Conditionals in

About Conditionals

Comparison operators

R has a set of operators that is the same as many other languages:

1 == 2 # FALSE
1 != 2 # TRUE
1 < 2 # TRUE
1 > 2 # FALSE
1 <= 2 # TRUE
1 >= 2 # FALSE

There is no infix operator for exclusive or (i.e. A OR B, but not both). The xor() function can be used in this case.

Branching

The basic form of an if-then-else is

if (x > 10) {
    y <- "big"
} else if (x > 2) {
    y <- "medium"
} else {
    y <- "tiny"
}

In this case, both parentheses () around the boolean and braces {} around the statement are required. A simple if statement can be shortened:

if (x == 0) return("success")

Using vectors in conditionals

Looking ahead to the vector-filtering concept: the conditional clause in parentheses () must evaluate to a single TRUE/FALSE. Including vectors in the comparison usually gives a vector of booleans, so these will need to be wrapped in an aggregating function such as all() or any().

> numbers <- c(4,5,7,9,10,11)
> numbers %% 3 == 0 # gives vector of booleans
[1] FALSE FALSE FALSE  TRUE FALSE FALSE

# use any() to get a single TRUE/FALSE
> if (any(numbers %% 3 == 0)) print("1 or more numbers are divisible by 3")
[1] "1 or more numbers are divisible by 3"

The ifelse function

An alternative if-else form may be useful, provided there is only a true/false outcome.

y = ifelse(x > 8, "big", "small")

# Don't do this:
z = ifelse(x > 100, 100, "small") # can lead to problems

ifelse() takes exactly three parameters: boolean clause, value if TRUE, value if FALSE.

Vector inputs are acceptable, and then a vector output is produced. In this case, ensure both branches result in the same type (mode) of data: numeric, character, boolean, etc. This will be covered in the vector-functions concept.