38 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: (None, this is the reading)

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38.1 Loops vs mapping

The core idea of map() is that it’s an alternative to writing a for loop. In a lot of for loops, we’re computing

## NOTE: No need to edit
v <- seq(1, 10)
v ^ 2 # Vectorized operation

##  [1]   1   4   9  16  25  36  49  64  81 100

However, if we have a non-vectorized function, we can still use map() to apply it to a vector:

## NOTE: No need to edit
f <- function(x) {x ^ 2}
map(v, f)

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## [1] 16
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The result prints in a weird way; that’s because we’ve made a list, rather than a vector. To specify that we want our output to be a vector of floating point values, we can use map_dbl():

## NOTE: No need to edit
f <- function(x) {x ^ 2}
map_dbl(v, f)

##  [1]   1   4   9  16  25  36  49  64  81 100

Function notation in R is a little weird; we don’t need to explicitly return our resulting value. Instead, the last line of the function will automatically be the return value:

f2 <- function(x) {
  x2 <- x * 2 # Do an intermediate calculation
  x2 ^ 2      # The last value of the function is automatically returned as the output
}

38.2 Formula notation

Rather than define a function(), 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.

38.2.1 q1 Add a prefix

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().

38.2.2 q2 Compute a log

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!"