43 Data: Map Basics

Purpose: The map() function and its variants are extremely useful for automating iterative tasks. We’ll learn the basics through this short exercise.

Reading: Introduction to Iteration and Map (you can skip the Case Study).

library(tidyverse)

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43.1 Formulas

The primer introduced map() as a way to apply a function to a list.

# NOTE: No need to change this example
map_dbl(c(1, 2, 3), log)

## [1] 0.0000000 0.6931472 1.0986123

This is very helpful when we have a builtin or otherwise defined function, but what about when we need a more special-purpose function for a specific case? In this instance we can use R’s formula notation. For example, to compute powers of 10, we could do:

# NOTE: No need to change this example
map_dbl(c(1, 2, 3), ~ 10 ^ .x)

## [1]   10  100 1000

The tilde ~ operator signals to R that we’re doing something special: defining a formula. The .x symbol is the argument for this new function. Basically, we are taking a formal function definition, such as

# NOTE: No need to change this example
pow10 <- function(x) {10 ^ x}

And defining a more compact version with ~ 10 ^ x.. We’ve actually already seen this formula notation when we use facet_grid() and facet_wrap(), though it’s used in a very different way in that context.

43.1.1 q1 Use map_chr() to prepend the string "N: " to the numbers in v_nums. Use formula notation with str_c() as your map function.

Hint: The function str_c() combines two or more objects into one string.

v_nums <- c(1, 2, 3)
v_q1 <- map_chr(v_nums, ~ str_c("N: ", .x))

v_q1

## [1] "N: 1" "N: 2" "N: 3"

Use the following test to check your work.

## NOTE: No need to change this!
assertthat::assert_that(setequal(v_q1, c("N: 1", "N: 2", "N: 3")))

## [1] TRUE

print("Great job!")

## [1] "Great job!"

Formula notation is another way to pass arguments to functions; I find this a little more readable than passing arguments to map().

43.1.2 q2 Use map_dbl() to compute the log with base = 2 of the numbers in v_nums. Use formula notation with log() as your map function.

v_q2 <- map_dbl(v_nums, ~ log(.x, base = 2))

v_q2

## [1] 0.000000 1.000000 1.584963

## NOTE: No need to change this!
assertthat::assert_that(setequal(v_q2, log(v_nums, base = 2)))

## [1] TRUE

print("Nice!")

## [1] "Nice!"