How to Use fread() in R to Import Files Faster

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In the R programming language, one of the most common tasks for data analysts and data scientists is importing data from external files. While functions like read.csv() in base R provide an easy-to-use approach for data importing, they can be relatively slow when handling large data files. Fortunately, there’s an alternative function that offers faster data importing: fread().

The fread() function is part of the data.table package, a powerful package in R that extends the data.frame structure, allowing for higher performance in terms of speed and memory usage. This function is often favored over base R functions for importing large datasets due to its efficiency.

In this comprehensive guide, we will dive deep into the usage of the fread() function and demonstrate how it can make data importing in R much faster and more efficient.

Overview of fread()

The fread() function is a faster and more flexible alternative to the base R read.table() and read.csv() functions. Here is the basic syntax:

fread(input, sep = "auto", header = "auto", nrows = -1L, skip = 0L, ...)
  • input: a character string specifying the file name or a system command to read from its standard output.
  • sep: the field separator character. The default is “auto”, which automatically detects the separator.
  • header: whether the first line (after skipping lines) should be used as column names. The default is “auto”, which automatically detects whether the first line is a header.
  • nrows: the maximum number of data rows to read. The default is -1, which means to read all rows.
  • skip: the number of lines to skip before reading data. The default is 0.
  • ...: other arguments passed on to data.table().

Installing and Loading the data.table Package

Before you can use fread(), you need to install and load the data.table package. You can install it using install.packages() and load it using library():

# Install the data.table package

# Load the data.table package

Once the package is loaded, you can use the fread() function.

Basic Usage of fread()

Using fread() to read a .csv file is straightforward. Here’s an example:

# Use fread() to read a .csv file
data <- fread("data.csv")

# Print the first few rows of the data

In this example, fread() reads the “data.csv” file and returns a data table, which is a high-performance version of a data frame.

Using fread() with Large Files

One of the main advantages of fread() is its speed, which makes it particularly well-suited for reading large files. The function is designed to efficiently read files with millions of rows, making it a powerful tool for big data analysis.

Here’s how you can read a large .csv file with fread():

# Use fread() to read a large .csv file
large_data <- fread("large_data.csv")

# Print the first few rows of the data

Despite the large size of the file, fread() can typically read it much faster than read.csv().

Selecting Specific Rows with fread()

If you want to read only a subset of rows from a file, you can use the nrows and skip arguments. The nrows argument specifies the maximum number of rows to read, and the skip argument specifies the number of lines to skip before reading data.

Here’s how you can read rows 101 to 200 from a .csv file:

# Use fread() to read rows 101 to 200
subset_data <- fread("data.csv", skip = 100, nrows = 100)

# Print the data

In this example, fread() skips the first 100 lines (rows 1 to 100) and then reads the next 100 lines (rows 101 to 200).


In this article, we’ve explored the fread() function from the data.table package in R. This function provides a fast and flexible way to import data from .csv files and other delimited text files. Its speed makes it particularly useful for large datasets. With the ability to select specific rows and automatically detect separators and headers, fread() offers a powerful tool for data importing in R.

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