如何将月份日和年份列转换为星期几

Question

我有3列：月，日和年。它们都是整数格式。例如，在一行中，year == 2013，month == 1，day == 1.我需要使用这3列来创建一个新列，该列说明它是星期几（即星期二）。感谢。

Answer 1

这可以帮助您从3列中获取日期。解决方案使用base库

# sample data to demonstrate conversion. Dates are random
df <- data.frame(year = c(2018, 2016, 2017, 2018),
                 month = c(7:10), 
                 day = c(9:12) )
df
#      year month day
#    1 2018     7   9
#    2 2016     8  10
#    3 2017     9  11
#    4 2018    10  12

# Convert and add additional column as date. Use of paste and sprintf.
df$date <- as.Date(paste(df$year, sprintf("%02d",df$month), sprintf("%02d", df$day) , sep = "-"), "%Y-%m-%d")
# Find and populate date-of-week
df$day_of_week <- weekdays(df$date)

#Result:
df
#      year month day       date day_of_week
#    1 2018     7   9 2018-07-09      Monday
#    2 2016     8  10 2016-08-10   Wednesday
#    3 2017     9  11 2017-09-11      Monday
#    4 2018    10  12 2018-10-12      Friday