CSC121 - R Console
Wendesday February 7th
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> # LISTS
> 
> # Lists are another R data structure that can store a group of values (just )
> # (just like a vector can)
> # However, unlike vectors, lists can store *multiple* data types at a time
> n <- 121
> s <- "csc"
> v <- c(432, 2, 95)
> t <- TRUE
> # We create a list using the list(..) function, which puts together a bunch of
> # elements into a list
> L <- list(n, s, v, t)
> L
[[1]]
[1] 121

[[2]]
[1] "csc"

[[3]]
[1] 432   2  95

[[4]]
[1] TRUE

> typeof(L)
[1] "list"
> 
> # A list is ordered, and in the console we can see the index of the elements
> # We can index the list using double square brackets
> L[[1]]
[1] 121
> L[[2]]
[1] "csc"
> L[[3]]
[1] 432   2  95
> # The third element is a vector, so we get back the entire vector, not just the first element of the vector
> # What if we index with one square bracket instead of two?
> L[2]
[[1]]
[1] "csc"

> typeof(L[2])
[1] "list"
> typeof(L[[2]])
[1] "character"
> # We don't get back the value at index 2 if we use square brackets
> # Instead, we get back another list, with only one element (the element we indexed)
> # You *must* use double square brackets to get the actual value of the element of the list
> 
> # Since we can create new lists using an index in single square brackets,
> # we can use themto create any sublist of L
> L[c(2, 4)]
[[1]]
[1] "csc"

[[2]]
[1] TRUE

> # L[c(2,4)] returns a list with only the 2nd and 4th elements
> # We can also index a list using logical values, the same way we did with vectors
> L[c(TRUE, FALSE, FALSE, TRUE)]
[[1]]
[1] 121

[[2]]
[1] TRUE

> # We cannot, however, use logical expressions as indexes in the same way we did for vectors
> L[L > 2]
Error: (list) object cannot be coerced to type 'double'
> L[[L > 2]]
Error: (list) object cannot be coerced to type 'double'
> L2 <- list(1, 2, 3)
> L2[L2 > 1]
[[1]]
[1] 2

[[2]]
[1] 3

> # If the list contains all numbers, than you can index using a logical expression involving numbers
> # Otherwise, the list might not be able to convert all of the values to another type
> # In some cases, you can convert the elements of the entire list to a specific type
> as.character(L)
[1] "121"           "csc"           "c(432, 2, 95)" "TRUE"         
> # But not in all cases
> as.double(L)
Error: (list) object cannot be coerced to type 'double'
> # R especially does not like converting lists with vectors
> L3 <- list(c(2,3,4))
> L3
[[1]]
[1] 2 3 4

> as.double(L3)
Error: (list) object cannot be coerced to type 'double'
> 
> # As an aside, we only use capital 'L' when dealing with arbitrary lists
> # Use the usual naming conventions for any other list name: studentList
> 
> # In lab today: Lists of lists
> K <- list(list(1, 2, "csc121"), list(3, 4, "csc207"))
> K
[[1]]
[[1]][[1]]
[1] 1

[[1]][[2]]
[1] 2

[[1]][[3]]
[1] "csc121"


[[2]]
[[2]][[1]]
[1] 3

[[2]][[2]]
[1] 4

[[2]][[3]]
[1] "csc207"


> # Notice that the output can be a bit confusing!
> # Since this is a list of lists, it needs to differentiate between the outer list
> # and the two inner lists.
> # The [[1]] and [[2]] which are on their own lines indicate the indexes of
> # the outer list (one index for each inner list)
> # [[1]][[1]] means the first index of the first inner list
> # [[1]][[2]] means the second index of the first inner list
> K[[1]][[1]]
[1] 1
> K[[1]][[2]]
[1] 2
> # [[2]][[1]] means the first index of the second inner list
> K[[2]][[1]]
[1] 3
> K[[2]][[3]]
[1] "csc207"
> K <- list(list(1, 2, "csc121"), list(3, 4, "csc207"))
> K[[2]]
[[1]]
[1] 3

[[2]]
[1] 4

[[3]]
[1] "csc207"

> # K[[2]] is a list
> 
> K[2]
[[1]]
[[1]][[1]]
[1] 3

[[1]][[2]]
[1] 4

[[1]][[3]]
[1] "csc207"


> # K[2] is a list that contains a list
> # Be careful with the indexes and number of square brackets! You will practise
> # in the lab today.
> 
> source('~/Desktop/for_loop_list.R')
3
three
4 56
> source('~/Desktop/for_loop_list.R')
Error in cat(e) : argument 1 (type 'list') cannot be handled by 'cat'
> source('~/printing_to_console.R')
[1] 4
> value
[1] 4
> source('~/printing_to_console.R')
[1] 4
> source('~/printing_to_console.R')
4
> source('~/printing_to_console.R')
> ReturnStatement(4)
[1] 4
> source('~/printing_to_console.R')
> source('~/printing_to_console.R')
[1] 4
4