5  Control flow

5.1 Quiz

5.1.1 Quiz 1

What is the difference between if and ifelse()?

Answers

The difference between these conditions are: - if() operates on a single logical value - ifelse() operates on a vector of logical values. Syntax is also different.

?Control
?ifelse
# if
x <- TRUE
if (x) {
  print("Hello")
}
[1] "Hello"
# ifelse
x <- c(TRUE, FALSE, TRUE, TRUE)
ifelse(
  x,
  print("Hello"),
  print("Goodbye")
)
[1] "Hello"
[1] "Goodbye"
[1] "Hello"   "Goodbye" "Hello"   "Hello"

5.1.2 Quiz 2

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

Answers

Outcome of all cases are: - x <- TRUE: y is assigned. - x <- FALSE: y is NULL - x <- NA: Error.

# Let's test
## x is TRUE
x <- TRUE
y <- if (x) 3
y
[1] 3
## x is FALSE
x <- FALSE
y <- if (x) 3
y
NULL
## x is NA
x <- NA
y <- if (x) 3
Error in `if (x) ...`:
! missing value where TRUE/FALSE needed
y
NULL

5.1.3 Quiz 3

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

Answers

switch evaluates the matched element or the first following non-missing argument. Since x is missing, the next argument y is evaluated, yielding 2.

switch("x",
  x = ,
  y = 2,
  z = 3
)
[1] 2

5.2 Choices

5.2.1 Ex. 1

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

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

Answers

# Evaluates to the TRUE arg, 1
ifelse(TRUE, 1, "no")
[1] 1
# Evaluates to the FALSE arg, no
ifelse(FALSE, 1, "no")
[1] "no"
# NA propagates, result is NA
ifelse(NA, 1, "no")
[1] NA

5.2.2 Ex. 2

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"

Answers

The code works because:

  • For the vector 1:10, its length is 10 and is equal to TRUE.
  • For the vector numeric(), its length is 10 and is equal to FALSE.
# Length of x is 10, equal to TRUE
x <- 1:10
length(x)
[1] 10
length(x) == TRUE
[1] FALSE
# Length of x is 0, equal to FALSE
x <- numeric()
length(x)
[1] 0
length(x) == FALSE
[1] TRUE

5.3 Loops

5.3.1 Ex. 1

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

Answers

This horrorshow works because:

  • vector() by default, creates an empty vector. This behaviour is intended.
  • 1:length(x) is a vector counting down from 1 to 0 (c(1, 0))
# Example case
x1 <- vector(
  mode = "list",
  length = numeric() %>%
    length()
)

# Default case
x2 <- vector(
  mode = "list"
)

identical(x1, x2)
[1] TRUE

During the first iteration, the following occurs:

  • The subsetting of x successfully returns NA as intended.
  • The out[1] list element is successfully set as intended.
x <- numeric()
out <- vector("list", length(x))

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

During the second iteration, the following occurs:

  • The subsetting of x returns a zero length vector not containing any data.
  • The subsetting of out returns a zero length vector not containing any data.
  • No data accessed or modified.
x <- numeric()
out <- vector("list", length(x))

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

In summary, all operations behaved as intended by the code and contained no errors.

5.3.2 Ex. 2

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

Answers

In every iteration, the calculated value is concatenated with the previous vector and attached to the end. The for looping section is evaluated prior to beginning the loop, which ensures that the loop ends in 3 iterations, even though the length of xs is increased by the loop.

xs <- c(1, 2, 3)
for (x in xs) {
  xs <- c(xs, x * 2)
  print(xs)
}
[1] 1 2 3 2
[1] 1 2 3 2 4
[1] 1 2 3 2 4 6

5.3.3 Ex. 3

for (i in 1:3) {
  i <- i * 2
  print(i)
}
[1] 2
[1] 4
[1] 6

Answers

In essence, there are two different i variables here:

  • The first i is outside of the loop and is responsible for counting the number of iterations (1, 2, 3)
  • The second i is inside the loop.

At every iteration, the outer i is passed in the loop, doubled, printed, then replaced with the outer i when the next iteration begins. The outer i is never modified.