Conditionals represent forks in the flow of a program. When a condition is fulfilled, one thing happens, but only then. Without conditional statements, programs would be much more boring.

The use of if

if is a keyword, that indicates such potential forks, such as this:

feedback <- TRUE
if(feedback) message("Feedback is printed to the console")

The example above is the simplest possible case for using a conditional. A message is printed to the console, when the value of feedback is TRUE, but only then.

if() expects a single logical value (either TRUE or FALSE), which we usually calculate using function that return a logical value (e.g. predicates) or with a logical operator.

# using the double equal logical operator

# TRUE case
a <- 12
if(a==12) message("If a is 12, feedback is printed to the console")

If a is 12, feedback is printed to the console

# FALSE case
a <- 8
if(a==12) message("If a is 12, feedback is printed to the console")

# using  function
# TRUE case
a <- NULL
if(is.null(a)) message("If a is NULL, feedback is printed to the console")

If a is NULL, feedback is printed to the console

# FALSE case
a <- TRUE
if(is.null(a)) message("If a is NULL, feedback is printed to the console")

Syntactic blueprint

These commands have the same general blueprint, which can be summarized this way (not actual R code, just placeholders!):

# Don't run
if( <condition> ) <command(s)>

The condition goes between the parentheses (()) which are followed by regular R statements.

If there is just one command that we want to execute (like the example above), we sometimes put statements on one line, as in the examples above.

If there are multiple commands that we want to condition, then they need to be put into braces ({}).

# Don't run
if( <condition> ){
	<command(s)>
	<command(s)>
	<command(s)>
}

The use of braces indicate to R that we want to execute commands as a single entity - they belong together and that the if condition appies to all instructions. The sets of commands that are tied together in such a way is called a block of code.

Special values in if() statements

Again, if() expects a single logical value. This means that using missing values (NA) or NULLs will trigger errors.

# Will trigger error 
if(NA) message("Impossible!")

# Will trigger error 
if(NULL) message("Impossible!")

Side note: numbers including Inf will work, because they can be coerced to logical TRUE values without problems. We will look at such conversions later.

Related exercises: