Converting multiple columns in an R dataframe to D

问题:

I have a large datafile where all the dates have been loaded as charaters. I would like to change all the Dates columns to date format. Most of the dates have "%y%m%d" format, some have "%Y%m%d" format. There are 25 columns of dates, so changing each one individually is inefficient.

I can do

df$DATE1 <- as.Date(df$DATE1, format ="%y%m%d")
df$DATE2 <- as.Date(df$DATE2, format ="%y%m%d")

etc., but very bad coding.

I tried the following code, but is is not working. This assumes all of the dates are of the format "%y%m%d". Using grep("DATE", names(df)) will get all the Dates columns

df[ , grep("DATE", names(df))] <- as.Date(df[ , grep("DATE", names(df))], "%y%m%d")

回答1:

Try:

df[, cols <- grep("^DATE", names(df))] <- lapply(df[, cols <- grep("^DATE", names(df))], as.Date, format = "%y%m%d")

Example:

df <- data.frame(DATE1 = c('910812', '900928'), DATE2 = c('890813', '890910'))
# Apply the above and you get:
# > df
#        DATE1      DATE2
# 1 1991-08-12 1989-08-13
# 2 1990-09-28 1989-09-10
# > class(df[, 1])
# [1] "Date"