Data Types

Question 1

getwd()

Answer 1

Returns the current working directory

Question 2

setwd()

Answer 2

sets the current working directory

Question 3

source()

Answer 3

loads the contents of a file

Question 4

ls()

Answer 4

lists objects in a given workspace

Question 5

dir()

Answer 5

lists files in the current working directory

Question 6

What happens when you try to include elements of multiple classes in a vector?

Answer 6

Certain elements are “coerced”, such that the class of that element is changed to a different type

This can have unexpected results

Question 7

What object can contain elements of different classes?

Answer 7

A list

Question 8

What are two functions used to create vectors, and what are their differences?

Answer 8

vector() and c()

With vector(), you have to pass a “mode” (atomic class type or “list”), and the vector length

By default c() combines its arguments to form a vector

Question 9

How do you force a number to be represented as an integer?

Answer 9

Append an “L” to it, e.g. 1L

Question 10

How is infinity represented in R

Answer 10

Inf

Question 11

How is “not a number” represented in R

Answer 11

NaN

Question 12

How are missing values represented in R?

Answer 12

NA

Question 13

What is a set of instructions typed at the R prompt called?

Answer 13

An expression

Question 14

What is the assignment operator?

Answer 14

<-

Question 15

How is a string represented?

Answer 15

As a character vector

Question 16

When you type the name of an object at the prompt and press Enter, what happens?

Answer 16

The value gets auto-printed

Question 17

How do you create a vector of integers from 1 to 10?

Answer 17

x <- 1:10

Question 18

How do you explicity coerce all elements of a vector to a given logical objects?

Answer 18

as.logical(x)

Question 19

How do you return the class of vector x?

Answer 19

class(x)

Question 20

What happens when you try to coerce a vector but certain elements cannot be coerced?

Answer 20

You get a set of NA values back

Question 21

What’s the core difference between a vector and a matrix?

Answer 21

A matrix is a vector with dimensions, specifically the “dim” attribute, a numeric vector

Question 22

How to you set the dimensions of a matrix m?

Answer 22

dim(m) <- c(1, 2)

Question 23

How do you return all attributes of a vector x?

Answer 23

attributes(x)

Question 24

Are matrices constructed column-wise or row-wise?

Answer 24

column-wise

Question 25

What’s a simple way to create a matrix from an existing vector?

Answer 25

x <- c(2, 2)

Question 26

What are factors?

Answer 26

factors are a special type of vector used to represent categorical data

factors can be ordered or unordered

treated specially by modelling functions

really an integer vector with a “levels” attribute

Question 27

How do you create factors?

Answer 27

x <- factor(c(“yes”, “no”))

Question 28

How do you order factors?

Answer 28

passing the “levels” argument, which is a character vector that specifies the ordering of the factors

Question 29

How do you find the missing elements of a vector x?

Answer 29

is.na(x)

Question 30

What are data frames?

Answer 30

a special type of list, where every element of the list has to have the same length

each element of the list here can be thought of as a column

length of each element is the number of rows

have special attributes called row.names

Question 31

What two functions are traditionally used to load data from disk and into a data frame?

Answer 31

read.table() and read.csv()

Question 32

How do you apply names to a standard, atomic object?

Answer 32

x <- 1:3
names(x) <- c(“foo”, “bar”, “baz”)

Question 33

How do you use cbind/rbind, and how do they work?

Answer 33

cbind(x, y) ; cbind binds vectors by column

rbind(x, y) ; rbind binds vectors by rows

Think of gluing two vectors together vertically (cbind) or horizontally (rbind)

Question 34

What’s the difference between an object’s mode and its class?

Answer 34

‘mode’ is a mutually exclusive classification of objects according to their basic structure. The ‘atomic’ modes are numeric, complex, charcter and logical. Recursive objects have modes such as ‘list’ or ‘function’ or a few others. An object has one and only one mode.

‘class’ is a property assigned to an object that determines how generic functions operate with it. It is not a mutually exclusive classification. If an object has no specific class assigned to it, such as a simple numeric vector, it’s class is usually the same as its mode, by convention.

Question 35

What’s one benefit to using factors in R for the purpose of graphing

Answer 35

Factors are used to group data

ggplot silently coerce character vectors to factors, as does read.table with the stringsAsFactors = TRUE argument

Question 36

What are categorical variables?

Answer 36

Variables which take on a limited number of different values