[R] Simple time series question with zoo

[Marc Girondot]

Le 27/10/11 22:18, Vinny Moriarty a écrit :
> New user here. My goal is pull daily averages from a long dataset.
>
> I've been working with some code I got from this list from
>
> https://stat.ethz.ch/pipermail/r-help/2009-March/191302.html
>
>
> The code how I have been using it is as follows:
>
> library(zoo)
> library(chron)
>
> DB<-read.table("/Users/me/Desktop/R/data.csv", sep=",", header=TRUE, as.is
> =TRUE)
> z<-zoo(LTER6$temp, chron(LTER6$Date, LTER6$Time))
> z.day=aggregate(z, trunc, mean) #This last line gives me daily averages for
> my data
>
>
> Simple and elegant- and it works. Thanks to the author the hard part is
> over. But I plan to tweak it so I have some questions about why this works
>
> 1- The data I have has the date and time format as a single string like this
> "2006-04-09 10:20:00". But the code was set up to read the data in two
> columns  ie- "2006-04-09"&  "10:20:00". Is this how the chrom package
> expects to have the data, or is there a way I can change the code to read
> the data as a single column. For now I am chopping up my date and time data
> manually before I run R.
 > strsplit("2006-04-09 10:20:00", " ")[[1]][1]
[1] "2006-04-09"
 > strsplit("2006-04-09 10:20:00", " ")[[1]][2]
[1] "10:20:00"

Then replace with

z<-zoo(LTER6$temp, chron(strsplit(chron(LTER6$DateTime, " ")[[1]][1], strsplit(LTER6$DateTime, " ")[[1]][2]))

>
> 2-  I've read the help on "as.is", and I'm not sure why I need that function
> in the first line of code. This is what my original data looks like (with
> header) if this helps answer this this question
>
> line.site,time_local,time_utc,reef_type_code,sensor_type,sensor_depth_m,temp
> 06,2006-04-09 10:20:00,2006-04-09 20:20:00,BAK,sb39, 2, 29.63
> 06,2006-04-09 10:40:00,2006-04-09 20:40:00,BAK,sb39, 2, 29.56
Don't know
>
> 3. Finally- how does the function "trunc" know to aggregate the data by day?
> If I wanted to do monthly averages I would need to specify with
> "as.yearmon", but I don't seem to need to specify "day" anywhere in the
> code.
Explanation (but not still a solution for month aggregate)
The numerical coding format for date-time is that integer part is the 
number of days since a reference and the decimal part is the time. Then 
if you use trunc, two different times of the same day will be identical.