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# Test answers for R 2020

(95) Last updated: February 12
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`1. Which R function fits a simple linear regression model of x predicting y?`

• lm(y ~ x)

• lm(x ~ y)

• proc reg; model y=x; run;

• regress y x

`2. What does the following R code output? Y <- c(1,2,3); X <- rep(c(1,length(Y)), times=2); X`

• 1 3 1 3

• 1 1

• 1 1 3 3

• 1 2 1 2

`3. Which R function provides a histogram of the numeric vector X?`

• plot X

• hist(X)

• plot(X)

• plot.hist(X)

`4. What's the output for command : rep(c(1:2), times = 3)`

• 1 2 1 2 1 2

• 1:2 1:2 1:2

• 3 6 9

• 1 1 1 2 2 2

`5. Which R function imports a comma separated file "X.csv"?`

• import("X.csv")

• import.csv("X.csv")

`6. What's the output for command: print("Hello World")`

• OMG

• Hello World

• 2

• NA

`7. What does the following R code output? seq(2,10, by=2)`

• 2 4 6 8 10

• 1 3 5 7 9

• 2

• 2 10

`8. Given the command: Z <- array(h, dim=c(3,4,2)), dim(Z) stands for:`

• the dimension vector c(3,4,2)

• the array of all zeros

• the data vector as it was in h

• the entire array as an array

`9. Which R code will sort the vector X <- c(1,5,3,9,7) from largest to smallest?`

• sort(X, decreasing=T)

• X[sort(X)]

• order(X)

• sort(X)

`10. What's the value of x after command: x <- 3 < 4`

• 4

• FALSE

• TRUE

• 3

`11. In the R data set X, what value will missing numeric data be assigned? X <- read.csv("X.csv")`

• NA

• 999

• .

• "missing"

`12. In order to apply lag operator for the data, you should:`

• convert the data into matrices

• convert the data into the time-series object

• convert the data into integer

• convert the data into vector

`13. Which R function provides basic descriptive summary statistics for a numeric vector x?`

• proc contents x;

• summarize(x)

• summary(x)

• summarize x

`14. The entities on which R operates are technically known as:`

• objects

• commands

• events

• mode

`15. What's the output for command is.nan(0/0)`

• TRUE

• FALSE

• -Inf

• NA

`16. How does one obtain the FIRST element of x when x = 1:9?`

• x\$0

• x

• x\$1

• x

`17. What is an appropriate syntax for a 'for' loop in R to run specific simulation code nnn times?`

• for(i=1, nnn, 1) { <simulation code> }

• for i in 1:nnn: <simulation code>

• for(i in 1:nnn) { <simulation code> }

• for(i - nnn) { <simulation code> }

`18. Which command is used to test if an object is a time series?`

• is.ts

• if.ts

• as.ts

• ts

`19. In the plot( ) function, what option is used to specify that the x-axis displays values from 0 to 1?`

• xlim=c(0, 1)

• xlim="0, 1"

• xlimit="0, 1"

• xlab=c(0, 1)

`20. What does the following R function output? abs(10 - 3 * 4)`

• 2

• NA

• -2

• 0

`21. What is the output of the following R code? x<-2; y <- c(1:3); paste("The value of x is", x, "and the value of y[x] is", y[x], sep=" ")`

• "The value of x is 2 and the value of y[x] is 2"

• "The value of x is 2 and the value of y[x] is c(1:3)"

• "The value of x is 2 and the value of y[x] is 6"

• "The value of x is 2 and the value of y[x] is 4"

`22. Which of the following is NOT a valid data import function?`

• scan

`23. What's the value of 'ind' after running command : z <- c(1:3,NA);  ind <- is.na(z)`

• FALSE

• FALSE TRUE

• TRUE

• FALSE FALSE FALSE TRUE

`24. What's the output for command: paste("O", "M", "G", sep = "")?`

• "omg"

• "O" "M" "G"

• "OMG"

• "O M G"

`25. Suppose numeric vectors X, Y, and Z are all of the same length. Which R command will create a matrix with rows X, Y, Z?`

• matrix(X, Y, Z, nrow=3)

• matrix(rbind(X, Y, Z), nrow=3)

• matrix(cbind(X, Y, Z), nrow=3)

• matrix(rbind(X, Y, Z), ncol=3)

`26. What R function can be used to tabulate values of categorical variable Y (columns) by categorical variable X (rows)?`

• table(X, Y)

• table(X*Y)

• tabulate(X, Y)

• table(Y ~ X)

`27. What is the value of Y? X <- c(1,2,2,2,3,3,4,4,5,6); Y <- unique(X[which(X<4)])`

• 1 2 2 2 3 3

• 1 2 3

• 3

• 1 2 3 4

`28. In order to decide the inequality between x and y, you can use the command:`

• x == y

• x =! y

• x =~ y

• x != y

`29. Which R function adds a line with slope 1 and intercept 0 into an existing plot of Y versus X?`

• lty=c(0, 1)

• lines(0, 1)

• abline(0, 1)

• smooth.spline(0, 1)

`30. What's the output for command: seq(from = 1, to = 5, by = 3)`

• 1 4 7

• 1 4

• 1 2 3 4 5

• 1 3 5

`31. What's the output for command:  paste0("O", "M", "G")?`

• "omg"

• "O M G"

• "O" "M" "G"

• "OMG"

`32. What does the option 'cex' do in the following R function? text(0, 1, "Hello", cex=2)`

• Surpresses printing of the word "Hello"

• Prints the word "Hello" in bold font

• Increases the size of the word "Hello" by a factor of 2

• Right justifies the word "Hello"

`33. Which R function will generate a mean for each row in a numeric matrix X with intermittent missing values?`

• by(X, 1, mean, na.rm=T)

• apply(X, 1, mean, na.rm=T)

• mean(X[1:length(X[,1]),], na.rm=T)

• apply(X, 1, mean)

`34. What is the output of the following R function? yyy <- c(1, 3, NA); fff <- function(xxx) {mean(xxx)}; fff(yyy)`

• 2

• 1.5

• NULL

• NA

`35. Which is NOT a valid R function for obtained the residuals from a simple linear regression model of X predicting Y?`

• lm(Y ~ X)\$residuals

• residuals(lm(Y ~ X))

• Y - lm(Y - X)\$fitted.values

• lm(Y ~ X)\$residuals - lm(Y ~ X)\$fitted.values

`36. In a linear regression model with outcome y and linear predictors x1, x2, and x3, which R code correctly includes an interaction term between x1 and x2?`

• lm(y ~ x1:x2 + x3)

• lm(y ~ x1:x3 + x1:x2)

• lm(y ~ x1 + x2 + x3 + x1*x2)

• lm(y ~ x1 + x2 + x3 + interaction(x1, x2))

`37. Which command is correct in order to load excel file into R?`

`38. What is the value of Y? Y <- 10 + 100 & !is.na(0)`

• 110

• False

• True

• 0

`39. Suppose X is the vector c(1:10). Which R code will swap the values of the 3rd element of X and the 7th element of X?`

• X[c(3,7)] <- X[c(7,3)]

• replace(X, c(3, 7), c(7,3)

• X <- X; X <- X

• X <- X; X <- X

`40. What is the output of dim(matrix(1:10, ncol=2))?`

• 20

• 5 2

• 10

• 2 5

`41. Which is an INCORRECT command to get an explanation on any specific named function?`

• ?rnorm

• ts?

• help(lm)

• ??log

`42. What's the output for command: is.nan(NA)`

• NA

• -Inf

• TRUE

• FALSE

`43. Let X be a 3x4 matrix with non-zero values. The result of: > apply(X, 2, mean), is:`

• equal to the result of: >c(mean(X[1,]), mean(X[2,]), mean(X[3,]))

• equal to the result of: >c(mean(X[,1]), mean(X[,2]), mean(X[,3]), mean(X[,4]))

• equal to the result of: >c(mean(X[1,]), mean(X[2,]), mean(X[3,]), mean(X[4,]))

• equal to the result of: >c(mean(X[,1]), mean(X[,2]), mean(X[,3]))

`44. What's the output for command:  is.na(0/0)?`

• NA

• FALSE

• TRUE

• -Inf

`45. Which of the following is a legal sort command?`

• sort(list(10, -3, 4))

• sort(c(10, -3,4))

• sort(data.frame(x = c(10, -3, 4)))

• sort(10, -3,4)

`46. What does the following R function output? xxx <- c(1, 1, 2, 2, 3, 3, NA, NA); yyy <- c(rep(0, 4), rep(1, 4)); cor(yyy, xxx)`

• R squared

• A correlation coefficient

• An error message

• NA

`47. Which R function fits a simple linear regression model of x predicting y without an intercept?`

• lm(y ~ x, intercept=F)

• regress y x, nointercept

• proc reg; model y=x; intercept=F; run;

• lm(y ~ -1 + x)

`48. Which R function can be used to produce a simple scatterplot of a numeric vector Y versus a numeric vector X?`

• scatter(Y, X)

• plot(Y, X)

• plot(X, Y)

• scatter(X, Y)

`49. Which R command is used to merge two data sets X and Y by the variable "ID" that includes all records from both data sets?`

• merge(X, Y, by="ID", all=T)

• merge(X, Y, id="ID", all=T)

• combine(X, Y, by="ID")

• merge(X, Y, by="ID")

`50. Suppose you need to randomly assign 100 study subjects into one of 5 groups such that there are exactly 20 subjects in each group. Which of the following R functions creates a variable Y that randomly distributes the 100 study subjects into 5 groups of equal size?`

• Y <- sample(rep(c(1:5), each=20), size=100)

• ytemp <- rnorm(100, 0, 1); Y <- cut(ytemp, breaks=c(-10, -2, -1, 1, 2, 10))

• Y <- rbinom(100, 5, 0.5)

• Y <- sample(c(1, 2, 3, 4, 5), 100, replace = TRUE)

`51. What's the output for command: rep(c(1:2), each = 3)`

• 3 6 9

• 1 1 1 2 2 2

• 1:2 1:2 1:2

• 1 2 1 2 1 2

`52. Which is NOT the right way to quit R program?`

• exit()

• quit()

• quit("yes")

• q()

`53. Given the command: Z <- array(h, dim=c(3,4,2)), Z[] with an empty subscript or Z with no subscript stands for:`

• the data vector as it was in h

• the dimension vector c(3,4,2)

• the array of all zeros

• the entire array as an array

`54. What is the output of matrix(1:10, nrow=2)[2,2]?`

• 4

• 3 4

• 2

• 2 4 6 8 10

`55. What is the result of the command:                              > A<-array(2:1,dim=(2:2)) > B<-array(2:3,dim=(2:2)) > A %*% B %*% A`

• [,1] [,2] [1,] 21 14

• [,1] [,2] [1,] 14 7

• [,1] [,2] [1,] 14 21

• [,1] [,2] [1,] 7 14

`56. Which is NOT the parameter of command seq()?`

• length

• times

• from

• by

`57. The command:                                                            > A<-array(3:2,dim=c(2,1)) > B <- t(A)                                                                       > nrow(B)                                                         will generate:`

• [,1] [1,] 3 [2,] 2

• [,1] [,2] [1,] 3 2

•  1

•  2

`58. What's the output of the command : x = c(T, T, NA); all(x, na.rm = TRUE)`

• NA

• TRUE

• TRUE TRUE FALSE

• FALSE

`59. Please fill in the blank of the following commands to extract elements X[1,3], X[2,2] and X[3,1] from a 4 by 5 array X and replace these entries in the array X by zeroes:                                                                  > x <- array(1:20, dim=c(4,5))                                                                    > i <-                  fill in the blank                                                                                           > x[i] <- 0`

• array(c(1:3,3:1), dim=c(3,2))

• array(c(3:1,1:3), dim=c(2,3))

• array(c(1:3,3:1), dim=c(2,3))

• array(c(3:1,1:3), dim=c(3,2))

`60. The command:>  state <- c("tas", "sa", "qld", "nsw", "nsw", "nt", "wa", "wa") >  statef <- factor(state) > incomes <- c(60, 49, 40, 61, 64, 60, 59, 54) > tapply(incomes, statef, mean) generates:`

• nsw nsw nt qld sa tas wa wa 61.0 64.0 60.0 40.0 49.0 60.0 54.0 59.0

• tas sa qld nsw nsw nt wa wa 60.0 49.0 40.0 61.0 64.0 60.0 59.0 54.0

• nsw nt qld sa tas wa 62.5 60.0 40.0 49.0 60.0 56.5

• tas sa qld nsw nt wa 60.0 49.0 40.0 62.5 60.0 56.5

`61. The command:                                                            > A<-array(3:2,dim=c(2,1)) > B <- t(A)                                                       will generate:`

• [,1] [1,] 3 [2,] 2

• [,1] [,2] [1,] 3 2

• [,1] [,2] [1,] 2 1

• [,1] [1,] 2 [2,] 1

`62. What values are displayed for the x-axis limits of the following plot?  plot(c(0, 2), c(0, 1), xaxt="n", yaxt="n"); axis(side=2, labels=c("0", "10"), at=c(0,1))`

• 0, 2

• No values are displayed

• 0, 1

• 0, 10

`63. What is the result of the command:                              >array(2:1,dim=(2:2))%*%array(2:3,dim=(2:2))`

• [,1] [,2] [1,] 4 3

• [,1] [1,] 8

• [,1] [,2] [1,] 6 2

• [,1] [1,] 7

`64. matrix(1:4, ncol = 2)[1,]`

• 1 3

• 1 4

• 1 2

• 2 4

`65. Which R function outputs the day of the month from X <- "2014-May-15"?`

• strsplit(X, "-")

• substr(X, 10, 11)

• date(X, "day")

• split(X, "-")

`66. What's the output for command: paste("O", "M", "G")`

• "O" "M" "G"

• "OMG"

• "O M G"

• "omg"

`67. What is the result of the command:                              >array(2:1,dim=(2:2))*array(2:3,dim=(2:2))`

• [,1] [,2] [1,] 6 2

• [,1] [1,] 8

• [,1] [1,] 7

• [,1] [,2] [1,] 4 3

`68. Which class avoids making copies of objects?`

• s3

• R5 (reference class)

• s4

`69. What is the output of the following calculation? t(c(1, 1)) %*% c(1, 1)`

• 4

• 1

• 2

• A 2x2 matrix with all cells equal to 1

`70. Let X be a n-dimension vector. Which test for:  "no value of X is NA"`

• all(!is.na(X))

• all(is.na(X))

• !all(is.na(X))

• !is.na(X)

`71. What is the output of: rrr <- 5.45667; sprintf("%1.f", round(rrr, 3))`

• 5

• 5.457

• "5"

• 5.5

`72. Which R function will generate an integer X from Y <- "9.145", where Y is stored as a text string?`

• X <- is.integer(Y)

• X <- as.integer(Y)

• X <- round(Y, 0)

• X <- as.numeric(Y)

`73. what's the result from following code a  = 2; fun = function(x) {a <<- a + 1;return(x + 1)}; z = fun(3); a + z;`

• NA

• 5

• 7

• 6

`74. If Y is a multi-modal vector of integers, which R function would return all modes of Y (the most frequent value(s) of Y)?`

• names(table(Y))[table(Y)==max(table(Y))]

• mode(Y)

• sort(table(Y), decreasing=T)

• sort(Y, decreasing=T)

`75. What is the result of command: labs <- paste(c("X","Y"), 1:10, sep="")?`

• the row vector ["X1" "Y1" "X2" "Y2" "X3" "Y3" "X4" "Y4" "X5" "Y5" "X6" "Y6" "X7" "Y7" "X8" "Y8"

• the row vector ["X1" "Y2" "X3" "Y4" "X5" "Y6" "X7" "Y8" "X9" "Y10"]

• the row vector ["Y1" "Y2" "Y3" "Y4" "Y5" "Y6" "Y7" "Y8" "Y9" "Y10"]

• the row vector ["X1" "X2" "X3" "X4" "X5" "X6" "X7" "X8" "X9" "X10"]

`76. R operates on named data structures. The simplest such structure is the numeric vector, which is a single entity consisting of an ordered collection of numbers. To set up a vector named x, consisting of 10.4, 5.6, 3.1, 6.4 and 21.7, which is the INCORRECT command:`

• assign("x", c(10.4, 5.6, 3.1, 6.4, 21.7))

• c(10.4, 5.6, 3.1, 6.4, 21.7) -> x

• x <- c(10.4, 5.6, 3.1, 6.4, 21.7)

• (All of these are correct)

`77. Given the command: Z <- array(h, dim=c(3,4,2)), Z[1:24] stands for:`

• the dimension vector c(3,4,2)

• the entire array as an array

• the array of all zeros

• the data vector as it was in h

`78. What is NOT the right answser about how to get help manual for function 'plot' in a running R session.`

• ?plot

• help(plot)

• help("plot")

• ?"plot"

• help.start(plot)

`79. Which of the following command only assumes a common continuous distribution?`

• wilcox.test(A, B)

• t.test(A, B, var.equal=TRUE)

• t.test(A, B)

• var.test(A, B)

`80. What's right about running following command: setClass("Test", contains = "VIRTUAL"); obj <- new("Test")`

• Error produced when create an instance for class "Test"

• A S3 class "Test" is defined

• Error produced when define the class.

• A new instance of class "Test" is generated.

`81. Given x <- c(1:3,NA), the command: (x+1)[(!is.na(x)) & x>0] -> z generates:`

• the row vector [2 3 4]

• the row vector [TRUE TRUE TRUE]

• the row vector [FALSE FALSE FALSE]

• the row vector [1 2 3]

`82. The command > Z <- array(h, dim=c(3,4,2)) would use h to set up 3 by 4 by 2 array in Z. However if h is shorter than 24, its values___________________.`

• makes Z an array of all zeros

• are just the same as the length of h

• would signal an error about mismatching length

• are recycled from the beginning again to make it up to size 24

`83. plot.lm is a function in R, what kind of programming style it is?`

• s3

• n6

• s4

• R5

`84. Which R code subsets a matrix Y to only those records (rows) where a variable x (in Y) is less than 100?`

• Y[which(x<100)]

• Y[,which(x<100)]

• Y[where(x<100),]

• Y[x<=100]

`85. Which one is the correct expression regarding outer product of two arrays?`

• > ab <- a %o% b

• > ab <- outer(a, b, *)

• > ab <- outer(a, b, %o%)

• > ab <- a * b

`86. What R function can be used to tabulate values of categorical variable Y (columns) by categorical variable X (rows), including missing values?`

• tabulate(X, Y, na.omit=F)

• table(X, Y, useNA="always")

• table(X*Y, na.rm=F)

• table(X, Y, na.include=T)

`87. The command: > fruit <- c(5, 10, 1, 20); > names(fruit) <- c("orange", "banana", "apple", "peach"); > lunch <- fruit[c("apple","orange")]; > lunch; generates:`

• apple orange 1 5

• 1 5

• apple orange

• 1 5 apple orange

`88. Which of the following is NOT one of R's basic vector types?`

• integer

• double

• (All of these are valid)

• logical

• complex

`89. Which of the following statements regarding mixed vector and array arithmetic is true?`

• As long as short vectors and arrays only are encountered, the arrays can have the different dim attribute or an error results

• Any vector operand shorter than a matrix or array operand generates an error

• The expression is scanned from right to left

• Any short vector operands are extended by recycling their values until they match the size of any other operands

`90. To test the residuals of a regression model X for heteroscedastisity, you can use this command:`

• bgtest(X)

• heteroscedastisity(X)

• gqtest(X)

• jarqueberaTest(residual(X))

`91. To test the residuals of model X for autocorrelation of first order, you can use this command:`

• bgtest(X)

• jarqueberaTest(residual(X))

• gqtest(X)

• autocorrelationtest(X)

`92. Which is the INCORRECT way to get the vector [1 1 1 3 3 3 5 5 5]?`

• x<-seq(1,5,by=2); rep(x, times=3)

• x<-seq(1,5,by=2); rep(x, each=3)

• x<-c(1 1 1 3 3 3 5 5 5)

• x<-seq(1,5,by=2); s<-rep(x, times=3); sort(s)

`93. What is the result of command: z <- c(1:3,NA); ind <- is.na(z); z ?`

• the row vector [FALSE FALSE FALSE TRUE]

• the row vector [TRUE TRUE TRUE FALSE]

• the row vector [1 2 3 FALSE]

• the row vector [1 2 3 NA]

`94. Let h be a numeric vector shorter than 24, the command: dim(h) <- c(3,4,2)`

• would signal an error about mismatching length

• is just the same as the length of h

• makes an array of all zeros

• is recycled from the beginning again to make it up to size 24

`95. Which is INCORRECT regarding the command: T=40; x=rnorm(T, 1,4)?`