R Misc

Question 1

What is the package that allows you to do ridge regression and lasso regression

Answer 1

library(‘glmnet’)

Question 2

How to implement CV when you are doing linear or logistics regression?

Answer 2

cv.glmnet() is a function that will do CV for you.

Question 3

Does glmnet library have predict function? What is glmnet for?

Answer 3

yes, it does however the regsubsets (for best subset, forward and backward LR) function does not have one.
Also glmnet is for lasso and ridge regression

Question 4

How to conduct lasso regression

Answer 4

library(glmnet)
# Do CV to get best tuning paramter
# Use alpha = 1 for lasso 
cv.out = cv.glmnet(x[train, ], y[train], alpha = 1)
bestlam = cv.out$lambda.min

Question 5

Print out character array one letter at at a time by looping

Answer 5

x = c(‘a’, ‘b’, ‘c’)
for(letter in x) {
print(letter)
}

Question 6

if control structure R

Answer 6

if() {
## do something
} else if() {
## do something different
} else {
## do something different
}

Question 7

easiest to initiate an infinite loop (on purpose)

Answer 7

repeat{

if(condition) {
break
}
}

Question 8

how to skip an interation in a loop

Answer 8

Use next
for(i in 1:100) {
if(i <= 20) {
## Skip the first 20 iterations
next
}
## Do something here
}

Question 9

while loops

Answer 9

while(count < 10) {
print(count)
count <- count + 1
}

Question 10

How to make a function

Answer 10

f ) {
## Do something interesting
}

Question 11

get list of arguments for a function

Answer 11

args()

ex: args(lm)

Question 12

describe the lazy evaluations of functions

Answer 12

Arguments to functions are evaluated lazily, so they are evaluated only as needed.
f <- function(a, b) {
a^2
}
f(2)
This function never actually uses the argument b, so calling f(2) will not produce an
error because the 2 gets positionally matched to a.

Question 13

What does … mean in a function defintion

Answer 13

1. when extending another function, but don’t want to copy entire argument list
2. … so that extra arguments can be passed to methods
3. … is necessary when the number of arguments cannot be known in advance, such as paste() and cat.

Question 14

Display the list of environment variables and packages R will iterate through to find a variable, in the order they will be searched

Answer 14

search()

Question 15

When user loads a library, where does it go on the search list?

Answer 15

it goes into second place, everything else goes down 1. the global environment of the users workspace is always number 1

Question 16

what are free variables?

Answer 16

f <- function(x, y) {
x^2 + y / z
}
This function has 2 formal arguments x and y. In the body of the function there is
another symbol z. In this case z is called a free variable.

Question 17

how do free variables get values?

Answer 17

they are not local arguments, can be defined outside the function. R will search for the free variable starting in the global environment

Question 18

Does R let you define a function inside another function?

Answer 18

Yes, lots of languages don’t let you do this

Question 19

Example of defining a function inside another function

Answer 19

make.power <- function(x) {
x^n
}
pow
}

Question 20

lapply vs sapply

Answer 20

lapply will apply a function over a list, whereas sapply will do the same thing, except simplify the output if possible

Question 21

apply -

Answer 21

str(apply)
function (X, MARGIN, FUN, …)
X is an Array
MARGIN is what margin should be retained - 1 is row, 2 is columns
FUN is what function do you want to apply
… is for other arguments to be passed to FUN

Question 22

shortcut functions equiavalent to apply

Answer 22

rowSums = apply(x, 1, sum)
rowMeans = apply(x, 1, mean)
colSums = apply(x, 2, sum)
colMeans = apply(x, 2, mean)

Question 23

Example of apply using the … arguments

Answer 23

apply(x, 1, quantile, probs = c(0.25, 0.75))

Question 24

tapply

Answer 24

used to apply a function over subset of a vector, like a group by statement, can apply a function over a factor

Question 25

split

Answer 25

can use to split a dataframe into pieces

s <- split(airquality, airquality$Month)

Question 26

return only complete cases of data frame

Answer 26

DataFrame[complete.cases(DataFrame), ]

na.omit(DataFrame)

Question 27

Add column to data frame

Answer 27

Carseats = data.frame(Carseats, High)

Question 28

convert confusion matrix table into percentages

Answer 28

prop.table(table(trainClass))

Question 29

Steps to follow when using the caret package

Answer 29

1. partition data *createDataPartition()
2. test for low variance variables *nearZeroVar()
3. test for multicolinearity *findcorrelation()
4. preprocess - center/scale and derive pca *preprocess()
5. Build and tune model *train()

Question 30

count occurences of each factor in a column

Answer 30

table(dataframe$columnname)

Question 31

profile data

Answer 31

use describe() in psych package

Question 32

install packages using command

Answer 32

install.packages(‘psych’)

Question 33

return predicted values for linear regression

Answer 33

fitted(modelobject)

Question 34

get the 95% confidence interval for regression coeeficient

Answer 34

confint(modelobject)