Using R: from plyr to purrr, part 0 out of however many

February 11, 2020
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This post is me thinking out loud about applying functions to vectors or lists and getting data frames back.

Using R is an ongoing process of finding nice ways to throw data frames, lists and model objects around. While tidyr has arrived at a comfortable way to reshape dataframes with pivot_longer and pivot_wider, I don’t always find the replacements for the good old plyr package as satisfying.

Here is an example of something I used to like to do with plyr. Don’t laugh!

Assume we have a number of text files, all in the same format, that we need to read and combine. This arises naturally if you run some kind of analysis where the dataset gets split into chunks, like in genetics, where chunks might be chromosomes.

## Generate vector of file names
files <- paste("data/chromosome", 1:20, ".txt", sep = "")

library(plyr)
library(readr)
genome <- ldply(files, read_tsv)

This gives us one big data frame, containing the rows from all those files.

If we want to move on from plyr, what are our options?

We can go old school with base R functions lapply and Reduce.

library(readr)

chromosomes <- lapply(files, read_tsv)
genome <- Reduce(rbind, chromosomes)

Here, we first let lapply read each file and store it in a list. Then we let Reduce fold the list with rbind, which binds the data frames in the list together, one below the other.

If that didn’t make sense, here it is again: lapply maps a function to each element of a vector or list, collecting the results in a list. Reduce folds the elements in a list together, using a function that takes in two arguments. The first argument will be the results it’s accumulated so far, and the second argument will be the next element of the list.

In the end, this leaves us, as with ldply, with one big data frame.

We can also use purrr‘s map_dfr. This seems to be the contemporary most elegant solution:

library(purrr)
library(readr)

genome <- map_dfr(files, read_tsv)

map_dfr, like good old ldply will map over a vector or list, and collect resulting data frames. The ”r” in the name means adding the next data frame as rows. There is also a ”c” version (map_dfc) for adding as columns.

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