthsum=accumarray(...      % the monthly sums as above
threemnth=sum(reshape(mnthsum,3,[])).';  % sum over columns of 3 row/each

The above relies on the linear storage order and that Matlab orders arrays/matrices in column-major order and the default action of functions such as sum is to operate over columns. This "trick" is useful for any such operation over a given evenly-divisible subset of any array.

ADDENDUM

If it's decided that the monthly sums aren't needed any longer, the above can be done in place, of course...

mnthsum=sum(reshape(mnthsum,3,[])).';