Overview

5.2 Choices

• if ... else
• if ... else if ... else
• ifelse
• switch

5.3 Loops

• for
• while
• repeat

5.2 Choices

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

5.2 Choices (2)

if (condition1) {
  true_action1 
} else if(condition2) {
  true_action2 
} 
# ...
else if(conditionN) {
  true_actionN 
} else { 
  false_action
}

if (condition1) {
  true_action1 
} 
if(condition2) {
  true_action2
}
# ...
if(conditionN) {
  true_actionN 
}

5.2 Choices (3)

Example: Grades

grade <- function(x) {
  if (x >= 90) {
    "A"
  } else if (x >= 80) {
    "B"
  } else if (x >= 70) {
    "C"
  } else {
    "F"
  }
}
grade(99)

## [1] "A"

grade(50)

## [1] "F"

5.2 Choices (4)

5.2.1 Invalid inputs

"The condition should evaluate to a single TRUE or FALSE".

if ("x") 1
if (logical()) 1
if (NA) 1

Except, logical vectors of length > 1:

if (c(TRUE, TRUE, FALSE)) 1

5.2 Choices (5)

5.2.2 Vectorised if (aka ifelse)

x <- 1:5
ifelse(x %% 2 == 0, "even", "odd")

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

Bonus: dplyr::case_when

dplyr::case_when(
  x %% 2 == 0 ~ "even",
  x %% 2 != 0 ~ "odd"
)

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

5.2 Choices (6)

5.2.3 Fancy "if-chain" (aka switch)

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

x_option <- function(x) {
  switch(x, 
         a = "option 1",
         b = "option 2",
         c = "option 3",
         stop("Invalid value"))
}
legs <- function(x) {
  switch(x, 
         cat = ,
         dog = 4,
         human = 2,
         stop("Invalid value"))
}

5.2 Choices (7)

5.2.4 Exercises

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

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

• 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"

5.3 Loops

for (item in vector) perform_action
while(condition) action
repeat(action)

5.3 Loops (2)

for loops

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

Terminate loop:

• next exists current iteration

for (i in 1:10) {
  if (i %% 2 == 0)
    next
  print(i)
}

## [1] 1
## [1] 3
## [1] 5
## [1] 7
## [1] 9

5.3 Loops (3)

Terminate loop (cont.):

• break exits the entire loop

for (i in 1:10) {
  if (i %% 5 == 0)
    break
  print(i)
}

## [1] 1
## [1] 2
## [1] 3
## [1] 4

5.3 Loops (4)

5.3.1 Common pitfalls

• Preallocate the output container (performance)

means <- c(1, 50, 20)
out <- vector("list", length(means))

• Instead of iterating over 1:length(x), use seq_along(x)

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

## [1] 1 0

seq_along(means)

## integer(0)

5.3 Loops (5)

5.3.2 Related tools

Flexibility: for < while < repeat

i <- 0
while (i < 10) {
  i <- i + 1
  if (i %% 2 == 0)
    next
  print(i)
}

## [1] 1
## [1] 3
## [1] 5
## [1] 7
## [1] 9

i <- 0
repeat({
  i <- i + 1
  if (i > 10) break
  if (i %% 2 == 0) next
  print(i)
})

## [1] 1
## [1] 3
## [1] 5
## [1] 7
## [1] 9

5.3 Loops (6)

5.3.3 Exercises

• 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

• 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

5.3 Loops (7)

5.3.3 Exercises

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

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

Conclusion

Can we put these concepts together in diagram form? Let’s work on improving these schematics!

Quiz

• 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?

y <- if (x) 3

x = TRUE

if (TRUE) 3

## [1] 3

x = FALSE

if (FALSE) 3

x = NA

if (NA) 3

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

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

## [1] 2