将星期几转换为R中的数字

Question

我在将工作日的.csv数据列转换为数字时遇到问题(因此1 =星期一,2 =星期二,3 =星期三等).我正在尝试使用如下所示的strptime功能:http://www.inside-r.org/r-doc/base/strftime

由于我想将工作日转换为数字,因此我使用了"%u"格式化选项.这是我的代码如下:

> newweekdaynum <- strptime(SFCrimeData$DayOfWeek, "%u")

其中SFCrimeData是一个数据集,我有一堆犯罪信息.在我运行语句后没有出现错误,但是当我想打印"newweekdaynum"时,所有出现的都是一个巨大的值表,都是"NA".

我究竟做错了什么？

Answer 1

strptime如果您有可以解析为完整日期/日期时间的内容,则可以使用.它将返回一个datetime对象.那不是你想要的.

相反,您可以使用有序因子:

#some example data
set.seed(42)
x <- factor(sample(c("Monday", "Tuesday", "Wednesday", 
                     "Thursday", "Friday", "Saturday", "Sunday"),
            20, TRUE))
# [1] Sunday    Sunday    Wednesday Saturday  Friday    Thursday  Saturday  Monday    Friday    Friday    Thursday  Saturday  Sunday   
#[14] Tuesday   Thursday  Sunday    Sunday    Monday    Thursday  Thursday 
#Levels: Friday Monday Saturday Sunday Thursday Tuesday Wednesday

#turn into ordered factor
x <- factor(x, levels = c("Monday", "Tuesday", "Wednesday", 
                          "Thursday", "Friday", "Saturday", "Sunday"),
            ordered = TRUE)
#[1] Sunday    Sunday    Wednesday Saturday  Friday    Thursday  Saturday  Monday    Friday    Friday    Thursday  Saturday  Sunday   
#[14] Tuesday   Thursday  Sunday    Sunday    Monday    Thursday  Thursday 
#Levels: Monday < Tuesday < Wednesday < Thursday < Friday < Saturday < Sunday

#extract underlying integer values
as.integer(x)
#[1] 7 7 3 6 5 4 6 1 5 5 4 6 7 2 4 7 7 1 4 4

(你真的不需要把它作为一个有序因子,一个以正确顺序指定的水平就足够了,但工作日在概念上是一个有序因子.)

Answer 2

df$Date <- as.Date(df$Date)  
df$wkdaynum <- format(df$Date,"%u")  
df$wkdaynum <- as.numeric(df$wkdaynum)

所以，你的错误是使用 strptime() 而不是 format()。