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Appendix C. Answers to Quizzes

Question 1-1

R is an open source reworking of the S programming language.

Question 1-2

Choices include imperative, object-oriented, and functional.

Question 1-3

8:27

Question 1-4

help.search (which does the same as ??)

Question 1-5

RSiteSearch

Question 2-1

%/%

Question 2-2

all.equal(x, pi) or, even better, isTRUE(all.equal(x, pi))

Question 2-3

At least two of the following:

<-
+ =
+ <<-
assign

Question 2-4

Just Inf and -Inf

Question 2-5

0, Inf, and -Inf

Question 3-1

numeric, integer, and complex

Question 3-2

nlevels

Question 3-3

as.numeric("6.283185")

Question 3-4

Any three of summary, head, str, unclass, attributes, or View. Bonus points if you’ve discovered tail, the counterpart to head that prints the last few rows.

Question 3-5

rm(list = ls())

Question 4-1

seq.int(0, 1, 0.25)

Question 4-2

Either by using name = value pairs when the vector is created, or by calling the names function afterward.

Question 4-3

Positive integers for locations to retrieve, negative integers for locations to avoid, logical values, or names of elements.

Question 4-4

3 * 4 * 5 = 60

Question 4-5

%*%

Question 5-1

3. The inner list counts as one element, and so does the NULL element.

Question 5-2

When passing arguments to functions, when calling formals, or in the global environment variable .Options.

Question 5-3

You can use matrix-style indexing with pairs of positive integers/negative integers/logical values/characters in single square brackets. You can also use list-style indexing with one index value inside single or double square brackets, or the dollar sign ($) operator. Thirdly, you can call the subset function.

Question 5-4

By passing check.names = FALSE to data.frame.

Question 5-5

rbind for appending vertically or cbind for appending horizontally.

Question 6-1

The user workspace.

Question 6-2

list2env is the best solution, but as.environment also works.

Question 6-3

Just type its name.

Question 6-4

formals, args, and formalArgs.

Question 6-5

do.call calls a function with its arguments in a list form.

Question 7-1

format, formatC, sprintf, and prettyNum are the main ones.

Question 7-2

Using alarm, or printing an a character to the console.

Question 7-3

factor and ordered

Question 7-4

The value is counted as missing (NA).

Question 7-5

Use cut to bin it.

Question 8-1

if will throw an error if you pass NA to it.

Question 8-2

ifelse will return NA values in the corresponding positions where NA is passed to it.

Question 8-3

switch will conditionally execute code based upon a character or integer argument.

Question 8-4

Insert the keyword break into your loop code.

Question 8-5

Insert the keyword next into your loop code.

Question 9-1

lapply, vapply, sapply, apply, mapply, and tapply were all discussed in the chapter, with eapply and rapply getting brief mentions too. Try apropos("apply") to see all of them.

Question 9-2

All three functions accept a list and apply a function to each element in turn. The difference is in the return value. lapply always returns a list, vapply always returns a vector or array as specified by a template, and sapply can return either.

Question 9-3

rapply is recursive, and ideal for deeply nested objects like trees.

Question 9-4

This is a classic split-apply-combine problem. Use tapply (or something from the plyr package).

Question 9-5

In a name like **ply, the first asterisk denotes the type of the first input argument and the second asterisk denotes the type of the return value.

Question 10-1

CRAN is by far the biggest package repository. Bioconductor, R-Forge, and RForge.net are others. There are also many packages on GitHub, Bitbucket, and Google Code.

Question 10-2

Both functions load a package, but library throws an error if it fails, whereas require returns a logical value (letting you do custom error handling).

Question 10-3

A package library is just a folder on your machine that contains R packages.

Question 10-4

.libPaths returns a list of libraries.

Question 10-5

R doesn’t do a great impression of Internet Explorer, but you can make it use Internet Explorer’s internet2.dll library for connecting to the Internet.

Question 11-1

POSIXct classes must be used. Dates don’t store the time information, and POSIXlt dates store their data as lists, which won’t fit inside a data frame.

Question 11-2

Midnight at the start of January 1, 1970.

Question 11-3

"%B %Y"

Question 11-4

Add 3,600 seconds to it. For example:

x <- Sys.time()
x + 3600

## [1] "2013-07-17 22:44:55 BST"

Question 11-5: The period will be longer, because 2016 is a leap year. A duration is always exactly 60 * 60 * 24 * 365 seconds. A period of one year will be 366 days in a leap year.
Question 12-1: Call the data function with no arguments.
Question 12-2: read.csv assumes that a decimal place is represented by a full stop (period) and that each item is separated by a comma, whereas read.csv2 assumes that a decimal place is represented by a comma and that each item is separated by a semicolon. read.csv is used for data created in locales where a period is used as a decimal place (most English-speaking locales, for example). read.csv2 is for data created in locales where a comma is used (most European locales, for example). If you are unsure, simply open your data file in a text editor.
Question 12-3: read.xlsx2 from the xlsx package is a good first choice, but there is also read.xlsx in the same package, and different functions in several other packages.
Question 12-4: You can simply pass the URL to read.csv, or use download.file to get a local copy.
Question 12-5: Currently SQLite, MySQL, PostgreSQL, and Oracle databases are supported.
Question 13-1: Read in the text as a character vector wih readLines, call str_count to count the number of instances in each line, and sum the total.
Question 13-2: with, within, transform, and mutate all allow manipulating columns and adding columns to data frames, as well as standard assignment.
Question 13-3: Casting. Not freezing!
Question 13-4: Use order or arrange.
Question 13-5: Define a function that reads TRUE when you have a positive number—for example, is.positive <- function(x) x > 0—and call Find(is.positive, x).
Question 14-1: min returns the single smallest value of all its inputs. pmin accepts several vectors that are the same length, and returns the smallest at each point along them.
Question 14-2: Pass the pch (“plot character”) argument.
Question 14-3: Use a formula of the form y ~ x.
Question 14-4: An aesthetic specifies a variable that you will look at a variation in. Most plots take an x and a y aesthetic for x and y coordinates, respectively. You can also specify color aesthetics or shape aesthetics (where more than two variables are to be looked at at once, for example).
Question 14-5: Histograms, box plots, and kernel density plots were all mentioned in the chapter. There are some other weirdly esoteric plots, like violin plots, rug plots, bean plots, and stem-and-leaf plots, that weren’t mentioned. Have 100 geek points for each of these that you guessed.
Question 15-1: Set the seed (with set.seed), generate the numbers, then reset the seed to the same value.
Question 15-2: PDF functions have a name beginning with d, followed by the name of the distribution. For example, the PDF for the binomial distribution is dbinom. CDF functions start with p followed by the name of the distribution, and inverse CDF functions begin with q followed by the name of the distribution.
Question 15-3: Colons represent an interaction between variables.
Question 15-4: anova, AIC, and BIC are common functions for comparing models.
Question 15-5: The R^2 value is available via summary(model)$r.squared.
Question 16-1: The warnings function shows the previous warnings.
Question 16-2: Upon failure, try returns an object of class try-error.
Question 16-3: The testthat equivalent of checkException is expect_exception. Say that 10 times fast.
Question 16-4: quote turns a string into a call, then eval evaluates it.
Question 16-5: Overload functions using the S3 system. A function print.foo will be called for objects of class foo.
Question 17-1: DESCRIPTION and NAMESPACE are compulsory.
Question 17-2: man and R are compulsory in all packages. src is required if you include C, C++, or Fortran code.
Question 17-3: CITATION files let you explain who made and maintains the package, if that information is too long and complicated to go in the DESCRIPTION file.
Question 17-4: roxygenise or roxygenize
Question 17-5: First warn the user that it is deprecated by adding a call to .Deprecated. Later, replace the body completely with a call to .Defunct.

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Table of Contents for C. Answers to Quizzes

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Appendix C. Answers to Quizzes

Table of Contents for
C. Answers to Quizzes