Day 1

Advent of Code: Worked Solutions

Setup

# Libraries
library(tidyverse)

# Read input from file
input <- read_lines("../input/day01.txt", skip_empty_rows = FALSE)

Part 1

Extract numbers from the text strings using regex and sum:

input |> 
  str_extract_all("\\d") |> 
  map(as.integer) |> 
  map_int(~ head(.x, 1) * 10 + tail(.x, 1)) |> 
  sum()

[1] 54390

Part 2

Add patterns to the regex for spelled-out digits. Search from the front of the string as usual, but for the last digit, search from the end by reversing all strings. Otherwise, regex will only recognize the first match in case of an overlap: “eightwo” needs to be recognized as 82, not just 8.

digits <- c("one", "two", "three", "four", "five", "six", "seven", "eight", "nine")
pattern_fwd <- str_c(c("\\d", digits), collapse = "|")
pattern_bwd <- str_c(c("\\d", stringi::stri_reverse(digits)), collapse = "|")

# Match the first digit from the front of the string
d1 <- input |> 
  str_extract(pattern_fwd)

# Match the second digit from the end of the string
d2 <- input |> 
  stringi::stri_reverse() |> 
  str_extract(pattern_bwd) |> 
  stringi::stri_reverse()

# Convert to integer values and sum
map2(d1, d2, c) |> 
  map(~ coalesce(parse_number(.x, na = digits), match(.x, digits))) |> 
  map_int(~ head(.x, 1) * 10 + tail(.x, 1)) |> 
  sum()

[1] 54277