Control 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

Arguments

cond	A length-one logical vector that is not NA. Conditions of length greater than one are currently accepted with a warning, but only the first element is used. An error is signalled instead when the environment variable _R_CHECK_LENGTH_1_CONDITION_ is set to true. Other types are coerced to logical if possible, ignoring any class.
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. As from R 4.0.0 this can be a long vector.
expr, cons.expr, alt.expr	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 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.

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)