Day 3

Advent of Code: Worked Solutions

Setup

Import libraries:

library(tidyverse)

Disable scientific notation to display all digits of large numbers:

options(scipen = 999)

Read text input from file:

input <- read_lines("../input/day03.txt") |> 
  str_split("") |> 
  map(as.numeric)

Part 1

For each number, take the largest possible single digit NOT in the final location, and combine it with the largest following digit:

input |> 
  map_dbl(\(x) {
    tens <- which.max(head(x, -1))
    ones <- max(tail(x, -tens))
    x[tens] * 10 + ones
  }) |> 
  sum()

Part 2

Define a recursive function which looks for the first through nth digits:

joltage <- function(input, n, init = "") {
  
  if (n == 0)
    return(as.numeric(init))
  
  posn <- input |> 
    discard_at(length(input) - seq_len(n - 1) + 1) |> 
    which.max()
  
  joltage(discard_at(input, 1:posn), n - 1, str_c(init, input[posn]))
  
}

Run on puzzle input with 12 digits and sum the result:

input |> 
  map_dbl(\(x) joltage(x, 12)) |> 
  sum()