R 中的减法时间

Question

I want to calculate the amount of time someone spends in bed at night. First column, j1bed (time goes to bed):

dput(head(subscales$j1bed, 20))

c("23:00:00", "01:00:00", "22:00:00", "00:00:00", "00:00:00", 
"23:00:00", "03:00:00", "23:00:00", "22:00:00", "23:00:00", "21:00:00", 
"23:30:00", "22:00:00", "22:00:00", "20:00:00", "22:00:00", "22:00:00", 
"21:00:00", "20:00:00", "23:00:00")

Second column, j3wake (time wakes up):

dput(head(subscales$j3wake, 20))

c("08:00:00", "08:00:00", "05:00:00", "08:00:00", "07:00:00", 
"07:00:00", "09:00:00", "09:00:00", "08:00:00", "06:00:00", "03:00:00", 
"08:30:00", "07:00:00", "08:00:00", "09:00:00", "07:00:00", "06:00:00", 
"07:00:00", "07:00:00", "12:00:00")

My code currently is as follows:

Hours_in_Bed<- as.numeric(difftime(strptime(subscales$j3wake, "%I:%M %p" ),strptime(subscales$j1bed, "%I:%M %p" ),units='hours'))

For some reason, this is coming up with only "NA". Any ideas?

Edit: I've tried the following three suggestions: 1)

result1<-strptime(subscales$j3wake, "%I:%M %p")

result2<-strptime(subscales$j1bed, "%I:%M %p")

Hours_in_Bed<-as.numeric(difftime(result2, result1, units='hours'))

This yields only "NA"s. 2)

 subscales$j3wake1=as.POSIXct(paste('1970-1-1',subscales$j3wake))
subscales$j1bed1 =as.POSIXct(paste('1970-1-1',subscales$j1bed))
as.numeric(difftime(subscales$j1bed1,subscales$j3wake1,units='hours'))

This yields "0"s. 3)

 all(grepl("[0-9]{2}:[0-9]{2} (AM|PM)", subscales$j3wake)) 

all(grepl("[0-9]{2}:[0-9]{2} (AM|PM)", subscales$j1bed))

Both yield "False".

Edit 2: that last tip was key: I think I should be using "%H:%M" instead:

Hours_in_Bed<- as.numeric(difftime(strptime(subscales$j3wake, "%H:%M" ),strptime(subscales$j1bed, "%H:%M" ),units='hours'))

If I then do:

Hours_in_Bed[!is.na(Hours_in_Bed) & Hours_in_Bed<0] <- 24 + Hours_in_Bed[!is.na(Hours_in_Bed) & Hours_in_Bed<0] 

I should be good to go. Let me know if I'm still off, and thanks for the help!

Answer 1

It's usually easier to use the POSIXct-type even if there isn't any real year/month/day. Example:

subscales$j3wake1 = as.POSIXct(paste('1970-1-1',subscales$j3wake))
subscales$j1bed1 = as.POSIXct(paste('1970-1-1',subscales$j1bed))
as.numeric(difftime(subscales$j1bed1,subscales$j3wake1,units='hours'))