从 R 中的数据帧中提取时间信息

Question

我在 R 中有一个时间图数据框，其中的日期完全是乱码。我正在寻找提取时间信息并替换正确的日期。但我运行遇到了一些问题。我已经展示了数据框的一部分 (Time_df)

Time_df = 
                  Time1               Time2      Time3      Time4      Time5
102 1970-01-01 07:00:00 1970-01-01 19:00:00 1970-01-01 1970-01-01 1970-01-01
103 1970-01-01 00:00:00 1970-01-02 00:00:00 1970-01-01 1970-01-01 1970-01-01
104 1970-01-01 00:00:00 1970-01-02 00:00:00 1970-01-01 1970-01-01 1970-01-01
105 1970-01-01 06:00:00 1970-01-01 22:00:00 1970-01-01 1970-01-01 1970-01-01
106 1970-01-01 08:00:00 1970-01-01 21:00:00 1970-01-01 1970-01-01 1970-01-01

dput(Time_df) =

 structure(list(Time1 = structure(c(43200, 18000, 18000, 39600, 
46800), class = c("POSIXct", "POSIXt"), tzone = ""), Time2 = structure(c(86400, 
104400, 104400, 97200, 93600), class = c("POSIXct", "POSIXt"), tzone = ""), 
    Time3 = structure(c(18000, 18000, 18000, 18000, 18000), class = c("POSIXct", 
    "POSIXt"), tzone = ""), Time4 = structure(c(18000, 18000, 
    18000, 18000, 18000), class = c("POSIXct", "POSIXt"), tzone = ""), 
    Time5 = structure(c(18000, 18000, 18000, 18000, 18000), class = c("POSIXct", 
    "POSIXt"), tzone = "")), .Names = c("Time1", "Time2", "Time3", 
"Time4", "Time5"), row.names = 102:106, class = "data.frame")

如果我查看结构，它都是 POSIXct 格式

str(Time_df)
'data.frame':   5 obs. of  5 variables:
 $ Time1: POSIXct, format: "1970-01-01 07:00:00" "1970-01-01 00:00:00" "1970-01-01 00:00:00" ...
 $ Time2: POSIXct, format: "1970-01-01 19:00:00" "1970-01-02 00:00:00" "1970-01-02 00:00:00" ...
 $ Time3: POSIXct, format: "1970-01-01" "1970-01-01" "1970-01-01" ...
 $ Time4: POSIXct, format: "1970-01-01" "1970-01-01" "1970-01-01" ...
 $ Time5: POSIXct, format: "1970-01-01" "1970-01-01" "1970-01-01" ...

但是当我试图提取时间时，它给我一个错误

times = strftime(Time_df, format="%H:%M:%S")
Error in as.POSIXlt.default(x, tz = tz) : 
  do not know how to convert 'x' to class “POSIXlt”

我可以为单个列提取时间，但不能为整个数据帧提取时间。我确实有 42 列，所以我希望有一种更简洁的方法。这就是我想要的：

Mon = rep("03/20/17", 6)
Tue = rep("03/21/17", 6)
Wed = rep("03/22/17", 6)
Thu = rep("03/23/17", 6)
Fri = rep("03/24/17", 6)
Sat = rep("03/25/17", 6)
Sun = rep("03/26/17", 6)

dates = c(Mon, Tue, Wed, Thu, Fri, Sat, Sun)
t = strftime(Time_df, format="%H:%M:%S")
x = paste(dates, t)

Answer 1

您必须按第 (2) 列使用 apply：

apply(Time_df,2,strftime, format="%H:%M:%S")

     Time1      Time2      Time3      Time4      Time5     
[1,] "07:00:00" "19:00:00" "00:00:00" "00:00:00" "00:00:00"
[2,] "00:00:00" "00:00:00" "00:00:00" "00:00:00" "00:00:00"
[3,] "00:00:00" "00:00:00" "00:00:00" "00:00:00" "00:00:00"
[4,] "06:00:00" "22:00:00" "00:00:00" "00:00:00" "00:00:00"
[5,] "08:00:00" "21:00:00" "00:00:00" "00:00:00" "00:00:00"

从 R 中的数据帧中提取时间信息

Extracting the time information from a dataframe in R

r

posixct