Solving Advent of Code 2020-03 with R
[Disclaimer] Obviously, this post contains a big spoiler about Advent of Code.
Instructions
Step 1
The input is a map (x, y), where
.is a square,#is a tree.We can move this way: 3 right (x + 3), 1 down (y - 1).
The y repeats itself indefinitely.
We start from the top left corner (coordinate x = nrow(map), and y = 1).
We move until we’ve reached x == 1, i.e by taking nrow(map) steps.
How many trees
#have we met along the way?
Step 2
Repeat this but modify the step schema.
Find the complete instructions at:https://adventofcode.com/2020/day/3.
R solution
Part one
library(purrr) n_trees <- function( by_x = 1, by_y = 3 ){ ipt <- read.delim( "2020-03-aoc.txt", header = FALSE, stringsAsFactors = FALSE) # Building the path # x need to go from 1 to nrow(ipt), 1 by 1 x <- seq(1, nrow(ipt), by = by_x) # y needs to start at 1, and go by 3 steps until we have length(x) steps y <- seq(1, by = by_y, length.out = length(x)) list( x = x, y = y ) %>% pmap_dbl(~ { # Which x step are we in? row <- ipt[..1,] # If the row isn't wide enough, expand it until it is # There is probably a better way to do that but I'm not # sure I want to spend 5 minutes trying stuff that will # save me half a micro second while(nchar(row) < ..2){ row <- paste0(row, row) } # Split the damn thing row <- strsplit(row, "")[[1]] # look for the y, is it a tree? if (row[..2] == ".") return(0) if (row[..2] == "#") return(1) } ) %>% sum() } n_trees() ## [1] 278 Part two
data.frame( x = c(1, 1, 1, 1, 2), y = c(1, 3, 5, 7, 1) ) %>% pmap_dbl(~ n_trees(..1, ..2)) %>% reduce(`*`) ## [1] 9709761600 
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