Advent of Code 2017 in #rstats: Day 2

After reading this puzzle, I was excited and concerned to see “spreadsheet”:

  1. Excited: R is good at rectangular numeric data, and I work with a lot of spreadsheets
  2. Concerned: Will I be starting with a single long string?  How do I get that into rectangular form?

After considering trying to chop up one long string, I copy-pasted my input to a text file and read.delim() worked on the first try.

Part 1 was simple, Part 2 was tricky.  The idea of row-wise iteration got in my head and I spent a lot of time with apply and lapply… I still hold a grudge against them from battles as a new R user, but having (mostly) bent them to my will I went to them here.  Maybe I should spend more time with the purrr package.

Part 1


Part 2

Got the idea quickly, but executing took a bit longer.  I decided right away that I would:

  1. Work on a function that tackled a row (as a vector) at a time
  2. The function would divide every value in the vector by every other value in the vector
  3. It would then pick out the integer that wasn’t 1, as well as its reciprocal, and divide them
  4. Then run this function row-wise on the input

Actually building up that function took ~20 minutes.  I felt confident the approach would work so didn’t stop to consider anything else… I’m excited to see other solutions after I publish this, as maybe there’s something elegant I missed.

Along the way I learned: is.integer doesn’t do what I thought it would.  Come on, base R.

I don’t work with matrices much (not a mathematician, don’t have big data) and was a somewhat vexed by the matrix -> data.frame conversion.


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