Control

R Base 3.6.2

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.

Syntax

if(cond) expr
if(cond) cons.expr  else  alt.expr<p></p><p>for(var in seq) expr
while(cond) expr
repeat expr
break
next</p>

Arguments

Parameter	Description
`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.
`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 }.

Return 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.

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

Examples

Example

# NOT RUN {
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)
# }