# [R] Summing data based on certain conditions

Steve Murray smurray444 at hotmail.com
Wed Mar 31 19:19:41 CEST 2010

Dear all,

I have a dataset of 1073 rows, the first 15 which look as follows:

> data[1:15,]
date year month day rammday thmmday
1   3/8/1988 1988     3   8    1.43    0.94
2  3/15/1988 1988     3  15    2.86    0.66
3  3/22/1988 1988     3  22    5.06    3.43
4  3/29/1988 1988     3  29   18.76   10.93
5   4/5/1988 1988     4   5    4.49    2.70
6  4/12/1988 1988     4  12    8.57    4.59
7  4/16/1988 1988     4  16   31.18   22.18
8  4/19/1988 1988     4  19   19.67   12.33
9  4/26/1988 1988     4  26    3.14    1.79
10  5/3/1988 1988     5   3   11.51    6.33
11 5/10/1988 1988     5  10    5.64    2.89
12 5/17/1988 1988     5  17   37.46   20.89
13 5/24/1988 1988     5  24    9.86    9.81
14 5/31/1988 1988     5  31   13.00    8.63
15  6/7/1988 1988     6   7    0.43    0.00

I am looking for a way by which I can create monthly totals of rammday (rainfall in mm/day; column 5) by doing the following:

For each case where the month value and the year are the same (e.g. 3 and 1988, in the first four rows), find the mean of the the corresponding rammday values and then times by the number of days in that month (i.e. 31 in this case).

Note however that the number of month values in each case isn't always the same (e.g. in this subset of data, there are 4 values for month 3, 5 for month 4 and 5 for month 5). Also the months will of course recycle for the following years, so it's not simply a case of finding a monthly total for *all* the 3s in the whole dataset, just those associated with each year in turn.

How would I go about doing this in R?

Any help will be gratefully received.

Many thanks,

Steve

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