Summarize dataframe by day from timestamp

Question

I have a dataset data that contains a timestamp and a suite of other variables with values at each timestamp. I am trying to use ddply within

Answer 1

You can use format to extract the day

 dat$DAY <- as.factor(format(dat$TIMESTAMP,'%d'))

 [1] 01 01 01 01 01 02 02 02 02 02 07 07 07 07 07
Levels: 01 02 07

For some reasons I get error if I don't remove the first column ( as said in comment in moment I am writing this)

ddply(data[,-1],.(DAY),summarise, V1 = mean(P), V2 = max(WS))
  DAY    V1    V2
1  01 992.4 6.259
2  02 992.4 4.023
3  07 239.0 3.129

converting the TIMESTAMP to POSIXct seems to correct the problem

 dat$TIMESTAMP <- as.POSIXct(dat$TIMESTAMP)

ddply(dat,.(DAY),summarise, V1 = mean(P), V2 = max(WS))
  DAY    V1    V2
1  01 992.4 6.259
2  02 992.4 4.023
3  07 239.0 3.129

EDIT no need to use format

Since Your TIMESTAMP column is a POSIXlt , it easy to retrieve day part from it, You can do this :

dat$TIMESTAMP$mday
 [1] 1 1 1 1 1 2 2 2 2 2 7 7 7 7 7