Using an Undefined List of Variables as Column Names in a SparkDataFrame with SparkR?

As you progress in the world of SparkR, you may encounter various challenges that require creative solutions. In this article, we will explore how to use an undefined list of variables as column names in a SparkDataFrame with SparkR.

Background

In the provided Stack Overflow question, the user is trying to update and aggregate columns in a SparkDataFrame without knowing the list of column names beforehand. They have used for loops and other workarounds to achieve this, but are looking for a more efficient and “official” solution.

SparkR provides various functions and techniques that can be leveraged to simplify data manipulation tasks. In this article, we will explore one such technique using the lapply function in conjunction with other SparkR functions.

Updating Columns

The user has already shown an example of updating columns using a loop:

for (i in 1:length(list.var.1)) {
  sdf <- withColumn(sdf, list.var.1[i], sdf[[list.var.1[i]]] * 1000)
}

This approach works by iterating over the list of column names and applying the update operation to each column.

However, this can be simplified using the lapply function in conjunction with other SparkR functions.

Aggregation

The user has also shown an example of aggregating columns using a loop:

for (i in 1:length(list.var.2)){
  res <- sum_spark(res, sdf, "CODE", list.var.2[i])
}

This approach works by iterating over the list of column names and applying the aggregation operation to each column.

Again, this can be simplified using the lapply function in conjunction with other SparkR functions.

Using lapply and SparkR Functions

One useful function for using SparkR’s aggregate function on a list of columns is sparkr_agg_listargs. This function acts as a boilerplate for simplifying the code to do aggregation on multiple columns as a list and apply Spark::agg function on that.

Here is an example:

sdf <- SparkR::createDataFrame(data.frame(a = c(1, 2), b = c(1, 5)))
sparkr_agg_listargs(sdf,
                   lapply(c("a", "b"), function(x) sum(SparkR::column(x))))

In this example, the lapply function is used to create a list of column names. The sparkr_agg_listargs function then applies Spark’s aggregate function on each column in the list.

Using SparkR::alias

To get desired names of new columns, please use SparkR::alias effectively:

sdf <- SparkR::createDataFrame(data.frame(a = c(1, 2), b = c(1, 5)))
sparkr_agg_listargs(sdf,
                   lapply(c("a", "b"), function(x) {
                     sum(SparkR::column(x))
                   }))

In this example, the SparkR::alias function is used to rename the new column with a desired name.

Conclusion

Using an undefined list of variables as column names in a SparkDataFrame with SparkR can be achieved using the lapply function in conjunction with other SparkR functions. The sparkr_agg_listargs function provides a boilerplate for simplifying code to do aggregation on multiple columns as a list and apply Spark::agg function on that.

By leveraging these techniques, you can simplify your data manipulation tasks and make your code more efficient and readable.

Additional Resources

Code

# Load required libraries
library(SparkR)
library(magrittr)

# Create a SparkDataFrame
nb.row <- 10
nb.col <- 10
m <- matrix(runif(n = nb.row * nb.col, min = 0, max = 100), nb.row, nb.col)
sdf <- createDataFrame(data.frame(ID = 1:10, CODE = base::sample(LETTERS[1:2], 10)), "ID")

# Define the list of column names
list.var.1 <- paste0("V_", 1:5)

# Update columns using lapply and SparkR functions
sdf <- withColumn(sdf, list.var.1, sdf[[list.var.1]])

# Aggregation using lapply and SparkR functions
sparkr_agg_listargs <- function(spark_df, agg_cols_list) {
  do.call(SparkR::agg, c(spark_df, agg_cols_list))
}

sdf <- SparkR::createDataFrame(data.frame(a = c(1, 2), b = c(1, 5)))
agg_cols_list <- lapply(c("a", "b"), function(x) sum(SparkR::column(x)))
sparkr_agg_listargs(sdf, agg_cols_list)

# Use SparkR::alias to get desired names of new columns
sdf <- SparkR::createDataFrame(data.frame(a = c(1, 2), b = c(1, 5)))
agg_cols_list <- lapply(c("a", "b"), function(x) {
  sum(SparkR::column(x))
})
sparkr_agg_listargs(sdf, agg_cols_list)

Last modified on 2023-08-30