Writing multiple data frames into .csv files using R

This is a common operation. You need to split the dataframe into a list of dataframes then write them to many separate csvs. I will demonstrate 2 approaches with base R, and 2 approaches with tidyverse.

base R

A for loop makes the iteration very explicit.

# example data.frame
df  <- data.frame(x = 1:4, y = c("a", "a", "b", "b"))

# split the dataframe into a list by the y column
l  <- split(df, df$y)

# make filepaths from list names, which are unique values of the y column
file_out <- paste0(names(l), ".csv")

# iterate over the list and the vector of list names to write csvs
for(i in 1:length(l)) {
  write_csv(l[[i]], file_out[i])
}

Or using mapply():

mapply(
  function(x, y) write_csv(x, y), 
  l, 
  file_out
)

tidyverse approach

library(tidyverse)

# we pass walk2 two inputs: a list of dataframes (.x) and filepaths (.y)
# `walk` is a silent `map` that doesn't print output to the console
walk2(l, file_out, ~write_csv(.x, .y))

Or, avoiding intermediate variables:

df %>% 
  group_split(y) %>% 
  walk(~write_csv(.x, paste0(.x$y[1], ".csv")))

Here's a self-contained example along the lines of Richard's comment, but uses the names of the dataframes in the list as filenames for the CSV files:

# Create a list of n data frames

n <- 10

my_list <- lapply(1:n, function(i)  data.frame(x = rnorm(10), y = rnorm(10)) )

# name the data frames

names(my_list) <- letters[1:n]

# save each new data frame as an individual .csv file based on its name

lapply(1:length(my_list), function(i) write.csv(my_list[[i]], 
                                      file = paste0(names(my_list[i]), ".csv"),
                                      row.names = FALSE))