Collapse / concatenate / aggregate a column to a single comma separated string within each group

R Aggregate R Faq

If you have a data frame and you want to aggregate a specific column into a single comma-separated string within each group defined by other variables, you can use various methods in R. In this article, we will explore different approaches to solve this problem using plyr, dplyr, and data.table libraries.

Method 1: Using plyr

The ddply function from the plyr library allows us to apply a summarization function on each group defined by the grouping variables. Here's an example:

library(plyr)
data <- data.frame(A = c(rep(111, 3), rep(222, 3)), B = rep(1:2, 3), C = c(5:10))

result <- ddply(data, .(A, B), summarise, test = paste(C, collapse = ", "))
result

This will give us the following output:

    A B  test
1 111 1 5, 7
2 111 2 6
3 222 1 9
4 222 2 8, 10

As you can see, the paste function is used within the summarise function to collapse the values of column C into a single comma-separated string.

Method 2: Using dplyr

The dplyr library provides a more modern and efficient approach to data manipulation. Here's how you can achieve the same result using dplyr:

library(dplyr)
data %>%
  group_by(A, B) %>%
  summarise(test = paste(C, collapse = ", "))

The above code snippet uses the group_by function to group the data by columns A and B, and then the summarise function with the paste function to collapse the values of column C into a single comma-separated string.

Method 3: Using data.table

The data.table library provides a powerful and efficient way to manipulate large datasets. Here's how you can accomplish this task using data.table:

library(data.table)
data <- data.table(A = c(rep(111, 3), rep(222, 3)), B = rep(1:2, 3), C = c(5:10))

result <- data[, .(test = paste(C, collapse = ", ")), by = .(A, B)]
result

This code snippet uses the [, .()] syntax to subset the data and calculate the desired result. The by = .(A, B) part defines the grouping variables, and paste function is used to collapse the values of column C into a single comma-separated string.

Conclusion

In this article, we have discussed three different methods to collapse a column into a single comma-separated string within each group in R. You can choose the method that best suits your needs depending on the size of your dataset and your familiarity with the libraries.

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