functions, factors, data frames, lists

Question 1

write a function

Answer 1

functionname <- function(arg1, arg2){

expressions

value_to_return

}

Question 2

How can you see the source code for function x?

Answer 2

Type x in console

Question 3

Write a function that takes another function as an argument

Answer 3

evaluate <- function(func, dat){

func(dat)

}

Question 4

what does lapply() do?

Answer 4

Note that write function without ()

It takes a list as input, applies a function to each element of the list, then returns a list of the same length as the original one

lapply(list, function)

Question 5

What does sapply do?

Answer 5

Basically the same thing as lapply, but instead of returning a list, it returns a vector (when each element of the list is of length 1) or matrix (when each element of the list is a vector of the same length >1)

s stands for simplify

Question 6

What does vapply do?

Answer 6

Tells R to return a character vector of length 1 for each column of the dataset

Takes a list and applies function to it. You can specify the form of the output,

eg. vapply(dataset, function, character(1))

Question 7

What does tapply do?

Answer 7

applies the mean function to the ‘animate’ variable separately for each of the ‘landmass’ groups

Allows you to split data into groups, then apply a function to the members of each group.

eg. tapply(flags$animate, flags$landmass, mean)

Question 8

Encode the following vector as factors:

sex_vector <- c(“Male”,”Female”,”Female”,”Male”,”Male”)

Answer 8

factor(sex_vector)

Question 9

Change the names of the levels of factor_vector

Answer 9

levels(factor_vector) <- c(“name1”, “name2”,…)

Question 10

Create an ordered factor for my_vector with levels “slow”, “medium”, “fast”

Answer 10

ordered_vector <- factor(my_vector, ordered=TRUE, levels=c(“slow”, “medium”, “fast”))

Question 11

Print out first 5 obs of a dataframe

Print out last 5 obs of a dataframe

Answer 11

head(dataframe)

tail(dataframe)

Question 12

What does str() tell you about a dataframe?

Answer 12

1. The total number of observations (e.g. 32 car types)
2. The total number of variables (e.g. 11 car features)
3. A full list of the variables names (e.g. mpg, cyl … )
4. The data type of each variable (e.g. num)
5. The first observations

Question 13

Create a dataframe from the following vectors and call it planets_df:

name <- c(“Mercury”, “Venus”, “Earth”, “Mars”, “Jupiter”, “Saturn”, “Uranus”, “Neptune”)
type <- c(“Terrestrial planet”, “Terrestrial planet”, “Terrestrial planet”,
“Terrestrial planet”, “Gas giant”, “Gas giant”, “Gas giant”, “Gas giant”)
diameter <- c(0.382, 0.949, 1, 0.532, 11.209, 9.449, 4.007, 3.883)
rotation <- c(58.64, -243.02, 1, 1.03, 0.41, 0.43, -0.72, 0.67)
rings <- c(FALSE, FALSE, FALSE, FALSE, TRUE, TRUE, TRUE, TRUE)

Answer 13

planets_df <- data.frame(name, type, diameter, rotation, rings)

Question 14

Two ways to select column1 from a dataframe

Answer 14

dataframe[,”column1”]

dataframe$column1

Question 15

How to subset a dataframe

Answer 15

subset(my_df, subset = some_condition)

Question 16

How to sort dataframe by column1

Answer 16

my_df[order(my_df$column1),]

Question 17

Create a list

Can a list hold different types of data in it?

Answer 17

my_list <- list(component1, component2…)

YES, a list can hold vectors, matrices, dataframes, and other lists

Question 18

Three ways to select the first component (comp1) from a list

Answer 18

my_list[[1]]

my_list[[“comp1”]]

my_list$comp1

Question 19

Add one component (compx) to my_list

Answer 19

ext_list <- c(my_list, compx)