R: Control Flow (original) (raw)
| Control {base} | R Documentation |
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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 NA. Other types are coerced to logical if possible, ignoring any class. (Conditions of length greater than one are an error.) |
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| 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 NULL. A factor value will be coerced to a character vector. This can be a long vector. |
| 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 { expr1 ; expr2 }. |
Details
break breaks out of a for, while or repeatloop; 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 andnext 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 } andelse to avoid a syntax error in entering a if ... elseconstruct 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. Ifseq has length zero the body of the loop is skipped. Otherwise the variable var is assigned in turn the value of each element ofseq. 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, orNULL invisibly if none was (which may happen if there is noelse).
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](../../base/help/Syntax.html) for the basic R syntax and operators,[Paren](../../base/help/Paren.html) for parentheses and braces.
[ifelse](../../base/help/ifelse.html), [switch](../../base/help/switch.html) 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"
[Package _base_ version 4.6.0 Index]