R

Front 1

What data type is a single number?
E.g. x <- 1

Back 1

A length-one vector - not a scalar

Front 2

Three properties of a vector

Back 2

type, length, attributes

Front 3

Four types of atomic vectors (plus two rare?)

Back 3

logical, integer, double, character (complex, raw)

Front 4

Talk to me about NA values in vectors and coercion

Back 4

Any NA in a vector will be coerced to the correct type, respecting the construction/storage rules for vectors

Front 5

What is the hierarchy of coercion (also, vectors & lists?)

Back 5

logical < integer < double < character

(vector + list) -> list gets coerced to list

Front 6

What are the three most important attributes (of a vector)?

Back 6

Name(s), class, length(s)

Front 7

What type of vector is a factor?

Back 7

Integer!

Front 8

What happens to a factor when you modify its levels?

Back 8

The levels are actually names, and apply arbitrarily to the values in the vector. Switching one doesn't switch the other, but switching both is identical to the original

Front 9

What's the difference between a matrix and an array?

Back 9

The number of dimensions (matrix is a special case of an array that has two)

Front 10

How are data frames like lists?

Back 10

They're actually a list of vectors, and can hold different data types, as long as the lists (i.e. columns) are all the same length ("exception" - as long as the row dimensionality is correct, you can store an array in a column)

Front 11

What are the three subsetting operators?

Back 11

$, [, @

Front 12

What is the result of subsetting a vector with positive integers, negative integers, a logical vector, or a character vector?

Back 12

Get some indices, remove some indices, get some indices, and get some named indices

Front 13

What's the difference between [, [[, and $ when applied to a list

Back 13

[ gets the list at an index, [[ the contents at an index, and $ is like [[

Front 14

How can you use a named vector to relabel categorical variables?

Back 14

A named character vector can act as a simple lookup table: c(x = 1, y = 2, z = 3)[c("y", "z", "x")]

Front 15

How do you get a simple vector from a data frame by subsetting? How do you get a small version of the data frame instead?

Back 15

matrix subsetting [,"x"], list subsetting ["a"]

Front 16

What's the difference between subsetting and preserving, and what are the consequences for each data type?

Back 16

preserving keeps the same type of variable. When simplified instead, atomic vectors lose their names, data frames lose all irrelevant columns and become vectors, lists return the object instead of a list (unless the object is a list), factors drop unused levels, and matrices/arrays drop unused dimensions

Front 17

how would you uncollapse a dataframe that has been aggregated by row

Back 17

Use the rep function to generate indices for the number of times a row has been repeated, and then subset the dataframe with those indices

Front 18

How would you order a data frame by row? by column?

Back 18

df[order(df$x),]

df[,order(names(df))]

Front 19

How would you match a list of (e.g.) integer values to a table describing the properties of those integers (e.g. grades to pass/fail and letter grade)?

Back 19

match from the grade vector to the data frame's grade vector - this gives positions with which to subset the dataframe

Front 20

What are the three parts of a function?

Back 20

Body, arguments, environment

Front 21

what are the four principles of lexical scoping

Back 21

fresh start, name masking, functions vs. variables, dynamic lookup

Front 22

How do you send a list of arguments to a function?

Back 22

do.call(mean,list(1:10, na.rm = TRUE))

Front 23

how do you define an infix function?

Back 23

'%...%' <-

Front 24

what is a replacement function and how do you define one?

Back 24

a replacement function modifies its argument (e.g. a vector) in place (e.g. changing the second value of a vector)

defined with 'name<-' <- function(x, ..., value)

All of that's necessary, and gives you access to the object itself as well as the value you want to replace one of its elements with

n.b. this is a modified copy, not an actual modification (this behavior is standard)

Front 25

how do you make sure that a certain portion of code executes before/as a function ends?

Back 25

on.exit() before the "risky" part of the function

Front 26

How is an environment different from a list?

Back 26

There are four ways: every object in an environment must have a name; order doesn’t matter; environments have parents; environments have reference semantics.

Also, rm() is the only way to get rid of things in an environment (not <-NULL)

Front 27

Talk to me about the global environment and parents and parentless environments

Back 27

The parent of the global environment is the last package that you loaded. The only environment that doesn’t have a parent is the empty environment.

Front 28

Which is the enclosing environment of a function and why does it matter?

Back 28

Wherever it was created, and this determines where it looks for values of variables

Front 29

How do you determine the environment from which a function was called?

Back 29

use parent.frame()

Front 30

How do you bind to the parent environment? How do you copy to the current environment

Back 30

This is the difference between <- and <<-, the former of which copies in the current environment

Front 31

What's the easiest way to view an environment's namespace and structure?

Back 31

ls() and ls.str() respectively

Front 32

How does subsetting with respect to an environment work?

Back 32

Exactly like you'd expect, as long as you have the environment and object's names (e$a[[1]])

Front 33

How do you look for particular bindings in an environment?

Back 33

get() or exists(), both of which can take inherits = false

Front 34

How do you figure out which environment a binding is tied to?

Back 34

where("name","parent to start with"

Front 35

What are the three relevant environments for a function?

Back 35

Binding (enclosing), executing, and calling.

Binding is where it is named (<-; special case: enclosing environment can be the same), executing is ephemeral and where it does its running, and calling is the place from which the execution was initiated

Front 36

What's the most useful/central function for debugging? What are some others?

Back 36

traceback()

breakpoint (shift+f9) or browser() (or options(error = browser) in a function

Front 37

What are the useful functions inside browser()?

Back 37

n, execute the next command; s, step into the next function; f, finish the current loop or function; c, continue execution normally; Q, stop the function and return to the console.

Front 38

What function can you use to avoid errors with some block of code?

Back 38

try()

Front 39

Talk to me about try()

Back 39

Useful for handling error-prone code - can produce a message on error, can have its output captured (error or not), can be particularly useful for lapply or assigning default values (default<-value; try(default<-riskybusiness)

Front 40

Talk to me about tryCatch()

Back 40

This allows you to assign particular behavior to each of the handlers (error, warning, etc.)

Front 41

How do you make a list of functions, and then call from it?

Back 41

This isn't do.call(), actually.

funlist <- list (name1 <- function(x) x/2, ...)

system.time(funlist[[1]](x))

Or, use lapply(vectorx, function(x) f(x))

Front 42

How would you create a function to create power-generating functions (x^y)?

Back 42

powers <- function(exp) {

function (x) {

x^^exp

}

}