# Control Structures

Nearly every operation in R can be written as a function, but it isn’t always convenient to do so. Therefore, R provides special syntax that you can use in common program structures. We’ve already described two important sets of constructions: operators and grouping brackets. This section describes a few other key language structures and explains what they do.

## Conditional Statements

Conditional statements take the form:

if ()`condition`

else`true_expression`

`false_expression`

or, alternatively:

if ()`condition`

`expression`

Because the expressions

,
`expression`

,
and `true_expression`

are not always evaluated, the function `false_expression`

`if`

has the type `special`

:

> typeof(`if`) [1] "special"

Here are a few examples of conditional statements:

> if (FALSE) "this will not be printed" > if (FALSE) "this will not be printed" else "this will be printed" [1] "this will be printed" > if (is(x, "numeric")) x/2 else print("x is not numeric") [1] 5

In R, conditional statements are not vector operations. If the

statement is a vector of more than one `condition`

`logical`

value, only the first item will be
used. For example:

> x <- 10 > y <- c(8, 10, 12, 3, 17) > if (x < y) x else y [1] 8 10 12 3 17 Warning message: In if (x < y) x else y : the condition has length > 1 and only the first element will be used

If you would like a vector
operation, use the `ifelse`

function
instead:

> a <- c("a","a","a","a","a") > b <- c("b","b","b","b","b") > ifelse(c(TRUE,FALSE,TRUE,FALSE,TRUE),a,b) [1] "a" "b" "a" "b" "a" ...

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