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.
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