How to Split a Vector into Chunks in R

Introduction

In data analysis, there are times when you need to split a vector into smaller chunks. Whether you’re managing large datasets or preparing data for parallel processing, breaking down vectors can be incredibly useful. In this post, we’ll explore how to achieve this in R using base R, dplyr, and data.table.

Examples

Using Base R

Base R provides a straightforward way to split a vector into chunks using the split function and a combination of other basic functions.

Example 1: Splitting a Vector into Chunks

Let’s say we have a vector x and we want to split it into chunks of size 3.

x <- 1:10
chunk_size <- 3
split_vector <- split(x, ceiling(seq_along(x) / chunk_size))
print(split_vector)

$`1`
[1] 1 2 3

$`2`
[1] 4 5 6

$`3`
[1] 7 8 9

$`4`
[1] 10

Explanation:

• x <- 1:10: Creates a vector x with values from 1 to 10.
• chunk_size <- 3: Defines the size of each chunk.
• seq_along(x): Generates a sequence of the same length as x.
• ceiling(seq_along(x) / chunk_size): Divides the sequence by the chunk size and uses ceiling to round up to the nearest integer, creating a grouping factor.
• split(x, ...): Splits the vector based on the grouping factor.

Using dplyr

The dplyr package, part of the tidyverse, offers a more readable and pipe-friendly approach to splitting vectors.

Example 2: Splitting a Vector into Chunks

Here’s how you can do it with dplyr.

library(dplyr)

x <- 1:10
chunk_size <- 3
split_vector <- x %>%
  as.data.frame() %>%
  mutate(group = ceiling(row_number() / chunk_size)) %>%
  group_by(group) %>%
  summarise(chunk = list(.)) %>%
  pull(chunk)
print(split_vector)

[[1]]
[1] 1 2 3

[[2]]
[1] 4 5 6

[[3]]
[1] 7 8 9

[[4]]
[1] 10

Explanation:

• as.data.frame(): Converts the vector to a data frame.
• mutate(group = ceiling(row_number() / chunk_size)): Adds a grouping column.
• group_by(group): Groups the data by the newly created group column.
• summarise(chunk = list(.)): Summarizes the groups into list columns using the . placeholder.
• pull(chunk): Extracts the list column as a vector of chunks.

Example 3: Splitting a Vector using group_split()

group_split() is another handy function from dplyr to split data into groups.

x <- 1:10
chunk_size <- 3
split_vector <- x %>%
  as.data.frame() %>%
  mutate(group = ceiling(row_number() / chunk_size)) %>%
  group_split(group)
print(split_vector)

<list_of<
  tbl_df<
    .    : integer
    group: double
  >
>[4]>
[[1]]
# A tibble: 3 × 2
      . group
  <int> <dbl>
1     1     1
2     2     1
3     3     1

[[2]]
# A tibble: 3 × 2
      . group
  <int> <dbl>
1     4     2
2     5     2
3     6     2

[[3]]
# A tibble: 3 × 2
      . group
  <int> <dbl>
1     7     3
2     8     3
3     9     3

[[4]]
# A tibble: 1 × 2
      . group
  <int> <dbl>
1    10     4

Explanation:

• as.data.frame(): Converts the vector to a data frame.
• mutate(group = ceiling(row_number() / chunk_size)): Adds a grouping column.
• group_split(group): Splits the data frame into a list of data frames based on the group column.

Using data.table

data.table is known for its efficiency with large datasets. Here’s how you can split a vector using data.table.

Example 4: Splitting a Vector into Chunks

library(data.table)

x <- 1:10
chunk_size <- 3
dt <- data.table(x = x)
dt[, group := ceiling(.I / chunk_size)]
split_vector <- dt[, .(chunk = list(x)), by = group]$chunk
print(split_vector)

[[1]]
[1] 1 2 3

[[2]]
[1] 4 5 6

[[3]]
[1] 7 8 9

[[4]]
[1] 10

Explanation:

• data.table(x = x): Converts the vector to a data.table.
• group := ceiling(.I / chunk_size): Creates a group column using the row index .I.
• .(chunk = list(x)), by = group: Groups by the group column and creates list columns.
• $chunk: Extracts the list column.

Your Turn!

These examples illustrate different ways to split vectors into chunks in R using base R, dplyr, and data.table. Each method has its own strengths, and you might prefer one over the others depending on your workflow and dataset size. Try these methods on your own data and see how they work for you. Experimenting with different chunk sizes and vector lengths can also help you understand the mechanics behind each approach better.

Happy coding!