Map .

R Map Vs Apply: Which One Should You Use?

Written by Pauline Lafleur Sep 30, 2022 · 4 min read
R Map Vs Apply: Which One Should You Use?

Table of Contents

Map with Data Labels in R
Map with Data Labels in R from powerbi.tips

Introduction

When it comes to data analysis in R, there are many functions that are commonly used. Two such functions are `map` and `apply`. They both serve the same purpose of applying a function to a set of data, but they have slightly different use cases. In this article, we will dive deeper into these functions and explore when to use which.

What is `map`?

`Map` is a function from the `purrr` package that is used to apply a function to each element of a list or vector. It is especially useful when you have a list of objects and you need to perform the same operation on each object. For example, if you have a list of data frames and you want to extract the first column of each data frame, you can use `map` to apply the `[,1]` function to each data frame.

Example:

``` library(purrr) list_of_df <- list(data.frame(a = 1:3, b = 4:6), data.frame(a = 7:9, b = 10:12)) map(list_of_df, ~.[,1]) ``` This will return a list with the first column of each data frame.

What is `apply`?

`Apply` is a base R function that is used to apply a function to either the rows or columns of a matrix or array. It is especially useful when you have a large matrix or array and you want to perform a simple operation on each row or column. For example, if you have a matrix where each row represents a person and each column represents a different variable (such as age, income, and education level), you can use `apply` to calculate the mean age, income, and education level.

Example:

``` matrix <- matrix(c(25, 50000, 12, 30, 60000, 16), nrow = 2) colnames(matrix) <- c("age", "income", "education_level") rownames(matrix) <- c("person1", "person2") apply(matrix, 2, mean) ``` This will return a vector with the mean age, income, and education level.

When to use `map`

As mentioned earlier, `map` is best used when you have a list of objects and you need to perform the same operation on each object. It is especially useful when you need to extract a specific element from each object or perform a complex operation on each object. For example, if you have a list of websites and you want to extract the title of each website, you can use `map` to apply a function that extracts the title from each website.

Example:

``` library(rvest) websites <- c("https://www.google.com", "https://www.facebook.com", "https://www.twitter.com") map(websites, ~read_html(.) %>% html_nodes("title") %>% html_text()) ``` This will return a list with the title of each website.

When to use `apply`

`Apply` is best used when you have a matrix or array and you need to perform a simple operation on each row or column. It is especially useful when you need to calculate summary statistics such as means or standard deviations. For example, if you have a matrix where each row represents a person and each column represents a different variable (such as age, income, and education level), you can use `apply` to calculate the mean age, income, and education level.

Example:

``` matrix <- matrix(rnorm(1000), nrow = 50) apply(matrix, 1, mean) ``` This will return a vector with the mean of each row.

Conclusion

In summary, `map` and `apply` are both useful functions for applying a function to a set of data. While they have some similarities, they have different use cases. `Map` is best used when you have a list of objects and you need to perform the same operation on each object, while `apply` is best used when you have a matrix or array and you need to perform a simple operation on each row or column. By understanding the differences between these functions, you can make an informed decision on which one to use in your data analysis projects.

Q&A

Q: Can `map` be used on a matrix or array?

A: No, `map` is designed to work with lists and vectors. If you want to apply a function to the rows or columns of a matrix or array, you should use `apply`.

Q: Can `apply` be used on a list or vector?

A: Yes, `apply` can be used on a vector, but it will treat the vector as a one-dimensional matrix. It cannot be used on a list. If you have a list of objects and you need to apply a function to each object, you should use `map`.
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