# 2021 Advent of Code Day 02 “Don’t Try This At Home” Edition

The Moderna booster level drained me all day on Dec 1 and did what jab two did during the overnight period (achy enough to wake me up and not get back to slumber easily). To try to wear myself down, I decided to practice a bit of R with the 2021 Advent of Code. There are plenty of superb R bloggers chronicling their daily katas that I do not feel compelled to post every one (truth be told, work and fam tasks/priorities will make devoting any time to this year’s daily puzzles a rare event).

Day 01 was very straightforward (even part 2 which I used {RcppRoll} despite hoping to stick to only base R 4.x) so it’s kinda not worth a post (for me), but Day 02 was kinda fun as I don’t have regular opportunities to use `scan()` and `get()`.

The input is a series of submarine commands:

``````forward 5
down 5
forward 8
up 3
down 8
forward 2
``````

with a set of rules that change between parts 1 and 2.

We can read in those commands with `scan()` which lets us specify a pattern for each line (`scan()` takes care of dealing with whitespace for you):

``````scan(
text = "forward 5
down 5
forward 8
up 3
down 8
forward 2",
what = list(character(0), integer(0))
) |>
setNames(c("command", "value")) -> input

str(input)
## List of 2
##  \$ command: chr [1:6] "forward" "down" "forward" "up" ...
##  \$ value  : int [1:6] 5 5 8 3 8 2
``````

The rules (link above) were pretty basic, increment/decrement some variables based on the command input, but I wanted to avoid a bunch of `if` statements. Since R has the `get()` function that enables searching by name for an object, we can make a series of functions that have the command as the identifier and then use `get()` to call the function:

``````up <- \(x) depth <<- depth - x
down <- \(x) depth <<- depth + x
forward <- \(x) position <<- position + x

position <- depth <- 0

for (idx in seq_along(input\$command)) {
get(input\$command[idx], mode = "function")(input\$value[idx])
}
``````

(the final answer is computed by `position` X `depth`).

While I find this to be a “fun” solution, I’d strongly suggest:

• avoiding using the new shortcut function declaration in mixed R version shops as it’s very new and likely to be confusing to new R users
• being wary of the `<<-` assignment operator as it’s introducing a side-effect (parent/global environment modification) which will come back to bite you in other projects some day
• ditching the `\$` referencing in favour of `[[]]` / `[]` to avoid partial name matching “gotchas”, and
• adding explicit documentation to what you’re doing with `get()` calls (provided you really have a good case for using `get()` to begin with)

The code only changes slightly for part 2, so I’ll refrain from adding even more unreadable code from this post.

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## 6 Comments →2021 Advent of Code Day 02 “Don’t Try This At Home” Edition

1. Martin Møller Skarbiniks Pedersen

You can solve 2021-day1 easy with tidyverse::lead()
eg. for part 2:
result 0)
cat(“2021-day01-part2:”, result, “\n”)

1. hrbrmstr

Or I can use base R and one package with much, much lighter dependencies :-)

``````sum(
diff(
RcppRoll::roll_sum(
scan(