在R数据框中的行之间比较日期

Question

novice R user here...I'm trying to compare dates for each id and determine which entry is earlier or later. The input data would look something like this:

id    date
101   18-Sep-12
101   21-Aug-12
102   25-Mar-13
102   15-Apr-13

And the output would look something like this:

id    date         Category
101   18-Sep-12    Late
101   21-Aug-12    Early
102   25-Mar-13    Early
102   15-Apr-13    Late

-Justin

Answer 1

If your data frame is df :

df$date <- as.Date(df$date, format="%d-%b-%y")
df = df[order(df$id, df$date),]
df$Category = c("Early", "Late")

Answer 2

You can use plyr here :

library(plyr)
loc <- Sys.setlocale("LC_TIME", "ENGLISH")
dat$date <- as.Date(dat$date, format = "%d-%b-%y")
ddply(dat, .(id), transform, cat = ifelse(date == min(date), "EARLY", "LATE"))
##    id       date   cat
## 1 101 2012-09-18  LATE
## 2 101 2012-08-21 EARLY
## 3 102 2013-03-25 EARLY
## 4 102 2013-04-15  LATE
Sys.setlocale("LC_TIME", loc)

Answer 3

I would probably look into using the "data.table" package.

The general approach I would use is to use order or rank to create your "category" column. The thing that's nice here is that you are not really limited by comparing two dates.

DT <- data.table(df)
DT[, category := order(date), by = id]
DT
#     id       date category
# 1: 101 2012-09-18        2
# 2: 101 2012-08-21        1
# 3: 102 2013-03-25        1
# 4: 102 2013-04-15        2

If you wanted text labels, you can use factor :

DT[, category := factor(category, labels = c("Early", "Late"))]
DT
#     id       date category
# 1: 101 2012-09-18     Late
# 2: 101 2012-08-21    Early
# 3: 102 2013-03-25    Early
# 4: 102 2013-04-15     Late

---

For convenience, this is the "df" that I started with:

df <- structure(list(id = c(101L, 101L, 102L, 102L), 
    date = structure(c(15601, 15573, 15789, 15810), class = "Date")), 
    .Names = c("id", "date"), row.names = c(NA, -4L), class = "data.frame")