 # Summary Basic Skills In Mathematics

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- Basic skills in mathematics
219 Flashcards & Notes
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Remember faster, study better. Scientifically proven. • ## 1 lecture 1

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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?
• ##  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)
• ##  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
• ##  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
• ##  "a" "b" "d"
• rep(v,3)
• ##  "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
• ##  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
• ##  11 22 33
• a * b # Multiply each element in a with the same element in b
• ##  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
• ##  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 # 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
• ##  TRUE TRUE FALSE FALSE FALSE
• How do you index the values of a variable?
Type datafilename\$variable