Control flow

Learning objectives

Understand the two primary tools for control flow: choices and loops
Learn best practices to void common pitfalls
Distinguish when to use if, ifelse(), and switch() for choices
Distinguish when to use for, while, and repeat for loops

Note

Basic familiarity with choices and loops is assumed.

Choices

`if` is the basic statement for a choice

Single line format

if (condition) true_action
if (condition) true_action else false_action

Compound statement within {

grade <- function(x) {
  if (x > 90) {
    "A"
  } else if (x > 80) {
    "B"
  } else if (x > 50) {
    "C"
  } else {
    "F"
  }
}

Results of `if` can be assigned

x1 <- if (TRUE) 1 else 2
x2 <- if (FALSE) 1 else 2

c(x1, x2)

#> [1] 1 2

Tip

Only recommended with single line format; otherwise hard to read.

`if` without `else` can be combined with `c()` or `paste()` to create compact expressions

if without else invisibly returns NULL when FALSE.

greet <- function(name, birthday = FALSE) {
  paste0(
    "Hi ", name,
    if (birthday) " and HAPPY BIRTHDAY"
  )
}
greet("Maria", FALSE)

#> [1] "Hi Maria"

greet("Jaime", TRUE)

#> [1] "Hi Jaime and HAPPY BIRTHDAY"

`if` should have a single `TRUE` or `FALSE` condition, other inputs generate errors

if ("x") 1

#> Error in if ("x") 1: argument is not interpretable as logical

if (logical()) 1

#> Error in if (logical()) 1: argument is of length zero

if (NA) 1

#> Error in if (NA) 1: missing value where TRUE/FALSE needed

if (c(TRUE, FALSE)) 1

#> Error in if (c(TRUE, FALSE)) 1: the condition has length > 1

Use `ifelse()` for vectorized conditions

x <- 1:10
ifelse(x %% 5 == 0, "XXX", as.character(x))

#>  [1] "1"   "2"   "3"   "4"   "XXX" "6"   "7"   "8"   "9"   "XXX"

ifelse(x %% 2 == 0, "even", "odd")

#>  [1] "odd"  "even" "odd"  "even" "odd"  "even" "odd"  "even" "odd"  "even"

Tip

Only use ifelse() if both results are of the same type; otherwise output type is hard to predict.

Use `dplyr::case_when()` for multiple condition-vector pairs

dplyr::case_when(
  x %% 35 == 0 ~ "fizz buzz",
  x %% 5 == 0 ~ "fizz",
  x %% 7 == 0 ~ "buzz",
  is.na(x) ~ "???",
  TRUE ~ as.character(x)
)

#>  [1] "1"    "2"    "3"    "4"    "fizz" "6"    "buzz" "8"    "9"    "fizz"

`switch()` is a special purpose equivalent to `if` that can be used to compact code

x_option <- function(x) {
  if (x == "a") {
    "option 1"
  } else if (x == "b") {
    "option 2" 
  } else if (x == "c") {
    "option 3"
  } else {
    stop("Invalid `x` value")
  }
}

x_option <- function(x) {
  switch(x,
    a = "option 1",
    b = "option 2",
    c = "option 3",
    stop("Invalid `x` value")
  )
}

`switch()` is a special purpose equivalent to `if` that can be used to compact code

Tip

The last component of a switch() should always throw an error, otherwise unmatched inputs will invisibly return NULL.
Only use switch() with character inputs. Numeric inputs are hard to read and have undesirable failure modes.

Caution

Like if, switch() can only take a single condition, not vector conditions

Avoid repeat outputs by leaving the right side of `=` empty

Inputs will “fall through” to the next value.

legs <- function(x) {
  switch(x,
    cow = ,
    horse = ,
    dog = 4,
    human = ,
    chicken = 2,
    plant = 0,
    stop("Unknown input")
  )
}
legs("cow")

#> [1] 4

legs("dog")

#> [1] 4

Loops

To iterate over items in a vector, use a `for` loop

for (item in vector) perform_action

for (i in 1:3) {
  print(i)
}

#> [1] 1
#> [1] 2
#> [1] 3

Note

Convention uses short variables like i, j, or k for iterating vector indices

`for` will overwrite existing variables in the current environment

i <- 100
for (i in 1:3) {}
i

#> [1] 3

Use `next` or `break` to terminate loops early

next exits the current iteration, but continues the loop
break exits the entire loop

Use `next` or `break` to terminate loops early

for (i in 1:10) {
  if (i < 3) 
    next

  print(i)
  
  if (i >= 5)
    break
}

#> [1] 3
#> [1] 4
#> [1] 5

Preallocate an output container to avoid slow loops

means <- c(1, 50, 20)
out <- vector("list", length(means))
for (i in 1:length(means)) {
  out[[i]] <- rnorm(10, means[[i]])
}

Tip

vector() function is helpful for preallocation

Use `seq_along(x)` instead of `1:length(x)`

1:length(x) causes unexpected failure for 0 length vectors
: works with both increasing and decreasing sequences

means <- c()
1:length(means)

#> [1] 1 0

seq_along(means)

#> integer(0)

Use `seq_along(x)` instead of `1:length(x)`

out <- vector("list", length(means))
for (i in 1:length(means)) {
  out[[i]] <- rnorm(10, means[[i]])
}

#> Error in rnorm(10, means[[i]]): invalid arguments

out <- vector("list", length(means))
for (i in seq_along(means)) {
  out[[i]] <- rnorm(10, means[[i]])
}
out

#> list()

Avoid problems when iterating over S3 vectors by using `seq_along(x)` and `x[[i]]`

loops typically strip attributes

xs <- as.Date(c("2020-01-01", "2010-01-01"))

for (x in xs) {
  print(x)
}

#> [1] 18262
#> [1] 14610

for (i in seq_along(xs)) {
  print(xs[[i]])
}

#> [1] "2020-01-01"
#> [1] "2010-01-01"

Use `while` or `repeat` when you don’t know the number of iterations

while(condition) action: perfoms action while condition is TRUE
repeat(action): repeats action forever (or until a break)

Always use the least-flexible loop option possible

Use for before while or repeat
In data analysis use apply() or purrr::map() before for

Quiz & Exercises

What is the difference between if and ifelse()?

if works with scalars; ifelse() works with vectors.

In the following code, what will the value of `y` be if `x` is `TRUE`? What if `x` is `FALSE`? What if `x` is `NA`?

y <- if (x) 3

When x is TRUE, y will be 3; when FALSE, y will be NULL; when NA the if statement will throw an error.

What does `switch("x", x = , y = 2, z = 3)` return?

switch(
  "x",
  x = ,
  y = 2,
  x = 3
)

This switch() statement makes use of fall-through so it will return 2.

What type of vector does each of the following calls to ifelse() return?

Read the documentation and write down the rules in your own words.

ifelse(TRUE, 1, "no")
ifelse(FALSE, 1, "no")
ifelse(NA, 1, "no")

What type of vector does each of the following calls to ifelse() return?

The arguments of ifelse() are named test, yes and no. In general, ifelse() returns the entry for yes when test is TRUE, the entry for no when test is FALSE and NA when test is NA.

ifelse(TRUE, 1, "no")

#> [1] 1

ifelse(FALSE, 1, "no")

#> [1] "no"

ifelse(NA, 1, "no")

#> [1] NA

What type of vector does each of the following calls to ifelse() return?

In practice, test is first converted to logical and if the result is neither TRUE nor FALSE, then as.logical(test) is returned.

ifelse(logical(), 1, "no")

#> logical(0)

ifelse(NaN, 1, "no")

#> [1] NA

ifelse(NA_character_, 1, "no")

#> [1] NA

ifelse("a", 1, "no")

#> [1] NA

ifelse("true", 1, "no")

#> [1] 1

Why does the following code work?

x <- 1:10
if (length(x)) "not empty" else "empty"

#> [1] "not empty"

x <- numeric()
if (length(x)) "not empty" else "empty"

#> [1] "empty"

if() expects a logical condition, but also accepts a numeric vector where 0 is treated as FALSE and all other numbers are treated as TRUE. Numerical missing values (including NaN) lead to an error in the same way that a logical missing, NA, does.

Why does this code succeed without errors or warnings?

x <- numeric()
out <- vector("list", length(x))
for (i in 1:length(x)) {
  out[i] <- x[i] ^ 2
}
out

#> [[1]]
#> [1] NA

Why does this code succeed without errors or warnings?

Subsetting behavior for out-of-bounds & 0 indices when using [<- and [
x[1] generates an NA. NA is assigned to the empty length-1 list out[1]
x[0] returns numeric(0). numeric(0) is assigned to out[0]. Assigning a 0-length vector to a 0-length subset doesn’t change the object.
Each step includes valid R operations (even though the result may not be what the user intended).

Walk-through

Setup

x <- numeric()
out <- vector("list", length(x))
1:length(x)

#> [1] 1 0

Walk-through

First Iteration

x[1]

#> [1] NA

x[1]^2

#> [1] NA

out[1]

#> [[1]]
#> NULL

out[1] <- x[1]^2
out[1]

#> [[1]]
#> [1] NA

Walk-through

Second Iteration

x[0]

#> numeric(0)

x[0]^2

#> numeric(0)

out[0]

#> list()

out[0] <- x[0]^2
out[0]

#> list()

Walk-through

Final Result

out

#> [[1]]
#> [1] NA

When the following code is evaluated, what can you say about the vector being iterated?

xs <- c(1, 2, 3)
for (x in xs) {
  xs <- c(xs, x * 2)
}
xs

#> [1] 1 2 3 2 4 6

In this loop x takes on the values of the initial xs (1, 2 and 3), indicating that it is evaluated just once in the beginning of the loop, not after each iteration. (Otherwise, we would run into an infinite loop.)

What does the following code tell you about when the index is updated?

for (i in 1:3) {
  i <- i * 2
  print(i) 
}

#> [1] 2
#> [1] 4
#> [1] 6

In a for loop the index is updated in the beginning of each iteration. Therefore, reassigning the index symbol during one iteration doesn’t affect the following iterations. (Again, we would otherwise run into an infinite loop.)

Control flow

Learning objectives

Choices

if is the basic statement for a choice

Results of if can be assigned

if without else can be combined with c() or paste() to create compact expressions

if should have a single TRUE or FALSE condition, other inputs generate errors

Use ifelse() for vectorized conditions

Use dplyr::case_when() for multiple condition-vector pairs

switch() is a special purpose equivalent to if that can be used to compact code

switch() is a special purpose equivalent to if that can be used to compact code

Avoid repeat outputs by leaving the right side of = empty

Loops

To iterate over items in a vector, use a for loop

for will overwrite existing variables in the current environment

Use next or break to terminate loops early

Use next or break to terminate loops early

Preallocate an output container to avoid slow loops

Use seq_along(x) instead of 1:length(x)

Use seq_along(x) instead of 1:length(x)

Avoid problems when iterating over S3 vectors by using seq_along(x) and x[[i]]

Use while or repeat when you don’t know the number of iterations

Always use the least-flexible loop option possible

Quiz & Exercises

What is the difference between if and ifelse()?

In the following code, what will the value of y be if x is TRUE? What if x is FALSE? What if x is NA?

What does switch("x", x = , y = 2, z = 3) return?

What type of vector does each of the following calls to ifelse() return?

What type of vector does each of the following calls to ifelse() return?

What type of vector does each of the following calls to ifelse() return?

Why does the following code work?

Why does this code succeed without errors or warnings?

Why does this code succeed without errors or warnings?

Walk-through

Walk-through

Walk-through

Walk-through

When the following code is evaluated, what can you say about the vector being iterated?

What does the following code tell you about when the index is updated?

`if` is the basic statement for a choice

Results of `if` can be assigned

`if` without `else` can be combined with `c()` or `paste()` to create compact expressions

`if` should have a single `TRUE` or `FALSE` condition, other inputs generate errors

Use `ifelse()` for vectorized conditions

Use `dplyr::case_when()` for multiple condition-vector pairs

`switch()` is a special purpose equivalent to `if` that can be used to compact code

`switch()` is a special purpose equivalent to `if` that can be used to compact code

Avoid repeat outputs by leaving the right side of `=` empty

To iterate over items in a vector, use a `for` loop

`for` will overwrite existing variables in the current environment

Use `next` or `break` to terminate loops early

Use `next` or `break` to terminate loops early

Use `seq_along(x)` instead of `1:length(x)`

Use `seq_along(x)` instead of `1:length(x)`

Avoid problems when iterating over S3 vectors by using `seq_along(x)` and `x[[i]]`

Use `while` or `repeat` when you don’t know the number of iterations

In the following code, what will the value of `y` be if `x` is `TRUE`? What if `x` is `FALSE`? What if `x` is `NA`?

What does `switch("x", x = , y = 2, z = 3)` return?