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# How to convert daily data to monthly?

I have huge number of precipitation daily data in a column (say for 60 years) for 400 meteorological stations. Now I need to convert it to monthly. How can I do that? thanks.

per isakson on 5 Dec 2012

Months have 28, 29, 30 or 31 days, which makes it a bit tricky.

Do you have a table with

`    date, value `

rather than just a column?

Does the length of the months matter?

my data format is Year Month Day data

that is in 4 column. Length of month is always 30. Does it help for providing your answer? thanks.

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Answer by Babak on 5 Dec 2012

Every 30 data rows, I would average them out and save it in 1 row.This way, 30 days shrinks down to 1 data (a month). Or another way, you can simply delete 29 data rows and keep only one from every 30 data rows.

Babak on 5 Dec 2012

you need to write what your variables are and we would know which format they are... I mean what type they are for example, string? double, or cell? moreover they are all in a matrix or a cell or how you ahev sorted them..

Answer by Andrei Bobrov on 5 Dec 2012
Edited by Andrei Bobrov on 5 Dec 2012

one way

M - your data matrix with 4 columns < Year Month Day data > - double type

```[a,~,c] = unique(M(:,1:2),'rows');
out = [a, accumarray(c,M(:,4),[],@mean)];
```

Answer by per isakson on 5 Dec 2012
Edited by per isakson on 5 Dec 2012

Given thirty days in every month try this

```    %%
daily_val = rand( 30*12, 1 );
%%
daily_val = reshape( daily_val, 30, [] );
%%
monthly_val = sum( daily_val, 1 );```

here I assume that the first value in daily_val is the first value of a month. This is intended as a start.