Join two numeric columns to create a Year-month variable

Question

I have two numeric columns. One contains a year, one a month number. I want to join these two and make a Year-month column that is of the proper Date type. To give a background,

Answer 1

You could consider the yearmon class in the zoo package:

library(zoo) zoo::as.yearmon(paste(year, month, sep='-'))

Answer 2

In order to be a proper date type, you need a month, day, and year. I usually just throw a "1" in there for the day, since every month has a first day...

df <- data.frame(year=c("2015", "2015", "2014"),
                 month=c("4", "2", "9"),
                 stringsAsFactors=FALSE)
df$date <- as.Date(paste(df$month, "01", df$year, sep="_"), format="%m_%d_%Y")
df <- df[order(df$date), ]

Answer 3

Just add a 01 in your paste and use as.Date

as.Date(paste(year, month, '01', sep= '-'))