r - Combine separate Year and Month columns into single Date Column

Question

I have a data frame (df) like this:

    code     year  month
1 YYOOGG    2011    8
2 YYOOGG    2011    1
3 YYOOGG    2011    4
4 YYOOGG    2011    3
5 YYOOGG    2011    12
6 YYOOGG    2011    9

and I need to create a 4th column with the Date like this:

    code     year  month  Date
1 YYOOGG    2011    8     2011-08
2 YYOOGG    2014    1     2014-01
3 YYOOGG    2016    4     2016-04
4 YYOOGG    2009    3     2009-03
5 YYOOGG    2000    12    2000-12
6 YYOOGG    2010    9     2010-09

I tried this:

  df$Date <- as.Date(paste(df$year, df$month, sep="-"), "%Y-%M")

but I get the following as the date:

2011-09-09

Answer 1

I'd use zoo::as.yearmon as follows

df$Date <- as.yearmon(paste(df$year, df$month), "%Y %m")

It will not look like the desired output (i.e. 2011-01).

However, IMO this approach is better than m0h3n's because df$Date will be saved as a yearmon object rather than a string. Therefore you can be handled it like a date. For example, if you save df$Date as a string you're going to have a hard time plotting your data over time, etc...