Conditionals: if and else

A conditional runs code only when a condition is true.

Real-world analogy: "If it's raining, take an umbrella; otherwise, wear sunglasses."

Syntax

if (condition) {
  # runs when condition is TRUE
} else if (other_condition) {
  # runs when the first was FALSE and this one is TRUE
} else {
  # runs otherwise
}

Example

temperature <- 18

if (temperature > 25) {
  cat("It's hot\n")
} else if (temperature > 15) {
  cat("It's pleasant\n")
} else {
  cat("It's cold\n")
}

Expected output

It's pleasant

if returns a value

age <- 20
status <- if (age >= 18) "adult" else "minor"
status

[1] "adult"

if is not vectorized — use ifelse for vectors

if needs a single TRUE or FALSE. To choose a value for every element of a vector, use ifelse():

ages <- c(12, 25, 37, 8)
ifelse(ages >= 18, "adult", "minor")

[1] "minor" "adult" "adult" "minor"

Passing a whole vector to if is an error:

ages <- c(12, 25)
if (ages >= 18) cat("adult\n")

Error in if (ages >= 18) cat("adult\n") : the condition has length > 1

Common mistakes

• Using & or | in an if condition with vectors. Use && and ||, which require single values.
• An if condition that's NA — for example, comparing a missing value — gives missing value where TRUE/FALSE needed. Check is.na() first.

Practice

Given scores <- c(45, 72, 90), use ifelse to label each score "pass" (50 or more) or "fail".

scores <- c(45, 72, 90)
ifelse(scores >= 50, "pass", "fail")

[1] "fail" "pass" "pass"