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A snapshot of the summary  Basic skills in mathematics

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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"
 Anything between single or double quotation marks is stored as a character, which can be used to encode strings:

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: nonnumeric argument to binary operator
 as.numeric("1") + 1
 ## [1] 2
 You can use functions named is.mode to test the mode of an object:

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
 You can perform elementwise operations on vectors:

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
 You can apply logical operations to vectors:

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
 You can select a subset of vector element using indexing

How do you index the values of a variable?Type datafilename$variable
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