| Control {base} | R Documentation |
Control Flow
Description
These are the basic control-flow constructs of the R language. They function in much the same way as control statements in any Algol-like language. They are all reserved words.
Usage
if(cond) expr
if(cond) cons.expr else alt.expr
for(var in seq) expr
while(cond) expr
repeat expr
break
next
x %||% y
Arguments
cond |
A length-one logical vector that is not |
var |
A syntactical name for a variable. |
seq |
An expression evaluating to a vector (including a list and
an expression) or to a pairlist or |
expr, cons.expr, alt.expr, x, y |
An expression in a formal sense. This is either a
simple expression or a so-called compound expression, usually
of the form |
Details
break breaks out of a for, while or repeat
loop; control is transferred to the first statement outside the
inner-most loop. next halts the processing of the current
iteration and advances the looping index. Both break and
next apply only to the innermost of nested loops.
Note that it is a common mistake to forget to put braces ({ .. })
around your statements, e.g., after if(..) or for(....).
In particular, you should not have a newline between } and
else to avoid a syntax error in entering a if ... else
construct at the keyboard or via source.
For that reason, one (somewhat extreme) attitude of defensive programming
is to always use braces, e.g., for if clauses.
The seq in a for loop is evaluated at the start of
the loop; changing it subsequently does not affect the loop. If
seq has length zero the body of the loop is skipped. Otherwise the
variable var is assigned in turn the value of each element of
seq. You can assign to var within the body of the loop,
but this will not affect the next iteration. When the loop terminates,
var remains as a variable containing its latest value.
The null coalescing operator %||% is a simple 1-line function:
x %||% y is an idiomatic way to call
if (is.null(x)) y else x
# or equivalently, of course,
if(!is.null(x)) x else y
Inspired by Ruby, it was first proposed by Hadley Wickham.
Value
if returns the value of the expression evaluated, or
NULL invisibly if none was (which may happen if there is no
else).
for, while and repeat return NULL invisibly.
for sets var to the last used element of seq,
or to NULL if it was of length zero.
break and next do not return a value as they transfer
control within the loop.
References
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.
See Also
Syntax for the basic R syntax and operators,
Paren for parentheses and braces.
ifelse, switch for other ways to control flow.
Examples
for(i in 1:5) print(1:i)
for(n in c(2,5,10,20,50)) {
x <- stats::rnorm(n)
cat(n, ": ", sum(x^2), "\n", sep = "")
}
f <- factor(sample(letters[1:5], 10, replace = TRUE))
for(i in unique(f)) print(i)
res <- {}
res %||% "alternative result"
x <- head(x) %||% stop("parsed, but *not* evaluated..")
res <- if(sum(x) > 7.5) mean(x) # may be NULL
res %||% "sum(x) <= 7.5"