Read All Files in Directory & Apply Function to Each Data Frame in R (Example)

In this tutorial you’ll learn how to import all data frames from a folder and apply a function to each of them in R programming.

Table of contents:

With that, let’s do this:

Example Data

Let’s first construct some example data:

data1 <- data.frame(x1 = 1:5,                           # Create data frames
                    x2 = 3:7,
                    x3 = 2)
data2 <- data.frame(y1 = c(6, 9, 2),
                    y2 = c(4, 2, 3))
data3 <- data.frame(z1 = c(3, 6, 7, 2),
                    z2 = c(1, 5, 2, 3),
                    z3 = 4:1)

The previous R code shows the structure of our example data frames: We’ve created three data frames with different numeric columns.

Now, let’s define the path of our directory…

my_path <- "C:/Users/... Your Path .../"
setwd(my_path)                                          # Set path to folder

…and let’s write our example data frames to this directory:

write.csv2(data1, "data1.csv", row.names = FALSE)       # Write data frames
write.csv2(data2, "data2.csv", row.names = FALSE)
write.csv2(data3, "data3.csv", row.names = FALSE)

Have a look at the directory that you have specified before. It should contain three data frames stored in csv files.

Example: Import Data Frames from Directory & Apply Function

The following R programming syntax illustrates how to open the data frames stored in our directory. First, we need to extract the file names of all csv files:

data_frame_names <- list.files(pattern = "*.csv")       # Get all file names
data_frame_names                                        # Return file names to console
# "data1.csv" "data2.csv" "data3.csv"

As you can see based on the previous output of the RStudio console, our folder contains the csv files data1, data2, and data3. Let’s read these three data frames to R:

data_frame_list <- lapply(data_frame_names, read.csv2)  # Read all data frames
data_frame_list                                         # List containing all data frames
# [[1]]
#   x1 x2 x3
# 1  1  3  2
# 2  2  4  2
# 3  3  5  2
# 4  4  6  2
# 5  5  7  2
# 
# [[2]]
#   y1 y2
# 1  6  4
# 2  9  2
# 3  2  3
# 
# [[3]]
#   z1 z2 z3
# 1  3  1  4
# 2  6  5  3
# 3  7  2  2
# 4  2  3  1

As you can see, we have stored our three data frames in a list. Now, we can use the lapply function to apply a function to each of our data frames. In this example, we’ll compute the rowSums of each data matrix:

data_frame_output <- lapply(data_frame_list, rowSums)   # Applying function to all data
data_frame_output                                       # Return result
# [[1]]
# [1]  6  8 10 12 14
# 
# [[2]]
# [1] 10 11  5
# 
# [[3]]
# [1]  8 14 11  6

The previous output of the RStudio console shows our final result.

Video & Further Resources

If you need further info on the R codes of this tutorial, you may want to have a look at the following video of my YouTube channel. I’m illustrating the R syntax of this tutorial in the video:

The YouTube video will be added soon.

Furthermore, you might have a look at the other articles of my homepage.

• Getting & Setting Working Directories
• apply() Function in R
• List All Files with Specific Extension in R
• Import & Merge Multiple csv Files in R
• The R Programming Language

In summary: This tutorial explained how to read data files from a computer in R programming. In case you have additional questions or comments, don’t hesitate to let me know in the comments section.