R Console
Wednesday, February 14
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> # More list manipulation
> 
> # Empty Lists
> # Unlike empty vectors which have the type name followed by (0), empty lists are made with no arguments
> emptyList <- list()
> emptyList
list()
> list(0)
[[1]]
[1] 0

> # We get a non-empty list with a value - 0, when we use list(0)
> 
> # Last time we saw the append() function
> L <- list("string", 43, c(3, 4))
> L <- append(L, 57)
> L
[[1]]
[1] "string"

[[2]]
[1] 43

[[3]]
[1] 3 4

[[4]]
[1] 57

> # What if we wanted to append a vector?
> append(L, c("first", "second"))
[[1]]
[1] "string"

[[2]]
[1] 43

[[3]]
[1] 3 4

[[4]]
[1] 57

[[5]]
[1] "first"

[[6]]
[1] "second"

> # Notice that the elements simply get added at the end as separate list elements.
> # We want *one* list element with the whole vector
> # How do we fix this? Enclose the vector in a list
> append(L, list(c("first", "second")))
[[1]]
[1] "string"

[[2]]
[1] 43

[[3]]
[1] 3 4

[[4]]
[1] 57

[[5]]
[1] "first"  "second"

> # Combining lists with the c(..)
> # We can combine the values of two separate lists using the c(..) function in much the same way that we did with vectors
> L1 <- list(2, c("hi", "hey"))
> L2 <- list(3, c("bye", "later"))
> c(L1, L2)
[[1]]
[1] 2

[[2]]
[1] "hi"  "hey"

[[3]]
[1] 3

[[4]]
[1] "bye"   "later"

> # We get one list with the elements of both L1 and L2 in the same order
> 
> # New logical operator: %in%
> # Usage: x %in% y
> # x %in% y returns TRUE iff x is an element of y
> # y can be any set, like a list or a vector
> v <- c(45, 32)
> 45 %in% v
[1] TRUE
> 47 %in% v
[1] FALSE
> L3 <- list(21, "hello")
> "hello" %in% L3
[1] TRUE
> "hey" %in% L3
[1] FALSE
> # Note: Only one-element vectors are supported. You can not properly check if a long vector is in a list
> L4 <- list(c(1,2,3))
> c(1, 2, 3) %in% L4
[1] FALSE FALSE FALSE
> # Does not give back the desired 'TRUE' output
> # Only use for single elements
> 
> # DICTIONARIES (in R, 'named lists')
> # In computer science, a dictionary is a data structure that allows us to 'look up' values using *names* rather than index numbers
> # We can name each element of a list and use it to look up that element in the list
> # An example of naming elements in a list:
> L5 <- list(a=4, b=5, c=6)
> L5
$a
[1] 4

$b
[1] 5

$c
[1] 6

> # We gave the names a, b, and c to the values inside L5
> # In dictionaries, these names are called 'keys'
> # The elements that the keys refer to are called 'values'
> # So we call these *key-value pairs*. Every key is paired with some value.
> L5 <- list(a=4, b=5, c=6)
> # key 'a' refers to the value 4
> # key 'b' refers to the value 5
> # key 'c' refers to the value 6
> # We can look up the value for each key in two ways:
> # First way is to index with a string 
> L5[["a"]]
[1] 4
> L5[["b"]]
[1] 5
> L5[["c"]]
[1] 6
> L5[["d"]]
NULL
> # Second way to index is to use the dollar sign $ notation
> L5$a
[1] 4
> L5$b
[1] 5
> # You don't need quotes for this method of indexing, BUT you also now can't use variable names to index the dictionary like you could by indexing with strings
> s <- "b"
> L5[[s]]
[1] 5
> L5$s
NULL
> 
> # You can get all the keys in a list (dictionary), using the names() function
> names(L5)
[1] "a" "b" "c"
> # You can change the keys by using the names() as well
> names(L5)[1] <- "w"
> L5
$w
[1] 4

$b
[1] 5

$c
[1] 6

> L5[["w"]]
[1] 4
> L5$w
[1] 4
> L5$a
NULL
> # As usual, you can change the value for a key in place
> L5[["w"]] <- c(76, 54)
> L5
$w
[1] 76 54

$b
[1] 5

$c
[1] 6

> # IMPORTANT: keys should be *unique*. You should not have two keys with teh same name in one list.
> # R let's you have two keys with the same name, but when you index it, you only get the *first* value with that key, not both.
> # It also doesn't help you look up a value.  The whole point is to look up one value with one key
> L6 <- list(a=2, b=3, a=4)
> L6[["a"]]
[1] 2
> # ^ Not what we want
> # Keys *must* be unique!
> 
> 
> 
> L <- list()
> append(L, c(1,2,3))
[[1]]
[1] 1

[[2]]
[1] 2

[[3]]
[1] 3

> append(L, list(c(1,2,3)))
[[1]]
[1] 1 2 3

> source('~/Desktop/list_functionss.R')
> ListOfSums(c(1,2,3), c(4,5,6))
[[1]]
[1] 1 4 5

[[2]]
[1] 2 5 7

[[3]]
[1] 3 6 9

> source('~/Desktop/list_functionss.R')
> AddToVectors(list(c(1,2), c(3,4)), 7)
[[1]]
[1] 1 2 7

[[2]]
[1] 3 4 7

> source('~/Desktop/list_functionss.R')
> PrintKeysAndValues(list(a=5, b=6, c=7))
Key: a Value: 5 
Key: b Value: 6 
Key: c Value: 7 
> list(a=5, b=6, c=7)
$a
[1] 5

$b
[1] 6

$c
[1] 7