Nesting

This concept is common in mathematical expressions: we can insert a pair of parentheses into another pair to indicate the order of operations.

b <- (1 + (12 + 4) * 3) * 4

In R blocks can nested as well, which are indicated by pairs of braces in other braces:

# this won't run, this is a just a structure
{
	<commandA>
	<commandB>
	<commandC>
	# here is a block of code, inside the block
	{
		<commandD>
		<commandE>
		<commandF>
	}
}

The idea is that R will think that the block of code is just one command.

Note the use of indentation! When we have lots of nested structures, we frequently use indentation to make our code readable.

Nesting if() statements

If you want’t one ore multiple statements to run conditionally that if fine. All you have to do is to put the if() before the command or block. Note that the only change to the previous blueprint is that the if() condition is now added the inserted block.

# this won't run, this is a just a structure
{
	<commandA>
	<commandB>
	<commandC>
	# here is a block of code, inside the block
	if( <condition> ){
		<commandD>
		<commandE>
		<commandF>
	}
}

In this example, commmands <commandA> <commandB> and <commandC> will run in all cases, but <commandD> <commandE> and <commandF> will run only, if condition is true.

This is the way how you can embed one if statement in another:

# this won't run, this is a just a structure
if( <condition1> ) {
	<commandA>
	<commandB>
	<commandC>
	# here is a block of code, inside the block
	if( <condition2> ){
		<commandD>
		<commandE>
		<commandF>
	}
}

Now the execution of the entire block is conditioned on <condition1>.

Here is an actual example in R:

input <- 12

# outer condition
if(is.numeric(input)){

	# command 1
	message("We have entered the first block!")
	message("The input is numeric!")

	# command 2 
	# Inner condition the whole block is one command!
	if( input > 0 ){
		message("The value is larger than 0!")
	}	
}

You can nest such blocks as deep as you want, but note that after 4-5 levels the code becomes practically unreadable and tend to be very-very long.

Related exercises:

• Nested if statements