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
)true_expression
elsefalse_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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