CSC121 - R Console
Wednesday January 10, 2018
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> # Let's work with assignment statements
> x <- 4
> x
[1] 4
> y <- 5
> y
[1] 5
> # When you refer to a variable (type it and press enter)
> # you always get the last value stored in it
> x <- 6
> x
[1] 6
> x <- x + 4
> # Step 1: Evaluate expression on the right hand side
> # x + 4 = 6 + 4 = 10
> # The value on the right hand side is 10
> # Step 2: Assign the value that we evaluated on the right hand side 
> # to the variable name on the left hand side
> x
[1] 10
> y
[1] 5
> y <- x + y
> # Step 1: Evaluate right hand side
> # x + y = 10 + 5 = 15
> # Step 2: Assign the value 15 to the variable y
> y
[1] 15
> # If we keep just reffering to y, R will always give back the same value
> y
[1] 15
> y
[1] 15
> y
[1] 15
> y <- 17
> y
[1] 17
> typeof(y)
[1] "double"
> typeof(x)
[1] "double"
> p <- 3
> # Non-integers:
> q <- 2.3
> t <- p + q
> t
[1] 5.3
> t <- p + as.integer(q)
> t
[1] 5
> # Let's see what happens when we change data types
> a <- 4.25
> a
[1] 4.25
> typeof(a)
[1] "double"
> as.integer(a)
[1] 4
> a
[1] 4.25
> # Still 4.25!
> # as.integer(a) does NOT change the value of a
> # The *only* way to change the value of a
> # is to use an assignment statement to assign a new value to the variable a
> a <- as.integer(a)
> a
[1] 4
> b <- 5
> c <- a + b
> c
[1] 9
> a <- 2
> a
[1] 2
> # Question: what is the value of c?
> c
[1] 9
> c <- a + b
> c
[1] 7
> # When we assigned c to be 9, we can't change its value until 
> # we assign a new value to it
> # So even though we changed a later
> # c's value wasn't assigned anything new
> # Remember: You can *only* change the value of a variable with an assignment statement. 
> 
> 
> ### FUNCTIONS ###
> 
> # In math, we can think of a function as a machine that:
> # 1. takes some input
> # 2. does something with that input
> # 3. produces some output
> 
> # For example: f(x) = 3x
> # It takes some input: x
> # Does something with that input: multiplies x by 3
> # produces some output: gives you back 3x
> 
> # Part of the structure of most proramming languages is the ability to use this kind of 'function' system
> # As we did with variables, let's see what can find out about functions in the console
> 
> # what looks like a function? 
> # as.integer(x)
> #   - just replace f in "f(x)" with as.integer
> 
> # how do we know as.integer is a function?
> # by our definition:
> # 1. takes some input: x (some number)
> # 2. does something with that input: finds the next lowest integer (or the same number if it is already an integer)
> # 3. produces some output: gives you back that lowest integer in the R console
> 
> # So, as.integer(x) behaves like a function
> # we can give it different inputs:
> as.integer(4.5)
[1] 4
> as.integer(6.352354545)
[1] 6
> as.integer(8)
[1] 8
> 
> # Whenever we give a function in R a value and press Enter,
> # we call this "calling the function"
> # as.integer(2.3) means "calling as.integer(x) with the value 2.3"
> # The value that is given back to us when the function is done being called
> # is called the "Return value"
> as.integer(2.3)
[1] 2
> # as.integer(2.3) "returned" 2
> 
> # R has many built-in functions
> # we already saw as.integer(x) and as.double(x)
> # Another one is sqrt(x)
> sqrt(9)
[1] 3
> # sqrt(x) returns the square root of the value it is called with
> sqrt(10 + 6)
[1] 4
> # typeof is also a function
> # typeof(x) returns the type of the value x
> 
> # Reminder: as.double(x) returns the 'double' version of x
> as.double(2.435)
[1] 2.435
> # It gives back the same value because 2.435 is already a double