[R] Extraction and replacement of data in a data frame

[Mike Marchywka]

----------------------------------------
> Date: Mon, 17 Jan 2011 12:51:43 -0800
> From: michael.hopgood at mrm.se
> To: r-help at r-project.org
> Subject: [R] Extraction and replacement of data in a data frame
>
>
> Dear R family,
> I am a relative newbie and have been dabbling with R for a little while.
> Simple things really, but my employers are beginning to see the benefits of
> using R instead of excel. We have a remote monitoring station measuring
> groundwater levels. We download the date as a .csv file and up until now,
> we have been using excel to analyse the data. It’s been a hassle trying to
> wrestle with that damn program as my boss wants to do things that excel was
> never meant to do, so I’ve convinced my boss to give R a chance. It’s been
> a steep learning curve, but I’m fairly confident I can reduce the amount of
> labour involved in producing and improving the graphs we show our clients.
>
> The groundwater levels are measured by pressure sensors lowered into the
> monitoring wells. After a certain time, the sensors were lowered further
> into the well, thus creating a disparity in the measurements.
>
> The data frame I import into R looks something like this:
> Date Waterhead (mm)
> 10-01-01 100
> 10-01-02 105
> 10-01-03 101
> 10-01-04 99
> 10-01-05 85
> 10-01-06 200
> 10-01-07 199
> 10-01-08 195
> 10-01-09 185
> 10-01-10 170
>
> For example, on the 10-10-06, the sensor was lowered by 115 mm.
> When I download the csv file, I download the data from the beginning of the
> measurement period. I then need to adjust the height by 115 mm to account
> for the lowering of the parameter. My question to you is how do I do that
> in R?
> I am after a formula or a manipulation that selects the first five
> measurements and adds a fixed amount. This is something that is added
> everytime I download the csv file and import it into R so that when I
> display my data, it is based on the following data frame:


See if this helps, I'm still learning how to do good R but this seems
to work. Just personal pref I converted your data to csv,

 254  cat xxx.txt | awk '{print "20"$1","$2}' > xxx.csv

I've neer used posix before, just copying what I've seen here but
it seemed to work as shown below, 

x<-read.table("xxx.csv",sep=",")
str(x)
x$V1=as.POSIXct(x$V1)
str(x)
y=(x$V1>as.POSIXct("2010-01-05"))
y
x$V2[y]=x$V2[y]+10000
x


output ends like 

5  2010-01-05   200
6  2010-01-06 10200
7  2010-01-07 10199
8  2010-01-08 10195
9  2010-01-09 10185
10 2010-01-10 10170



>
> Date Waterhead (mm)
> 10-01-01 215
> 10-01-02 220
> 10-01-03 216
> 10-01-04 214
> 10-01-05 200
> 10-01-06 200
> 10-01-07 199
> 10-01-08 195
> 10-01-09 185
> 10-01-10 170
>
> In short, I want to select a fixed number of rows of a column from my data
> frame, add a constant to these, and insert the new values into their
> respective rows without affecting the subsequent rows. I hope I have
> produced a reproducible example. I have been searching high and low for a
> solution, but have come up against a brick wall. I feel I have read
> something that tackles this some time in the past, but can’t find it again.
> Thanks in advance!
>
> --
> View this message in context: http://r.789695.n4.nabble.com/Extraction-and-replacement-of-data-in-a-data-frame-tp3221261p3221261.html
> Sent from the R help mailing list archive at Nabble.com.
>
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