Solving the Problem of Aggregating Multiple Variables per Group in R

When working with data frames in R, there may be situations where you need to aggregate or summarize multiple variables simultaneously. For example, you might want to calculate the sum or mean of multiple variables within each group defined by one or more grouping variables. In this article, we will explore different approaches to solve this problem using the example data provided.

Understanding the Problem

Let's start by understanding the problem statement and the sample data at hand. The given data frame df1 contains information about dates, years, months, and two variables x1 and x2. The goal is to aggregate or summarize x1 and x2 simultaneously based on the grouping variables year and month.

Approach 1: Using the aggregate() function

One way to solve this problem is by using the aggregate() function in R. The aggregate() function allows us to apply a specified function (such as sum, mean, max etc.) to one or more variables while grouping them based on one or more grouping variables.

Here is the code to simultaneously aggregate x1 and x2 variables from df1 by year and month:

df2 <- aggregate(cbind(x1, x2) ~ year + month, data = df1, sum)

The above code creates a new data frame df2 where the variables x1 and x2 are aggregated by grouping them based on the year and month variables. The sum function is applied to both x1 and x2 within each group to calculate their respective sums.

The output of head(df2) will give you the first few rows of the aggregated data frame:

  year month         x1           x2
1 2000     1  -8.413382  0.335080538
2 2000     2  15.674935  0.111161131
3 2000     3  59.280883 -2.756456487
4 2000     4   7.207184 -1.839681525
5 2000     5  12.735084 -1.389263697
6 2000     6 243.821585 -4.661839057

Approach 2: Using the dplyr package

Another popular approach for data manipulation and summarization in R is to use the dplyr package. The dplyr package provides a set of functions designed to make data manipulation tasks easier and more readable.

To solve our problem using the dplyr package, we can follow these steps:

  1. Load the dplyr package using library(dplyr).
  2. Use the group_by() function to specify the grouping variables year and month.
  3. Use the summarize() function to apply the desired aggregation functions (such as sum(), mean(), etc.) to the variables x1 and x2.

Here is an example code using the dplyr package:


df2 <- df1 %>%
  group_by(year, month) %>%
  summarize(x1_sum = sum(x1), x2_sum = sum(x2))


The above code uses the %>% operator to pipe the data frame df1 into a sequence of operations. The group_by() function groups the data by year and month, and the summarize() function calculates the sum of x1 and x2 within each group. The resulting summarized data frame is stored in df2.

The output of head(df2) will give you the first few rows of the summarized data frame:

# A tibble: 6 x 4
# Groups:   year [1]
   year month     x1    x2
  <dbl> <dbl> <dbl> <dbl>
1  2000     1 ___ ___
2  2000     2 ___ ___
3  2000     3 ___ ___
4  2000     4 ___ ___
5  2000     5 ___ ___
6  2000     6 ___ ___

Please note that the values in the x1 and x2 columns have been aggregated and should be filled in the respective blanks in the above output.


In this article, we have discussed how to solve the problem of aggregating or summarizing multiple variables per group in R. We explored two approaches using the aggregate() function and the dplyr package.

Both approaches allow you to simultaneously aggregate multiple variables based on one or more grouping variables. The aggregate() function is part of base R and provides a simple way to aggregate variables, while the dplyr package offers a more versatile and readable syntax for data manipulation tasks.

By using these techniques, you can easily summarize and analyze data in R based on your specific requirements.