具有不等长度级别的id.vars变量的dcast数据

Question

我有一个像这样的数据框架：

  date_time            id        Temp
2012-02-27 00:12:30   |  Sens_1  |  25.2
2012-02-27 00:13:00   |  Sens_1  |  25.5
2012-02-27 00:13:30   |  Sens_1  |  26.3
2012-02-27 00:14:00   |  Sens_1  |  26.3
2012-02-27 00:14:30   |  Sens_1  |  26.3
           :               :          :
2012-02-27 00:12:30   |  Sens_2  |  35.4
2012-02-27 00:13:00   |  Sens_2  |  36.5
2012-02-27 00:13:30   |  Sens_2  |  38.8
2012-02-27 00:14:30   |  Sens_2  |  36.3
          :                 :         :
2012-02-27 00:12:30   |  Sens_3  |  15.4
2012-02-27 00:13:00   |  Sens_3  |  15.5
2012-02-27 00:13:30   |  Sens_3  |  16.8
2012-02-27 00:14:30   |  Sens_3  |  16.3
          :                 :         :  

我想要得到另一个数据框架，看起来是这样的：

      date_time           Sens_1 |  Sens_2  |  Sens_3
2012-02-27 00:12:30   |  25.2    |  35.4    |  15.4
2012-02-27 00:13:00   |  25.5    |  36.5    |  15.5
2012-02-27 00:13:30   |  26.3    |  38.8    |  16.8
2012-02-27 00:14:00   |  26.3    |   NA     |   NA
2012-02-27 00:14:30   |  26.3    |  36.3    |  16.3
           :               :          :          :

我怎样才能做到这一点？尽管如此，在df中，首先，我把三个变量都得到了相同的"date_time“值(在大多数情况下也是如此)，在某些行中，任何一个变量子组都可以有某个"date_time”值，而其余的则不会。

Answer 1

这是一条线：

library( 'data.table' )
setDT( df1 )  # convert data frame to data table by reference
dcast( df1, date_time ~ id, value.var = 'Temp', fun = function( x ) x, fill = NA_real_ )
#              date_time Sens_1 Sens_2 Sens_3
# 1: 2012-02-27 00:11:00   23.0   34.6   14.6
# 2: 2012-02-27 00:11:30   23.4   33.4   14.4
# 3: 2012-02-27 00:12:00   34.6     NA   13.6
# 4: 2012-02-27 00:12:30   35.4     NA   15.4
# 5: 2012-02-27 00:13:00   25.5   36.5   15.5
# 6: 2012-02-27 00:13:30   26.3   38.8   16.8

数据：

df1 <- structure(list(date_time = c("2012-02-27 00:11:00", "2012-02-27 00:11:30", 
                                    "2012-02-27 00:12:00", "2012-02-27 00:12:30", "2012-02-27 00:13:00", 
                                    "2012-02-27 00:13:30", "2012-02-27 00:11:00", "2012-02-27 00:11:30", 
                                    "2012-02-27 00:12:00", "2012-02-27 00:12:30", "2012-02-27 00:13:00", 
                                    "2012-02-27 00:13:30", "2012-02-27 00:11:00", "2012-02-27 00:11:30", 
                                    "2012-02-27 00:12:00", "2012-02-27 00:12:30", "2012-02-27 00:13:00", 
                                    "2012-02-27 00:13:30"), 
                      id = c("Sens_1", "Sens_1", "Sens_1", "Sens_1", "Sens_1", "Sens_1", "Sens_2", "Sens_2",
                             "Sens_1", "Sens_1", "Sens_2", "Sens_2", "Sens_3", "Sens_3", "Sens_3", "Sens_3", 
                             "Sens_3", "Sens_3"),
                      Temp = c(23, 23.4, 24.6, 25.2, 25.5, 26.3, 34.6, 33.4, 34.6, 35.4, 36.5, 38.8, 14.6,
                               14.4, 13.6, 15.4, 15.5, 16.8)), .Names = c("date_time", "id", "Temp"),
                 row.names = c(NA, -18L), class = "data.frame")