Often you may want to combine two columns into one in R. For example, suppose you have a data frame with three columns:

  month year  value
   10   2019   15
   10   2020   13
   11   2020   13
   11   2021   19
   12   2021   22

You may wish to combine the month and year column into a single column called date:

    date   value
   2019_10   15
   2020_10   13
   2020_11   13
   2021_11   19
   2021_12   22

This tutorial explains two ways to quickly do this in R.

Method 1: Use the Paste Function from Base R

The following code shows how to use the paste function from base R to combine the columns month and year into a single column called date:

#create data frame
data <- data.frame(month=c(10, 10, 11, 11, 12),
                   year=c(2019, 2020, 2020, 2021, 2021),
                   value=c(15, 13, 13, 19, 22))

#view data frame
data

#combine year and month into one column
data$date <- paste(data$year, data$month, sep="_")

#view new data frame 
data

  month year value    date
1    10 2019    15 2019_10
2    10 2020    13 2020_10
3    11 2020    13 2020_11
4    11 2021    19 2021_11
5    12 2021    22 2021_12

Once we’ve combined the two columns, we can remove the old ones if we’d like:

data_new <- data[c("date", "value")]

data_new

     date value
1 2019_10    15
2 2020_10    13
3 2020_11    13
4 2021_11    19
5 2021_12    22

Method 2: Use the Unite Function from Tidyr

The following code shows how to use the unite function from the tiydr package to combine the columns month and year into a single column called date:

#load tidyr package
library(tidyr)

#create data frame
data <- data.frame(month=c(10, 10, 11, 11, 12),
                   year=c(2019, 2020, 2020, 2021, 2021),
                   value=c(15, 13, 13, 19, 22))

#combine year and month into one column
unite(data, date, c(year, month))

     date value
1 2019_10    15
2 2020_10    13
3 2020_11    13
4 2021_11    19
5 2021_12    22

Notice that both methods produce identical results.

You can find the complete documentation for the unite function here.