More R Key Actions

This page has the following sections:

Value and Side Effects
Three types of function
Operator precedence
Some simple data creation
Resources

Value and Side Effect

There are two possible results from a function:

• It can return a value
• It can produce one or more side effects

The mean function returns a value, and it has no side effects.

The plot function has the side effect of creating a plot on the current graphics device. If there is not an active graphics device in the current session, it also has the side effect of starting one.

The print function has the side effect of printing a representation of its first argument. It also returns a value (which is the same object that it prints).

Note that “value” is singular. A function will return only one object (though that object may have many components).

Three types of function

The three types of function are:

• standard functions
• operators
• assignment functions

Here is a small example:

> x <- runif(26) + 5
> names(x) <- letters

This example has one of each type of function.

runif is a typical function — it is called by following its name with arguments surrounded by parentheses.

+ is an operator. In this case it is a binary operator — it appears between its two arguments.

The use of names in this case is as an assignment function. It appears on the left of the assignment operator. It is precipitating a change to an existing object rather than creating a new object.

There are only a select few functions that can be used as assignment functions.

There is one particular function that novices tend to try to use as an assignment function that may not be. The attempt boils down to something like:

> paste("x", 1, sep="") <- 1 # WRONG

(In the context where this comes up, the advice is generally to create one list rather than a number of individual objects.)

Operator precedence

Operators in R have a precedence order — some are done before others.

For example in the command:

> 4 * 3 + 8 / 2
[1] 16

the multiplication (*) and division (/) are done before the addition.

If you want the addition done first, then use parentheses:

> 4 * (3 + 8) / 2
[1] 22

If you aren’t sure of the precedence, then use parentheses.

You can see the precedence table with the command:

> ?Syntax

Some simple data creation

Here we meet three types of simple data creation. These are done with:

• c
• : or seq
• rep

Combine

The c function combines values into a vector:

> x <- c(4, 84, -318, 22)

Sequences

The : operator creates simple sequences. For example:

> x <- 3:12

creates a vector containing each integer from 3 to 12, inclusive. The seq function is more flexible in the sequences it produces.

Repetition

The rep function creates repetitions. For instance:

> x <- rep(1:2, 10)

creates a vector of length 20 whose elements alternate between 1 and 2.

Resources

• An Introduction to R

This includes a discussion of functions, of looping and control structures, of subscripting, and many other topics.

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