r將字母月份轉換為數字

Question

我想將數據表中月份名稱（一月，二月等）的向量轉換為它們的數字等效項（1、2，...）。 編碼

which(month.abb == month)

按預期工作； 那就是which(month.abb == "Feb")返回2。但是在數據表中使用的相同代碼返回警告，並且test向量包含的數字不是我期望的數字。

dt <- data.table(month = sort(c("Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec", "Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec")))
dt[, test :=  which(month.abb == month)]

Answer 1

嘗試這個：

dt[, test := match(month, month.abb)]

你會得到：-

    month test
 1:   Apr    4
 2:   Apr    4
 3:   Aug    8
 4:   Aug    8
 5:   Dec   12
 6:   Dec   12
 7:   Feb    2
 8:   Feb    2
 9:   Jan    1
10:   Jan    1
11:   Jul    7
12:   Jul    7
13:   Jun    6
14:   Jun    6
15:   Mar    3
16:   Mar    3
17:   May    5
18:   May    5
19:   Nov   11
20:   Nov   11
21:   Oct   10
22:   Oct   10
23:   Sep    9
24:   Sep    9

Answer 2

只需使用向量化查找表：

month_trans <- setNames(1:12, month.abb)

set.seed(2017-11-15)

month_column <- sample(month.abb, 50, replace=TRUE)
month_column
##  [1] "Feb" "Mar" "Jan" "Jul" "Oct" "Aug" "Oct" "Aug" "May" "Jan" "Aug" "Apr" "Jan" "May"
## [15] "May" "Mar" "Dec" "Oct" "Nov" "Nov" "Aug" "Jul" "Mar" "Feb" "Oct" "Feb" "Feb" "Dec"
## [29] "Dec" "Sep" "Nov" "May" "Jan" "Oct" "Feb" "Oct" "Nov" "Feb" "Mar" "Mar" "Nov" "Jun"
## [43] "May" "Mar" "Jul" "Nov" "Jul" "Dec" "Oct" "Feb"

month_trans[month_column]
## Feb Mar Jan Jul Oct Aug Oct Aug May Jan Aug Apr Jan May May Mar Dec Oct Nov Nov Aug Jul 
##   2   3   1   7  10   8  10   8   5   1   8   4   1   5   5   3  12  10  11  11   8   7 
## Mar Feb Oct Feb Feb Dec Dec Sep Nov May Jan Oct Feb Oct Nov Feb Mar Mar Nov Jun May Mar 
##   3   2  10   2   2  12  12   9  11   5   1  10   2  10  11   2   3   3  11   6   5   3 
## Jul Nov Jul Dec Oct Feb 
##   7  11   7  12  10   2 

unname(month_trans[month_column])
## [1]  2  3  1  7 10  8 10  8  5  1  8  4  1  5  5  3 12 10 11 11  8  7  3  2 10  2  2 12
## [29] 12  9 11  5  1 10  2 10 11  2  3  3 11  6  5  3  7 11  7 12 10  2