CSC121 - R Console
Friday February 9th
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> # Recall our list from last time
> n <- 121
> s <- "csc"
> v <- c(432, 2, 98)
> t <- TRUE
> L <- list(n, s, v, t)
> L
[[1]]
[1] 121

[[2]]
[1] "csc"

[[3]]
[1] 432   2  98

[[4]]
[1] TRUE

> 
> # Adding elements to a list
> # We can add elements to a list using the 'append' function
> # 'append' means to add to the end of the list
> append(L, "appended")
[[1]]
[1] 121

[[2]]
[1] "csc"

[[3]]
[1] 432   2  98

[[4]]
[1] TRUE

[[5]]
[1] "appended"

>  Notice that this only returns a copy of L with the appended element
Error: unexpected symbol in " Notice that"
> # It does NOT change L
> L
[[1]]
[1] 121

[[2]]
[1] "csc"

[[3]]
[1] 432   2  98

[[4]]
[1] TRUE

> # We must use an assignment statement
> L <- append(L, "appended")
> L
[[1]]
[1] 121

[[2]]
[1] "csc"

[[3]]
[1] 432   2  98

[[4]]
[1] TRUE

[[5]]
[1] "appended"

> # We can also add elements at different points in the list by adding an extra argument to append()
> L3 <- list(1, "2")
> append(L3, "appended", 1)
[[1]]
[1] 1

[[2]]
[1] "appended"

[[3]]
[1] "2"

> # In this case, the last argument in appended is 1, which indicates *after* which index we want to append the element (in this case, after index 1)
> # It is NOT at which index the element will be when added
> # If we wanted it to be added at index 1 (beginning of list), we would give an argument of 0
> append(L3, "appended", 0)
[[1]]
[1] "appended"

[[2]]
[1] 1

[[3]]
[1] "2"

> length(L)
[1] 5
> length(L3)
[1] 2
> # You can find the length of a list with length(), just like vectors
> L3 <- append(L3, "appended", 0)
> L3
[[1]]
[1] "appended"

[[2]]
[1] 1

[[3]]
[1] "2"

> length(L3)
[1] 3
> 
> # Changing elements at an index
> # Just like vectors, we can change a list in place
> L3[2] <- 45
> L3
[[1]]
[1] "appended"

[[2]]
[1] 45

[[3]]
[1] "2"

> L3[[2]] <- 77
> L3
[[1]]
[1] "appended"

[[2]]
[1] 77

[[3]]
[1] "2"

> # Remove element
> # One way is just to index without that element present
> L3
[[1]]
[1] "appended"

[[2]]
[1] 77

[[3]]
[1] "2"

> L3[2:length(L3)]
[[1]]
[1] 77

[[2]]
[1] "2"

> # To remove an element at *any* index, you can assign it the value NULL
> L3
[[1]]
[1] "appended"

[[2]]
[1] 77

[[3]]
[1] "2"

> L3[2] <- NULL
> L3
[[1]]
[1] "appended"

[[2]]
[1] "2"

> # Notice that the element at index 3 originally was pushed up to index 2 after the removal
> L3[-2]
[[1]]
[1] "appended"

> # Negative indexes also work, but you need to assign the resulting list. Assigning the element to NULL is the way to change the list in place
> L3
[[1]]
[1] "appended"

[[2]]
[1] "2"

> L3[2] <- NULL
> L3
[[1]]
[1] "appended"
>