combine month and day into one date column

Question

Using R, I\'d like to combine into one column (date) the month nb (month) and the day nb (day) contained in two different columns and use the created column in a date format.

Answer 1

I assume here that by "date format" you just mean a string that looks like a date, rather than a proper date class in R.

data <- data.frame( Month=c(1,5,2), Day=c(1,2,3) )
data$MonthDay <- paste( month.abb[data$Month], data$Day, sep="-" )
data
#   Month Day MonthDay
# 1     1   1    Jan-1
# 2     5   2    May-2
# 3     2   3    Feb-3

Answer 2

Try this...

df <- data.frame( Month = sample(1:12 , 10 , repl = TRUE ) , Day = sample(1:30 , 10 , repl = TRUE ) )
df$Date <- as.Date( paste( df$Month , df$Day , sep = "." )  , format = "%m.%d" )

df
#      Month Day       Date
#   1      1   8 2013-01-08
#   2      1  17 2013-01-17
#   3      7  23 2013-07-23
#   4     11  21 2013-11-21
#   5      3  30 2013-03-30
#   6     12  15 2013-12-15
#   7      2  30       <NA>
#   8      7  10 2013-07-10
#   9      1  16 2013-01-16
#   10     8   1 2013-08-01

I got some NAs because I made up random dates, some of which don't exist, such as 30th Feb