extract data based on datetime - r

Question

I have two dataframes:
dat is a 9752x8 dataframe that contains some POSIXlt dates
trips.df is a 35772x28 dataframe that contains hourly temperature
data
I would like to save the corresponding temperature for each dates in dat.
I have tried:
trips.df$temperature<-lapply(trips.df$fin, function(x){
dat_meteo[dat_meteo$Date.Heure==round(x,"hours"),7]})
But I got this error, which makes me think that x is not passed as a datetime variable
Error in round(x, "hours") :
non-numeric argument to mathematical function
I have also tried this:
merge(trips.df,dat_meteo[,c(1,7)])
But I also got an error:
Error: cannot allocate vector of size 653.8 Mb
Any advice on how to retrieve data on dat_meteo by dates?
I am using R version 3.4.0 with RStudio Version 1.0.143 on Windows 10
And here are an exercpt of my data:
> head(trips.df$fin)
[1] "2013-06-25 16:34:16 EDT" "2013-06-25 16:34:16 EDT" "2013-06-26 13:00:05 EDT"
[4] "2013-06-29 12:52:21 EDT" "2013-06-29 15:34:13 EDT" "2013-06-29 17:39:29 EDT"
> dat_meteo[1870:1875,c(1,7)]
Date.Heure Temp...C.
1870 2013-03-19 18:00:00 -1,2
1871 2013-03-19 19:00:00 -1,7
1872 2013-03-19 20:00:00 -2,1
1873 2013-03-19 21:00:00 -2,8
1874 2013-03-19 22:00:00 -3,0
1875 2013-03-19 23:00:00 -3,7

Answer 1

You may want to take a slightly different approach and use data.table.
trips.dt <- data.table(trips.df)
dat <- data.table(dat)
trips.dt <- trips.dt[ , dates.a := strptime(as.POSIXct(fin,format='%m/%d/%Y %H:%M:%S'),format='%m/%d/%Y')][,dates.b := dates.a]
dat <- dat[dates.dat.a := strptime(as.POSIXct(Date.Heure, format = '%m/%d/%Y %H:%M:%S'),format='%m/%d/%Y')][, dates.dat.b := dates.dat.a]
setkey(trips.dt, id, dates.a, dates.b)
setkey(dat , id, dates.dat.a, dates.dat.b)
combo <- foverlaps(trips.df, dat, type = "within")
This creates date ranges for both trip.df and dat after converting it to a data.table, then merges trips.df to dat and stores the result as combo

Answer 2

Make sure that the two time columns you want to match have the same format (POSIXct). It is more straightforward to use the POSIXct format within a dataframe, as the POSIXlt format actually corresponds to a list of named elements whereas POSIXct is in vector form.
dat_meteo$Date.Heure=as.POSIXct(dat_meteo$Date.Heure,format="%Y-%m-%d %H:%M:%S")
Create a column in trips.df of times rounded to the closest hours, converting it to POSIXct too, as round converts POSIXct to POSIXlt:
trips.df$fin_r=as.POSIXct(round(trips.df‌​$fin,"hours"))
Then use merge:
res=merge(trips.df,dat_meteo[,c(1,7)],by.x="fin_r",by.y ="Date.Heure")