B Test Exam

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Test Exam SS 2019 DOWNLOAD

B.1 Additional Exercises

B.1.1 Exercise I

The following function makemat was found in the file function.final.final2.R as part of an old undocumented project. Therefore its purpose is unknown. Being a young and ambitious R developer you were asked to answer the following questions.

set.seed(124)
num <- round(runif(9, min = 0, max = 0.6), 2)
num
#R> [1] 0.05 0.25 0.31 0.24 0.13 0.18 0.35 0.29 0.55
kat <- c(1, 2, 3)
val <- c(0.2, 0.4, 0.6)
mat <- matrix(num, ncol = 3, nrow = 3)

for ( i in 1:prod(dim(mat)) ){
  for ( j in val){
    state <- (mat[i] < j)
    if (state){
      mat[i] <- kat[j == val]
    }
  }
}
  
mat[(mat % 2) == 1] <- 1

mat

  1. While executing and trying to load the function above the following message appears. Explain why the message is returned and correct the error in the provided source code.

    Error: unexpected input in "mat[(mat % 2) == 1] <- 1"

  2. Make yourself familiar with the provided code and briefly describe in a generic way what the code does (line-by-line, one full iteration of the outer loop). Ensure that your description is short but precise and allows the reader to gain a deep understanding of the data manipulation process.

  3. Write down the content of the variable mat after fixing the error and executing the code above.

B.2 Exercise II

The following three code snippets were found in the file misc.support.final.final2.R as part of an old undocumented project. Therefore their purpose is unknown. Being a young and ambitious R developer you were asked to explain the mysterious content of the following three snippets.

### CODE SNIPPET 1
x <- 100
counter <- 0

while( x > 0){
 x <- x/2
 counter <- counter + 1
}

counter
  1. Briefly describe what the code snippet above does (line-by-line).
  2. Please give your opinion on the provided code and comment on time of execution.
### CODE SNIPPET 2
res <- list()

for (i in 1:10){
  res[[i]] <- sample(seq(from=0,to=1,by=0.2))
}

sum(sapply(res,FUN = sum))
length(res)
typeof(res[[1]][1])
  1. Please write down what the last three lines of code snippet 2 return when executing the code above.
### CODE SNIPPET 3
a <- array(sort(rep(1:3,4)),dim=c(2,2,3))

for ( i in 1:dim(a)[1] ){
  for ( j in 1:dim(a)[2] ){
    for ( k in 1:dim(a)[3] ){
      a[i,j,k]  <- ifelse(a[i,j,k] %% 2, a[i,j,k], NA)
    }
  }
}

a
  1. Please write down the content of the variable a when snippet 3 is executed.

B.2.1 Exercise III

The following function genlist was found in the file list-maker.final.final2.R as part of an old undocumented project. Therefore its purpose is not known. Being a young and ambitious R developer you were asked to answer the following questions.

genlist <- function(x=sample(x=1:5, size=5)){
  n <- 0
  for (i in (1:length(x))){
    if (x[i] % 2 == 1){
      n <- n + 1
    }
  }
  res <- list(n=n, x=x, y=rev(x), z=sort(x))
  return(res)
}

  1. While executing and trying to load the function above the following message appears. Explain why the message is returned and correct the error in the provided source code.

    Error: unexpected input in:

    " for (i in (1:length(x))){

    if (x[i] % 2 == 1){"

  2. Briefly describe what the function genlist does (line-by-line) and returns. Ensure that your description is precise and allows the reader to gain a deep understanding of how the values in the returned list are generated.

  3. Provide the results to the following function call based on your corrected code.

genlist()
genlist(1:7)
  1. Please come up with a single line of code that returns the type of each list element. Name the respective datatypes and answer the question if these can be different from values returned by class.

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Auguie, Baptiste. 2017. GridExtra: Miscellaneous Functions for "Grid" Graphics. https://CRAN.R-project.org/package=gridExtra.

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Wickham, Hadley. 2016. Ggplot2: Elegant Graphics for Data Analysis. Springer-Verlag New York. https://ggplot2.tidyverse.org.

Wickham, Hadley, and Maximilian Girlich. 2022. Tidyr: Tidy Messy Data. https://CRAN.R-project.org/package=tidyr.

Wilkinson, L. 2005. The Grammar of Graphics. 2nd ed. New York: Springer.

Wilkinson, Leland. 2010. “The Grammar of Graphics.” Wiley Interdisciplinary Reviews. Computational Statistics 2 (6): 673–77.