# ▸ R (Programming Language) | LinkedIn Skill Assessment Quiz Solutions

1. How does a matrix differ from a data frame?

• A matrix may contain numeric values only.
• A matrix must not be singular.
• A data frame may contain variables that have different modes.
• A data frame may contain variables of different lengths.

1. What value does this statement return?
`unclass(as.Date("1971-01-01"))`

• 1
• 365
• 4
• 12

1. What do you use to take an object such as a data frame out of the workspace?

• remove()
• erase()
• detach()
• delete()

1. Review the following code. What is the result of line 3?

``````xvect<-c(1,2,3)
xvect <- "2"
xvect
``````
•  1 2 3
•  “1” 2 “3”
•  “1” “2” "3"
•  7 9

1. The variable height is a numeric vector in the code below. Which statement returns the value 35?

• `height(length(height))`
• `height[length(height)]`
• `height[length[height]]`
• `height(5)`

1. In the image below, the data frame is named rates. The statement `sd(rates[, 2])` returns 39. As what does R regard Ellen’s product ratings? • sample with replacement
• population
• trimmed sample
• sample <-- not sure

1. Which choice does R regard as an acceptable name for a variable?

• `Var_A!`
• `\_VarA`
• `.2Var_A`
• `Var2_A`

1. What is the principal difference between an array and a matrix?

• A matrix has two dimensions, while an array can have three or more dimensions.
• An array is a subtype of the data frame, while a matrix is a separate type entirely.
• A matrix can have columns of different lengths, but an array’s columns must all be the same length.
• A matrix may contain numeric values only, while an array can mix different types of values.

1. Which is not a property of lists and vectors?

• type
• length
• attributes
• scalar

1. In the image below, the data frame on lines 1 through 4 is names StDf. State and Capital are both factors. Which statement returns the results shown on lines 6 and 7? • StDf[1:2,-3]
• StDf[1:2,1]
• StDf[1:2,]
• StDf[1,2,]

1. Which function displays the first five rows of the data frame named pizza?

• BOF(pizza, 5)
• first(pizza, 5)
• top(pizza, 5)

1. You accidentally display a large data frame on the R console, losing all the statements you entered during the current session. What is the best way to get the prior 25 statements back?

• console(-25)
• console(reverse=TRUE)
• history()
• history(max.show = 25)

1. d.pizza is a data frame. It’s column named temperature contains only numbers. If u extract temperature using the [] accessors, its class defaults to numeric. How can you access temperature so that it retains the class of data.frame?

``````> class( d.pizza[ , "temperature" ] )
> "numeric"
``````
• `class( d.pizza( , "temperature" ) )`
• `class( d.pizza[ , "temperature" ] )`
• `class( d.pizza\$temperature )`
• `class( d.pizza[ , "temperature", drop=F ] )`

1. What does c contain?

``````a <- c(3,3,6.5,8)
b <- c(7,2,5.5,10)
c <- a < b
``````
•  NaN
•  -4
•  4 -1 -1 2
•  TRUE FALSE FALSE TRUE

1. Review the statements below. Does the use of the dim function change the class of y, and if so what is y’s new class?

``````> y <- 1:9
> dim(y) <- c(3,3)
``````
• No, y’s new class is “array”.
• Yes, y’s new class is “matrix”.
• No, y’s new class is “vector”.
• Yes, y’s new class is “integer”.

1. What is `mydf\$y` in this code?
`mydf <- data.frame(x=1:3, y=c("a","b","c"), stringAsFactors=FALSE)`

• list
• string
• factor
• character vector

1. How does a vector differ from a list?

• Vectors are used only for numeric data, while list are useful for both numeric and string data.
• Vectors and lists are the same thing and can be used interchangeably.
• A vector contains items of a single data type, while a list can contain items of different data types.
• Vectors are like arrays, while lists are like data frames.

1. What statement shows the objects on your workspace?

• list.objects()
• print.objects()
• getws()
• ls()

1. What function joins two or more column vectors to form a data frame?

• rbind()
• cbind()
• bind()
• coerce()

1. Review line 1 below. What does the statement in line 2 return?

``````1 mylist <- list(1,2,"C",4,5)
2 unlist(mylist)
``````
•  1 2 4 5
• “C”
•  “1” “2” “C” “4” "5"
•  1 2 C 4 5

1. What is the value of y in this code?

``````x <- NA
y <- x/1
``````
• Inf
• Null
• NaN
• NA

1. Two variable in the mydata data frame are named Var1 and Var2. How do you tell a bivariate function, such as cor.test, which two variables you want to analyze?

• `cor.test(Var1 ~ Var2)`
• `cor.test(mydata\$(Var1,Var2))`
• `cor.test(mydata\$Var1,mydata\$Var2)`
• `cor.test(Var1,Var2, mydata)`

1. A data frame named d.pizza is part of the DescTools package. A statement is missing from the following R code and an error is therefore likely to occur. Which statement is missing?

``````library(DescTools)
deliver <- aggregate(count,by=list(area,driver), FUN=mean)
print(deliver)
``````
• `attach(d.pizza)`
• `summarize(deliver)`
• `mean <- rbind(d.pizza,count)`
• `deliver[!complete.cases(deliver),]`

1. How to name rows and columns in DataFrames and Matrices F in R?

• data frame: names() and rownames() matrix: colnames() and row.names()
• data frame: names() and row.names() matrix: dimnames() (not sure)
• data frame: colnames() and row.names() matrix: names() and rownames()
• data frame: colnames() and rownames() matrix: names() and row.names()

1. Which set of two statements-followed by the cbind() function-results in a data frame named vbound?

• A

``````v1<-list(1,2,3)
v2<-list(c(4,5,6))
vbound<-cbind(v1,v2)
``````
• B

``````v1<-c(1,2,3)
v2<-list(4,5,6))
vbound<-cbind(v1,v2)
``````
• C

``````v1<-c(1,2,3)
v2<-c(4,5,6))
vbound<-cbind(v1,v2)
``````

1. ournames is a character vector. What values does the statement below return to Cpeople?
`Cpeople <- ournames %in% grep("^C", ournames, value=TRUE)`

• records where the first character is a C
• any record with a value containing a C
• TRUE or FALSE, depending on whether any character in ournames is C
• TRUE or FALSE values, depending on whether the first character in an ournames record is C

1. What is the value of names(v)?

``````v <- 1:3
names(v) <- c("a", "b", "c")
v <- 4
``````
• ""
• d
• NULL
• NA

1. Which of the following statements doesn’t yield the code output below. Review the following code. What is the result of line 3?

``````x <- c(1, 2, 3, 4)
Output:  2 3 4
``````
• x[c(2, 3, 4)]
• x[-1]
• x[c(-1, 0, 0, 0)]
• x[c(-1, 2, 3, 4)]

1. Given DFMerged <- merge(DF1, DF2) and the image below, how manu rows are in DFMerged?

``````DF1(data frame 1): DF2(data frame 2):
VarA VarB VarA VarD
1 1 2 1 18 21
2 4 5 2 19 22
3 7 8 3 20 23
``````
• 6
• 9
• 3
• 0

CREDITS: (Source)

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