R help - Jan 2013 - How to extract value for specific rows in an array?

Hi everyone,

I have a dataset that contains depths measurements every 30 sec for several
days.

The subset of data that I am working on looks like this:

Date            Day           Depth
18442.00    29              41.0
18442.00    29              43.5
...
18442.04    29              40.3
18442.04    29              35.1
...
18443.00    30              16.5
...

Since my data is collected every 30 sec, I want to find the maximum depth
recorded for each day and recorded daily maximum depths into a new array.
Now, I would like to also sample the date for each maximum depth value. 

So far I have been able to sample the max daily depth in a 1-column array
using the loop below. I tried creating a new empty array for recording the
date for each daily max depth value, but it doesn't seem to be working...
Any ideas or suggestions?

DF <- data.frame(date,day,depth)
samplesperday <- 2*60*24  # number of datapoints per day (2880 here)
nbdays<- nrow(DF)/samplesperday  # number of days in the dataset (5 here)
MaxDepth <- array(NaN,c(nbdays,1))  # create empty array where max depth
values will be stored
Date <- array(NaN,c(nbdays,1))  # create empty array where date for max
depth values will be stored

for (i in 1:nbdays){
  dayIndex1 <- samplesperday*(i-1)+1 # start of day period (value 1 of 2880)
  dayIndex2 <- samplesperday*(i) # end of day period (value 2880 of 2880)    
  MaxDepth[i] <- max(DF$depth[dayIndex1:dayIndex2])  # extract max depth
value from the 2880 datapoints collected per day
  Date [i] <- DF$date[MaxDepth[i]]  ### NOT WORKING...
}

Another way to do this, since I am getting one daily max depth value, would
be to sample the date every day. Since I have 2880 measurements per day, I
would just have to sample the date every 2881 rows. This said, I am stuck
and can't figure out out to do this... 

Your help would be most appreciated! :)
Thanks,
Fabrice




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Hello,

If you want the maximum depth for each day, ?aggregate will do it. If 
you also want the date column, use ?merge after it.


dat <- read.table(text = "
Date            Day           Depth
18442.00    29              41.0
18442.00    29              43.5
18442.04    29              40.3
18442.04    29              35.1
18443.00    30              16.5
", header = TRUE)

agg <- aggregate(Depth ~ Day, data = dat, FUN = max)
merge(agg, dat)


Hope this helps,

Rui Barradas
Em 12-01-2013 05:32, fjaine escreveu:
> Hi everyone,
>
> I have a dataset that contains depths measurements every 30 sec for several
> days.
>
> The subset of data that I am working on looks like this:
>
> Date            Day           Depth
> 18442.00    29              41.0
> 18442.00    29              43.5
> ...
> 18442.04    29              40.3
> 18442.04    29              35.1
> ...
> 18443.00    30              16.5
> ...
>
> Since my data is collected every 30 sec, I want to find the maximum depth
> recorded for each day and recorded daily maximum depths into a new array.
> Now, I would like to also sample the date for each maximum depth value.
>
> So far I have been able to sample the max daily depth in a 1-column array
> using the loop below. I tried creating a new empty array for recording the
> date for each daily max depth value, but it doesn't seem to be
working...
> Any ideas or suggestions?
>
> DF <- data.frame(date,day,depth)
> samplesperday <- 2*60*24  # number of datapoints per day (2880 here)
> nbdays<- nrow(DF)/samplesperday  # number of days in the dataset (5
here)
> MaxDepth <- array(NaN,c(nbdays,1))  # create empty array where max depth
> values will be stored
> Date <- array(NaN,c(nbdays,1))  # create empty array where date for max
> depth values will be stored
>
> for (i in 1:nbdays){
>    dayIndex1 <- samplesperday*(i-1)+1 # start of day period (value 1 of
2880)
>    dayIndex2 <- samplesperday*(i) # end of day period (value 2880 of
2880)
>    MaxDepth[i] <- max(DF$depth[dayIndex1:dayIndex2])  # extract max
depth
> value from the 2880 datapoints collected per day
>    Date [i] <- DF$date[MaxDepth[i]]  ### NOT WORKING...
> }
>
> Another way to do this, since I am getting one daily max depth value, would
> be to sample the date every day. Since I have 2880 measurements per day, I
> would just have to sample the date every 2881 rows. This said, I am stuck
> and can't figure out out to do this...
>
> Your help would be most appreciated! :)
> Thanks,
> Fabrice
>
>
>
>
> --
> View this message in context:
http://r.789695.n4.nabble.com/How-to-extract-value-for-specific-rows-in-an-array-tp4655323.html
> Sent from the R help mailing list archive at Nabble.com.
>
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