Chapter 21 - Iteration
library(tidyverse)
library(stringr)
library(purrr)21.2 For loops
Problem 1
Write a for loop to compute the mean of every column in mtcars.
means <- vector(mode = "double", length = ncol(mtcars))
for (i in seq_along(mtcars)) {
means[[i]] <- mean(mtcars[[i]])
}Write a for loop to determine the type of each column in nycflights13::flights.
types <- vector(mode = "character", length = ncol(nycflights13::flights))
for (i in seq_along(nycflights13::flights)) {
types[[i]] <- typeof(nycflights13::flights[[i]])
}Write a for loop to compute the number of unique values in each column of iris.
uniques <- vector(mode = "integer", length = ncol(iris))
for (i in seq_along(iris)) {
uniques[[i]] <- length(unique(iris[[i]]))
}Write a for loop to generate 10 random normals for each of μ=−10, 0, 10, and 100.
mu <- c(-10, 0, 10, 100)
means <- vector(mode = "list", length = length(mu))
for (i in seq_along(mu)) {
means[[i]] <- rnorm(10, mu[[i]])
}Problem 2
Eliminate the for loop in each of the following examples by taking advantage of an existing function that works with vectors:
out <- ""
for (x in letters) {
out <- stringr::str_c(out, x)
}str_c(letters, collapse = "")## [1] "abcdefghijklmnopqrstuvwxyz"x <- sample(100)
sd <- 0
for (i in seq_along(x)) {
sd <- sd + (x[i] - mean(x)) ^ 2
}
sd <- sqrt(sd / (length(x) - 1))x <- sample(100)
sd(x)## [1] 29.01149x <- runif(100)
out <- vector("numeric", length(x))
out[1] <- x[1]
for (i in 2:length(x)) {
out[i] <- out[i - 1] + x[i]
}x <- runif(100)
cumsum(x)## [1] 0.7992565 1.7608118 2.0568895 2.7632569 3.1874423 3.4156319
## [7] 3.7932377 4.5010074 4.7897675 5.2268024 5.7017297 5.8936281
## [13] 6.5535643 6.9500150 7.2025988 8.1761619 8.2140256 8.3230613
## [19] 8.6160396 9.0065614 9.8620361 10.7127212 11.1803565 11.8688522
## [25] 12.1589534 12.8618798 13.6073635 13.8465202 14.6820606 15.5347900
## [31] 15.6181820 16.2615141 16.3094982 16.9174266 17.4108490 18.0282809
## [37] 18.6796008 19.3726228 19.9166126 20.8239177 21.7560005 22.6184446
## [43] 22.7806640 23.5676463 23.6948960 24.2016778 25.0481692 25.7883647
## [49] 26.4821228 26.8559495 27.3206825 27.7898423 27.7932023 28.3234821
## [55] 29.1914478 29.5327613 30.3893470 30.3923624 31.1120492 31.3565176
## [61] 31.8913102 32.3702339 32.4786353 32.5455526 33.0663031 33.8657784
## [67] 34.4458223 34.5220059 34.8413884 35.6982702 36.2742193 36.4214467
## [73] 36.6967912 37.2807661 37.5067842 38.1225982 38.9413877 39.5963806
## [79] 40.0266886 40.5622833 41.0330933 41.7829160 42.0595204 42.9576707
## [85] 43.5532956 44.3516510 45.2313919 46.0113575 46.1073148 46.2408325
## [91] 46.8870112 47.7124623 48.0929501 48.2655434 48.2890810 48.8839214
## [97] 49.8599138 50.0684173 50.8891799 51.6686330Problem 3
Write a for loop that prints() the lyrics to the children’s song “Alice the camel”.
humps <- c("five", "four", "three", "two", "one", "no")
for (i in seq_along(humps)) {
for (j in 1:3) {
print(str_glue("Alice the camel has {humps[i]} humps."))
}
if (i < 6) {
# str_glue is unecessary. it drops [1]
print(str_glue("So go, Alice, go."))
} else {
print(str_glue("Now Alice is a horse"))
}
}## Alice the camel has five humps.
## Alice the camel has five humps.
## Alice the camel has five humps.
## So go, Alice, go.
## Alice the camel has four humps.
## Alice the camel has four humps.
## Alice the camel has four humps.
## So go, Alice, go.
## Alice the camel has three humps.
## Alice the camel has three humps.
## Alice the camel has three humps.
## So go, Alice, go.
## Alice the camel has two humps.
## Alice the camel has two humps.
## Alice the camel has two humps.
## So go, Alice, go.
## Alice the camel has one humps.
## Alice the camel has one humps.
## Alice the camel has one humps.
## So go, Alice, go.
## Alice the camel has no humps.
## Alice the camel has no humps.
## Alice the camel has no humps.
## Now Alice is a horseConvert the nursery rhyme “ten in the bed” to a function. Generalise it to any number of people in any sleeping structure.
n_in_the_bed <- function(occupants = 10L) {
for (i in seq(from = occupants, to = 1)) {
if (i > 1) {
print(str_glue(
"There were {i} in a bed
And the little one said
'Roll over, roll over'
So they all rolled over
And one fell out
"
))
} else if (i == 1) {
print(str_glue("There was one in a bed
And the little one said
'Good night!'"))
}
}
}
n_in_the_bed()## There were 10 in a bed
## And the little one said
## 'Roll over, roll over'
## So they all rolled over
## And one fell out
##
## There were 9 in a bed
## And the little one said
## 'Roll over, roll over'
## So they all rolled over
## And one fell out
##
## There were 8 in a bed
## And the little one said
## 'Roll over, roll over'
## So they all rolled over
## And one fell out
##
## There were 7 in a bed
## And the little one said
## 'Roll over, roll over'
## So they all rolled over
## And one fell out
##
## There were 6 in a bed
## And the little one said
## 'Roll over, roll over'
## So they all rolled over
## And one fell out
##
## There were 5 in a bed
## And the little one said
## 'Roll over, roll over'
## So they all rolled over
## And one fell out
##
## There were 4 in a bed
## And the little one said
## 'Roll over, roll over'
## So they all rolled over
## And one fell out
##
## There were 3 in a bed
## And the little one said
## 'Roll over, roll over'
## So they all rolled over
## And one fell out
##
## There were 2 in a bed
## And the little one said
## 'Roll over, roll over'
## So they all rolled over
## And one fell out
##
## There was one in a bed
## And the little one said
## 'Good night!'Convert the song “99 bottles of beer on the wall” to a function. Generalise to any number of any vessel containing any liquid on any surface.
bottles_of_beer <- function(bottles = 99L) {
for (i in seq(from = bottles, to = 0)) {
bottles_left <- ifelse(i == 0, "no more", i)
print(str_glue(
"{i} bottle of beer on the wall, {i} bottle of beer.
Take one down and pass it around, {bottles_left} bottles of beer on the wall."
)
)
}
}
bottles_of_beer()## 99 bottle of beer on the wall, 99 bottle of beer.
## Take one down and pass it around, 99 bottles of beer on the wall.
## 98 bottle of beer on the wall, 98 bottle of beer.
## Take one down and pass it around, 98 bottles of beer on the wall.
## 97 bottle of beer on the wall, 97 bottle of beer.
## Take one down and pass it around, 97 bottles of beer on the wall.
## 96 bottle of beer on the wall, 96 bottle of beer.
## Take one down and pass it around, 96 bottles of beer on the wall.
## 95 bottle of beer on the wall, 95 bottle of beer.
## Take one down and pass it around, 95 bottles of beer on the wall.
## 94 bottle of beer on the wall, 94 bottle of beer.
## Take one down and pass it around, 94 bottles of beer on the wall.
## 93 bottle of beer on the wall, 93 bottle of beer.
## Take one down and pass it around, 93 bottles of beer on the wall.
## 92 bottle of beer on the wall, 92 bottle of beer.
## Take one down and pass it around, 92 bottles of beer on the wall.
## 91 bottle of beer on the wall, 91 bottle of beer.
## Take one down and pass it around, 91 bottles of beer on the wall.
## 90 bottle of beer on the wall, 90 bottle of beer.
## Take one down and pass it around, 90 bottles of beer on the wall.
## 89 bottle of beer on the wall, 89 bottle of beer.
## Take one down and pass it around, 89 bottles of beer on the wall.
## 88 bottle of beer on the wall, 88 bottle of beer.
## Take one down and pass it around, 88 bottles of beer on the wall.
## 87 bottle of beer on the wall, 87 bottle of beer.
## Take one down and pass it around, 87 bottles of beer on the wall.
## 86 bottle of beer on the wall, 86 bottle of beer.
## Take one down and pass it around, 86 bottles of beer on the wall.
## 85 bottle of beer on the wall, 85 bottle of beer.
## Take one down and pass it around, 85 bottles of beer on the wall.
## 84 bottle of beer on the wall, 84 bottle of beer.
## Take one down and pass it around, 84 bottles of beer on the wall.
## 83 bottle of beer on the wall, 83 bottle of beer.
## Take one down and pass it around, 83 bottles of beer on the wall.
## 82 bottle of beer on the wall, 82 bottle of beer.
## Take one down and pass it around, 82 bottles of beer on the wall.
## 81 bottle of beer on the wall, 81 bottle of beer.
## Take one down and pass it around, 81 bottles of beer on the wall.
## 80 bottle of beer on the wall, 80 bottle of beer.
## Take one down and pass it around, 80 bottles of beer on the wall.
## 79 bottle of beer on the wall, 79 bottle of beer.
## Take one down and pass it around, 79 bottles of beer on the wall.
## 78 bottle of beer on the wall, 78 bottle of beer.
## Take one down and pass it around, 78 bottles of beer on the wall.
## 77 bottle of beer on the wall, 77 bottle of beer.
## Take one down and pass it around, 77 bottles of beer on the wall.
## 76 bottle of beer on the wall, 76 bottle of beer.
## Take one down and pass it around, 76 bottles of beer on the wall.
## 75 bottle of beer on the wall, 75 bottle of beer.
## Take one down and pass it around, 75 bottles of beer on the wall.
## 74 bottle of beer on the wall, 74 bottle of beer.
## Take one down and pass it around, 74 bottles of beer on the wall.
## 73 bottle of beer on the wall, 73 bottle of beer.
## Take one down and pass it around, 73 bottles of beer on the wall.
## 72 bottle of beer on the wall, 72 bottle of beer.
## Take one down and pass it around, 72 bottles of beer on the wall.
## 71 bottle of beer on the wall, 71 bottle of beer.
## Take one down and pass it around, 71 bottles of beer on the wall.
## 70 bottle of beer on the wall, 70 bottle of beer.
## Take one down and pass it around, 70 bottles of beer on the wall.
## 69 bottle of beer on the wall, 69 bottle of beer.
## Take one down and pass it around, 69 bottles of beer on the wall.
## 68 bottle of beer on the wall, 68 bottle of beer.
## Take one down and pass it around, 68 bottles of beer on the wall.
## 67 bottle of beer on the wall, 67 bottle of beer.
## Take one down and pass it around, 67 bottles of beer on the wall.
## 66 bottle of beer on the wall, 66 bottle of beer.
## Take one down and pass it around, 66 bottles of beer on the wall.
## 65 bottle of beer on the wall, 65 bottle of beer.
## Take one down and pass it around, 65 bottles of beer on the wall.
## 64 bottle of beer on the wall, 64 bottle of beer.
## Take one down and pass it around, 64 bottles of beer on the wall.
## 63 bottle of beer on the wall, 63 bottle of beer.
## Take one down and pass it around, 63 bottles of beer on the wall.
## 62 bottle of beer on the wall, 62 bottle of beer.
## Take one down and pass it around, 62 bottles of beer on the wall.
## 61 bottle of beer on the wall, 61 bottle of beer.
## Take one down and pass it around, 61 bottles of beer on the wall.
## 60 bottle of beer on the wall, 60 bottle of beer.
## Take one down and pass it around, 60 bottles of beer on the wall.
## 59 bottle of beer on the wall, 59 bottle of beer.
## Take one down and pass it around, 59 bottles of beer on the wall.
## 58 bottle of beer on the wall, 58 bottle of beer.
## Take one down and pass it around, 58 bottles of beer on the wall.
## 57 bottle of beer on the wall, 57 bottle of beer.
## Take one down and pass it around, 57 bottles of beer on the wall.
## 56 bottle of beer on the wall, 56 bottle of beer.
## Take one down and pass it around, 56 bottles of beer on the wall.
## 55 bottle of beer on the wall, 55 bottle of beer.
## Take one down and pass it around, 55 bottles of beer on the wall.
## 54 bottle of beer on the wall, 54 bottle of beer.
## Take one down and pass it around, 54 bottles of beer on the wall.
## 53 bottle of beer on the wall, 53 bottle of beer.
## Take one down and pass it around, 53 bottles of beer on the wall.
## 52 bottle of beer on the wall, 52 bottle of beer.
## Take one down and pass it around, 52 bottles of beer on the wall.
## 51 bottle of beer on the wall, 51 bottle of beer.
## Take one down and pass it around, 51 bottles of beer on the wall.
## 50 bottle of beer on the wall, 50 bottle of beer.
## Take one down and pass it around, 50 bottles of beer on the wall.
## 49 bottle of beer on the wall, 49 bottle of beer.
## Take one down and pass it around, 49 bottles of beer on the wall.
## 48 bottle of beer on the wall, 48 bottle of beer.
## Take one down and pass it around, 48 bottles of beer on the wall.
## 47 bottle of beer on the wall, 47 bottle of beer.
## Take one down and pass it around, 47 bottles of beer on the wall.
## 46 bottle of beer on the wall, 46 bottle of beer.
## Take one down and pass it around, 46 bottles of beer on the wall.
## 45 bottle of beer on the wall, 45 bottle of beer.
## Take one down and pass it around, 45 bottles of beer on the wall.
## 44 bottle of beer on the wall, 44 bottle of beer.
## Take one down and pass it around, 44 bottles of beer on the wall.
## 43 bottle of beer on the wall, 43 bottle of beer.
## Take one down and pass it around, 43 bottles of beer on the wall.
## 42 bottle of beer on the wall, 42 bottle of beer.
## Take one down and pass it around, 42 bottles of beer on the wall.
## 41 bottle of beer on the wall, 41 bottle of beer.
## Take one down and pass it around, 41 bottles of beer on the wall.
## 40 bottle of beer on the wall, 40 bottle of beer.
## Take one down and pass it around, 40 bottles of beer on the wall.
## 39 bottle of beer on the wall, 39 bottle of beer.
## Take one down and pass it around, 39 bottles of beer on the wall.
## 38 bottle of beer on the wall, 38 bottle of beer.
## Take one down and pass it around, 38 bottles of beer on the wall.
## 37 bottle of beer on the wall, 37 bottle of beer.
## Take one down and pass it around, 37 bottles of beer on the wall.
## 36 bottle of beer on the wall, 36 bottle of beer.
## Take one down and pass it around, 36 bottles of beer on the wall.
## 35 bottle of beer on the wall, 35 bottle of beer.
## Take one down and pass it around, 35 bottles of beer on the wall.
## 34 bottle of beer on the wall, 34 bottle of beer.
## Take one down and pass it around, 34 bottles of beer on the wall.
## 33 bottle of beer on the wall, 33 bottle of beer.
## Take one down and pass it around, 33 bottles of beer on the wall.
## 32 bottle of beer on the wall, 32 bottle of beer.
## Take one down and pass it around, 32 bottles of beer on the wall.
## 31 bottle of beer on the wall, 31 bottle of beer.
## Take one down and pass it around, 31 bottles of beer on the wall.
## 30 bottle of beer on the wall, 30 bottle of beer.
## Take one down and pass it around, 30 bottles of beer on the wall.
## 29 bottle of beer on the wall, 29 bottle of beer.
## Take one down and pass it around, 29 bottles of beer on the wall.
## 28 bottle of beer on the wall, 28 bottle of beer.
## Take one down and pass it around, 28 bottles of beer on the wall.
## 27 bottle of beer on the wall, 27 bottle of beer.
## Take one down and pass it around, 27 bottles of beer on the wall.
## 26 bottle of beer on the wall, 26 bottle of beer.
## Take one down and pass it around, 26 bottles of beer on the wall.
## 25 bottle of beer on the wall, 25 bottle of beer.
## Take one down and pass it around, 25 bottles of beer on the wall.
## 24 bottle of beer on the wall, 24 bottle of beer.
## Take one down and pass it around, 24 bottles of beer on the wall.
## 23 bottle of beer on the wall, 23 bottle of beer.
## Take one down and pass it around, 23 bottles of beer on the wall.
## 22 bottle of beer on the wall, 22 bottle of beer.
## Take one down and pass it around, 22 bottles of beer on the wall.
## 21 bottle of beer on the wall, 21 bottle of beer.
## Take one down and pass it around, 21 bottles of beer on the wall.
## 20 bottle of beer on the wall, 20 bottle of beer.
## Take one down and pass it around, 20 bottles of beer on the wall.
## 19 bottle of beer on the wall, 19 bottle of beer.
## Take one down and pass it around, 19 bottles of beer on the wall.
## 18 bottle of beer on the wall, 18 bottle of beer.
## Take one down and pass it around, 18 bottles of beer on the wall.
## 17 bottle of beer on the wall, 17 bottle of beer.
## Take one down and pass it around, 17 bottles of beer on the wall.
## 16 bottle of beer on the wall, 16 bottle of beer.
## Take one down and pass it around, 16 bottles of beer on the wall.
## 15 bottle of beer on the wall, 15 bottle of beer.
## Take one down and pass it around, 15 bottles of beer on the wall.
## 14 bottle of beer on the wall, 14 bottle of beer.
## Take one down and pass it around, 14 bottles of beer on the wall.
## 13 bottle of beer on the wall, 13 bottle of beer.
## Take one down and pass it around, 13 bottles of beer on the wall.
## 12 bottle of beer on the wall, 12 bottle of beer.
## Take one down and pass it around, 12 bottles of beer on the wall.
## 11 bottle of beer on the wall, 11 bottle of beer.
## Take one down and pass it around, 11 bottles of beer on the wall.
## 10 bottle of beer on the wall, 10 bottle of beer.
## Take one down and pass it around, 10 bottles of beer on the wall.
## 9 bottle of beer on the wall, 9 bottle of beer.
## Take one down and pass it around, 9 bottles of beer on the wall.
## 8 bottle of beer on the wall, 8 bottle of beer.
## Take one down and pass it around, 8 bottles of beer on the wall.
## 7 bottle of beer on the wall, 7 bottle of beer.
## Take one down and pass it around, 7 bottles of beer on the wall.
## 6 bottle of beer on the wall, 6 bottle of beer.
## Take one down and pass it around, 6 bottles of beer on the wall.
## 5 bottle of beer on the wall, 5 bottle of beer.
## Take one down and pass it around, 5 bottles of beer on the wall.
## 4 bottle of beer on the wall, 4 bottle of beer.
## Take one down and pass it around, 4 bottles of beer on the wall.
## 3 bottle of beer on the wall, 3 bottle of beer.
## Take one down and pass it around, 3 bottles of beer on the wall.
## 2 bottle of beer on the wall, 2 bottle of beer.
## Take one down and pass it around, 2 bottles of beer on the wall.
## 1 bottle of beer on the wall, 1 bottle of beer.
## Take one down and pass it around, 1 bottles of beer on the wall.
## 0 bottle of beer on the wall, 0 bottle of beer.
## Take one down and pass it around, no more bottles of beer on the wall.Problem 4
It’s common to see for loops that don’t preallocate the output and instead increase the length of a vector at each step:
output <- vector("integer", 0)
for (i in seq_along(x)) {
output <- c(output, lengths(x[[i]]))
}
outputHow does this affect performance? Design and execute an experiment.
library(microbenchmark)
grow_vector <- function() {
# create a character vector
x <- rep("abc", 1000)
# create an empty vector of length 0 for the output
output <- vector("integer", 0)
# find the length of each observation in x and grow and concatenate it to
# output
for (i in seq_along(x)) {
output <- c(output, lengths(x[[i]]))
}
output
}
microbenchmark(grow_vector())## Unit: milliseconds
## expr min lq mean median uq max neval
## grow_vector() 1.900363 2.13041 3.498677 2.936309 3.768595 32.02081 100This first chunk grows a vector from length zero to length 1,000.
preallocate_vector <- function() {
# create a character vector
x <- rep("abc", 1000)
# create an empty vector of length length(x) for the output
output <- vector("integer", length(x))
# find the length of each observation in x and assignment it the corresponding
# location in the output vector
for (i in seq_along(x)) {
output[i] <- lengths(x[[i]])
}
output
}
microbenchmark(preallocate_vector())## Unit: microseconds
## expr min lq mean median uq max
## preallocate_vector() 637.998 646.7515 749.7371 653.1365 670.594 5219.469
## neval
## 100This second chunk preallocates the space in the vector and then populates it. It is an order of magnitude faster.
21.3 - For loop variations
Problem 1
Imagine you have a directory full of CSV files that you want to read in. You have their paths in a vector, files <- dir("data/", pattern = "\\.csv$", full.names = TRUE), and now want to read each one with read_csv(). Write the for loop that will load them into a single data frame.
library(tidyverse)
files <- dir("data/", pattern = "\\.csv$", full.names = TRUE)
output <- vector("list", length(files))
for (i in seq_along(files)) {
output[[i]] <- read_csv(files[[i]])
}
output <- flatten_df(output)Problem 2
What happens if you use for (nm in names(x)) and x has no names?
x <- c(1, 2, 3)
for (nm in names(x)) {
print(names(x[nm]))
}What if only some of the elements are named?
It does not matter. It still iterates along the entire vector.
x <- c("a" = 1, 2, "c" = 3)
for (nm in names(x)) {
print(names(x[nm]))
}## [1] "a"
## [1] NA
## [1] "c"What if the names are not unique?
It works.
x <- c("a" = 1, "a" = 2, "c" = 3)
for (nm in names(x)) {
print(names(x[nm]))
}## [1] "a"
## [1] "a"
## [1] "c"Problem 3
Write a function that prints the mean of each numeric column in a data frame, along with its name. For example, show_mean(iris) would print:
show_mean(iris)
#> Sepal.Length: 5.84
#> Sepal.Width: 3.06
#> Petal.Length: 3.76
#> Petal.Width: 1.20(Extra challenge: what function did I use to make sure that the numbers lined up nicely, even though the variable names had different lengths?)
print_means <- function(data = iris, digits = 2) {
max_str_length <- max(str_length(names(data)))
for (i in seq_along(data)) {
if (is.numeric(data[[i]])) {
cat("#> ",
str_pad(str_c(names(data)[[i]], ": "), width = max_str_length + 2, side = "right"),
format(mean(data[[i]]), digits = digits, nsmall = 2),
"\n",
sep = "")
}
}
}
print_means()## #> Sepal.Length: 5.84
## #> Sepal.Width: 3.06
## #> Petal.Length: 3.76
## #> Petal.Width: 1.20Problem 4
What does this code do? How does it work?
>>>>>>> aaron
trans <- list(
disp = function(x) x * 0.0163871,
am = function(x) {
factor(x, labels = c("auto", "manual"))
}
)
for (var in names(trans)) {
mtcars[[var]] <- trans[[var]](mtcars[[var]])
}The list trans contains two functions: disp() and am(). disp() takes its input and multiplies by 0.0163871 and am() takes its input and creates a factor variable with the value “auto” or “manual.” The for loop loops over the two functions in the list, uses the variables “disp” and “am” in the dataset mtcars as inputs, and outputs a new mtcars dataset.
21.4 For loops vs. functionals
Problem 1
Read the documentation for apply(). In the 2d case, what two for loops does it generalise?
Read the documentation for apply(). In the 2d case, what two for loops does it generalise?
It generalizes a for loop along a vector inside of a for loop along a series of columns in a data frame or matrix.
Problem 2
Adapt col_summary() so that it only applies to numeric columns You might want to start with an is_numeric() function that returns a logical vector that has a TRUE corresponding to each numeric column.
col_summary <- function(df, fun) {
out <- vector("double", length(df))
for (i in seq_along(df)) {
if (is_numeric(df[[i]])) {
out[i] <- fun(df[[i]])
}
}
out
}
col_summary(mtcars, median)## [1] 19.200 6.000 196.300 123.000 3.695 3.325 17.710 0.000
## [9] 0.000 4.000 2.00021.5 The map functions
<<<<<<< HEAD ### Problem 1
Write code that uses one of the map functions to compute the mean of every column in mtcars.
mtcars %>%
map_dbl(mean)## mpg cyl disp hp drat wt
## 20.090625 6.187500 230.721875 146.687500 3.596563 3.217250
## qsec vs am gear carb
## 17.848750 0.437500 0.406250 3.687500 2.812500Write code that uses one of the map functions to determine the type of each column in nycflights13::flights
nycflights13::flights %>%
map_chr(typeof)## year month day dep_time sched_dep_time
## "integer" "integer" "integer" "integer" "integer"
## dep_delay arr_time sched_arr_time arr_delay carrier
## "double" "integer" "integer" "double" "character"
## flight tailnum origin dest air_time
## "integer" "character" "character" "character" "double"
## distance hour minute time_hour
## "double" "double" "double" "double"Write code that uses one of the map functions to compute the number of unique values in each column of iris
iris %>%
map_int(~n_distinct(.))## Sepal.Length Sepal.Width Petal.Length Petal.Width Species
## 35 23 43 22 3Write code that uses one of the map functions to generate 10 random normals for each of μ = -10, 0, 10, and 100.
c(-10, 0, 10, 100) %>%
map(rnorm, n = 10)## [[1]]
## [1] -8.613927 -10.861591 -10.509092 -9.621524 -11.265569 -9.559204
## [7] -10.645219 -8.926115 -8.283552 -9.792644
##
## [[2]]
## [1] 0.8410627 -0.5900651 -0.1973340 0.4830072 0.7880175 0.3395677
## [7] 0.6817573 -0.2498184 -0.1152174 -0.3149770
##
## [[3]]
## [1] 10.963085 9.409694 11.771349 11.784008 11.862400 11.362329 10.703007
## [8] 10.791937 9.934156 9.021313
##
## [[4]]
## [1] 100.50982 100.82160 100.64492 99.57490 101.21460 101.74406 100.68118
## [8] 102.20611 98.92326 100.688880.0.1 Problem 2
How can you create a single vector that for each column in a data frame indicates whether or not it’s a factor?
iris %>%
map_lgl(is.factor)## Sepal.Length Sepal.Width Petal.Length Petal.Width Species
## FALSE FALSE FALSE FALSE TRUE0.0.2 Problem 3
What happens when you use the map functions on vectors that aren’t lists? What does map(1:5, runif) do? Why?
The function iterates across each value, 1:5, in the vector and returns a list with five vectors containing between one and five random deviates:
map(1:5, runif)## [[1]]
## [1] 0.6802404
##
## [[2]]
## [1] 0.1940716 0.4242075
##
## [[3]]
## [1] 0.5138952 0.4181993 0.9047920
##
## [[4]]
## [1] 0.097138551 0.095828098 0.167626012 0.002122607
##
## [[5]]
## [1] 0.67009965 0.81967810 0.46746453 0.41498014 0.06425946If the vector 1:5 is a list, the function does not iterate across five values but rather takes one value, the length five vector, as its sole input. Since the input is not a single value, the map function uses the length of the vector as its input:
map(list(1:5), runif)## [[1]]
## [1] 0.90291553 0.59047650 0.04802083 0.28696725 0.74672489And as a result, the above function is equivalent to:
map(list(5), runif)
map(5, runif)
map(list(5:9), runif)0.0.3 Problem 4
What does map(-2:2, rnorm, n = 5) do? Why? What does map_dbl(-2:2, rnorm, n = 5) do? Why?
map(-2:2, rnorm, n = 5) returns a list with five vectors of length five with values draw from the standard normal distribution with means ranging from -2 to 2.
map_dbl(-2:2, rnorm, n = 5) returns the error Error: Result 1 is not a length 1 atomic vector because vectors can’t be nested inside of vectors.
The first works because map() returns a list and the second doesn’t work because map_dbl() returns a vector.
0.0.4 Problem 5
Rewrite map(x, function(df) lm(mpg ~ wt, data = df)) to eliminate the anonymous function.
map(x, ~lm(mpg ~ wt, data = .))21.9 Other patterns of for loops
0.0.5 Problem 1
Implement your own version of every() using a for loop. Compare it with purrr::every(). What does purrr’s version do that your version doesn’t?
x <- list(1:5, letters, list(10))
my_every <- function(x, f) {
out <- vector("logical", 1)
for (i in seq_along(x)) {
out[[i]] <- f(x[[i]])
}
!FALSE %in% out
}
x %>%
my_every(is.numeric)## [1] FALSEpurrr:every() is clever. It doesn’t create a vector and store output. Instead, it tests every object and then returns a FALSE when the first vector fails the test. This is more efficient than my code.
Unlike dplyr’s every(), my function does not support shortcuts for anonymous functions
iris %>%
select_if(is.numeric) %>%
every(~mean(.) > 3)0.0.6 Problem 2
Create an enhanced col_sum() that applies a summary function to every numeric column in a data frame.
col_summary <- function(df, fun) {
df_num <- keep(df, is.numeric)
map_dbl(df_num, fun)
}
col_summary(iris, mean)## Sepal.Length Sepal.Width Petal.Length Petal.Width
## 5.843333 3.057333 3.758000 1.199333Problem 3
A possible base R equivalent of col_sum() is:
col_sum3 <- function(df, f) {
is_num <- sapply(df, is.numeric)
df_num <- df[, is_num]
sapply(df_num, f)
}But it has a number of bugs as illustrated with the following inputs:
df <- tibble(
x = 1:3,
y = 3:1,
z = c("a", "b", "c")
)
# OK
col_sum3(df, mean)
# Has problems: don't always return numeric vector
col_sum3(df[1:2], mean)
col_sum3(df[1], mean)
col_sum3(df[0], mean)What causes the bugs?
The bug stems from sapply, which returns a list rather than a vector when given a dataframe with no variables. Here is a more consistent approach in base R:
df <- tibble(
x = 1:3,
y = 3:1,
z = c("a", "b", "c")
)
col_sum3 <- function(df, f) {
is_num <- vapply(df, is.numeric, logical(1))
df_num <- df[, is_num]
sapply(df_num, f)
}
col_sum3(df, mean)## x y
## 2 2