Accessing List Value by looking up value with same index - r

Question

I was following this blog post:
https://www.robert-hickman.eu/post/dixon_coles_1/
And in a number of places, he gets a value from a list by putting in a value with the equivalent index, rather like using it as a key in a python dictionary. This would be an example in the link:
What I understand he's done is basically this:
teams <- c("a","b","c")
values <- c(1,2,3)
example_df <- data.frame(teams,values)
example_list <- as.list(example_df)
example_list$values[a]
Error: object 'a' not found
But I get an NA value for this - am I missing something ?
Thanks in advance !

Answer 1

The way a list works in R, makes it not really practical to address a list like that, as the values in the list aren't associated like that.
Which leads to this:
teams <- c("a","b","c")
values <- c(1,2,3)
example_df <- data.frame(teams,values)
example_list <- as.list(example_df)
#Gives NULL
example_list[example_list$teams == "a"]$values
#Gives 1, 2, 3
example_list[example_list$teams == "b"]$values
#Gives NULL
example_list[example_list$teams == "b"]$values
You can see that this wouldn't work, because the syntax you would expect to work in this case, throws an error "incorrect amount of dimensions":
example_list[example_list$teams == "b", ]$values
However, it is really easy to address a data frame, or any matrix like structure in the way you want to:
teams <- c("a","b","c")
values <- c(1,2,3)
example_df <- data.frame(teams,values)
#Gives 1
example_df[example_df$teams == "a", ]$values
#Gives 2
example_df[example_df$teams == "b", ]$values
#Gives 3
example_df[example_df$teams == "b", ]$values
What I think is happening in the tutorial you shared is something else. As far as I can see, there are no names passed through to the list, but variables. It is not giving the value of a higher dimensional thing, but rather the value of the list itself.
That also makes a lot more sense, as that is what the syntax is doing. "teams[1]" Simply returns the first value in the list called "teams" (even if that value is a vector or whatever) Of course, teams[i], where i is a variable, also works. What I mean is this:
teams = list(A = 1, B = 2, C = 3, D = 4)
#Gives A
teams[1]
If you want to understand why one of them works and the other one doesn't, here is both together. Throw it in RStudio, and look through the Environment.
## One dimensional
teams = list(A = "a", B = "very", C = "good", D = "example")
#Gives "very"
teams[2]
## Two dimensional
teams <- c("a","b","c")
values <- c(1,2,3)
teams2 <- list(teams, values)
#Gives "a, b, c"
teams2[1]
#Gives NULL
teams2[3]