[R] to extract particular date/data

[Richard.Cotton at hsl.gov.uk]

> If I have this daily rainfall data, how do call a particular day?
> Year,Month,Day,Amount
> 1900,12,22,1.3
> 1900,12,23,0
> 1900,12,24,0
> 1900,12,25,0
> 1900,12,26,0
> 1900,12,27,0
> 1900,12,28,0
> 1900,12,29,4.8
> 1900,12,30,0.3
> 1900,12,31,0.5
> 1901,1,1,0
> 1901,1,2,3
> 1901,1,3,0
> 1901,1,4,0.5
> 1901,1,5,0
> 1901,1,6,0
> ...
> I used to use julian.date in S-Plus.

I read your data in as the following data frame:
> rainfall
   Year Month Day Amount
1  1900    12  22    1.3
2  1900    12  23    0.0
3  1900    12  24    0.0
4  1900    12  25    0.0
5  1900    12  26    0.0
6  1900    12  27    0.0
7  1900    12  28    0.0
8  1900    12  29    4.8
9  1900    12  30    0.3
10 1900    12  31    0.5
11 1901     1   1    0.0
12 1901     1   2    3.0
13 1901     1   3    0.0
14 1901     1   4    0.5
15 1901     1   5    0.0
16 1901     1   6    0.0

You can access only the days with rainfall$Day, but you might also like to 
convert all the dates to POSIX format.  This code will do the trick:
posixdates <- with(rainfall, ISOdate(Year, Month, Day))

posixdates is a vector of class POSIXct, which means it is the number of 
seconds since the start of 1970, with some methods to make it print nicely 
and behave itself in time series plots.

See also ?julian, and the chron package.

Regards,
Richie.

Mathematical Sciences Unit
HSL


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