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• Explain what different object modes are?

  • There are different types of objects in R: - numeric, e.g., 2, 0.03 - character, e.g., ‘a’, “b” - logical, TRUE, FALSE 
  • These are called modes 
  • To request the mode of object x use mode(x) 
    
  • a mode is the category an object belongs to.

• What are characters and strings?

  • Anything between single or double quotation marks is stored as a character, which can be used to encode strings: 
    
  • strings are kind of naming objects a different name.
  • a character is the object, and the string is the second name that is given to that character, what it's also reffered by.
  • characterObject <- 'this is a string' 
  • characterObject2 <- "this is also a string" 
    

  You can not do mathematical expressions with character strings, even when the string looks like a number

• Describe the logical mode.

  • The logical mode indicates a Boolean object which can only be true (TRUE) or false (FALSE): 
  • logicalObject <- TRUE 
    
  • these logical objects can be used in compariosn tests;
  • 1 == 1 # Is 1 equal to 1?

  • ## [1] TRUE

• How can you test and transform modes?

  • You can use functions named is.mode to test the mode of an object: 
    
  • a <- 1.23 is.logical(a) 
  • ## [1] FALSE 
  • You can use functions named as.mode to transform objects into a different mode: 
    
  • "1" + 1 ## Error in "1" + 1: non-numeric argument to binary operator 
  • as.numeric("1") + 1 
  • ## [1] 2 
    

• What does the rep function do?

  • A vector containing copies of the same object can be created using rep(object,number_of_repetitions): 
  • For example, rep(1,3) creates the vector [1 1 1]
  • Create a vector with n copies:

  • v <- c('a','b','d')
  • v
  • ## [1] "a" "b" "d"
  • rep(v,3)
  • ## [1] "a" "b" "d" "a" "b" "d" "a" "b" "d"

• What does the sample function do?

  You can create a vector of n random samples from another vector using sample(vector_to_sample_from, n)
  

  • v1 <- seq(1, 4, 0.5) 
  • v2 <- sample(v1, 4) 
  • v2 
  • ## [1] 2.0 3.5 4.0 1.5  

• How can you perform mathematical computations with vectors?

  • You can perform elementwise operations on vectors:
    

  • a <- c(10,20,30)
  • b <- 1:3
  • a + b # Add each element in a with the same element in b
  • ## [1] 11 22 33
  • a * b # Multiply each element in a with the same element in b
  • ## [1] 10 40 90

• How can you apply logical implications to vectors?

  • You can apply logical operations to vectors:
    

  • a <- c(10,20,30)
  • b <- c(5,20,50)
  • a < 20 # Test each element of a if it is smaller than 20
  • ## [1] TRUE FALSE FALSE

• What is indexing and how do you do it?

  • You can select a subset of vector element using indexing
    

  • Using square brackets [ and ], you can indicate which elements to return as:

  • A vector containing numbers of the elements you wish to keep
  • a[5] # Get the fifth element of vector a
    
  • a[c(1,5)] # Get the first and fifth element of vector a
  • A minus sign followed by the element you wish to omit
  • a[-5] # Get everything except the fifth element of vector a
    
  • A vector containing a TRUE or FALSE for each element, TRUE indicating you want to keep the element
  • a < 5 
  • ## [1] TRUE TRUE FALSE FALSE FALSE 
    

• How do you index the values of a variable?

  Type datafilename$variable