---
title: set intersection with & - Advent of Code 2022 - Day 3 with Ruby
slug: set-intersection-with-advent-of-code-2022-day-3-with-ruby
published_at: 2022-12-04 20:30:02 +0000
updated_at: 2026-03-04 20:12:59 +0000
summary: 
description: In today's exercise, you'll see how to use the & operator to find the intersection of two sets to solve day 3 of the advent of code 2022.  Challenge: https://adventofcode.com/2022/day/3 Solution: https://gist.github.com/cjavdev/864bec6b1b6bc423be487d234009f9bf  #ruby #adventofcode
tags: [cjav_dev, web development tutorials, web development for beginners, vim, ruby, advent of code, advent of code 2022, advent of code ruby, advent of code 2022 ruby, advent of code 2022 day 3, advent of code tutorial, ruby tutorial, intersection, set intersection]
views: 300
author: CJ Avilla
url: https://www.cjav.dev/videos/set-intersection-with-advent-of-code-2022-day-3-with-ruby
youtube_url: https://www.youtube.com/watch?v=wksc9evwjHA
youtube_id: wksc9evwjHA
embed_url: https://www.youtube.com/embed/wksc9evwjHA
thumbnail_url: https://i.ytimg.com/vi/wksc9evwjHA/hqdefault.jpg
type: video
---

# set intersection with & - Advent of Code 2022 - Day 3 with Ruby

*Published: December 04, 2022*
*Views: 300*

## Watch

[Watch on YouTube](https://www.youtube.com/watch?v=wksc9evwjHA)

[![set intersection with & - Advent of Code 2022 - Day 3 with Ruby](https://i.ytimg.com/vi/wksc9evwjHA/hqdefault.jpg)](https://www.youtube.com/watch?v=wksc9evwjHA)

## Description

In today's exercise, you'll see how to use the & operator to find the intersection of two sets to solve day 3 of the advent of code 2022.

Challenge: https://adventofcode.com/2022/day/3
Solution: https://gist.github.com/cjavdev/864bec6b1b6bc423be487d234009f9bf

#ruby #adventofcode

## Transcript

what's up welcome back this is day three of the Advent of code and you're gonna see how to solve it with Ruby so today's problem is called rucksack reorganization and the input looks scary it's like all of these random numbers but as you read through it we're still on day three I think these just continue to get more and more challenging and you know increasing in in difficulty as we go throughout all the Advent of code this is day three of 25 so it should still be relatively easy so let's jump into it so each of these rows represents a rucksack and each rucksack has two compartments in it so you know when you're like opening up the zippers for your pockets I know my kids like they're like Dad where did you put my snacks is it in the small front pack or the big back compartment or is it in this other little pocket these rucksacks of the elves are carrying they just have two giant zipper compartments and they're the same size this is like all of the items that are in the first half and this is all the items that are in the second half something happened where everything is not in the right compartment and so what we need to do to help the elves reorganize is figure out which items appear in both compartments and then we're going to just like figure out basically what we want to do is figure out which letters are in the first half and the second half of each rucksack and there should only be one occurrence so for this first rucksack if we look at the first compartment that is like v j r all the way through R so that's going to be this first half of the rucksack and the second half is going to be HCS blah blah and there's only one item that appears in both the first half and the second half and that is lowercase p so the first part of the challenge is go through each of these rows and we want to split them up into the first compartment second compartment and then figure out what is like the one thing that's on the left and on the right so we're going to copy this let's head over to our terminal and we're going to make a new day three directory here that is a directory day three and then we're going to open up day three day three to RB and let's just dump this in again at the end as our rucksacks so what we want to say is our data is going to be data.readlines and then we can say let's just do like P data dot or maybe a result is data dot we want to remove all the new lines map Chomp and then we want to say for each of the rucksacks I think we want to split them into all of the individual characters so yeah let's do like dot map can we do map chars chars like that does this give us what we want I don't know let's say p result and we'll come over here and say Ruby day03 day three and okay this is looking pretty good so this gives us an array of arrays where the inner arrays are each of the rucksacks with their entire compartment right and so now what we want to do is take that and cut it in half so we want the first half and we want the second half so what I think we can do here is then map it onto another thing where we can say like one dot each slice where each slice is the length of half of the result so under one dot length and I think that should give us or should split it up into a bunch of half things I guess that gives us back in the numerator so we just call 2A on that and then I don't know it's whatever we'll call that good all right so this is giving us now an array of arrays or it's an array of arrays that are each two elements long so here is the full thing wait no that's wrong so this should that's an array of arrays that are the full length so we need length divided by two because we want to split it in half so let's try this again there we go okay so now each inner array has like this first part and a second part and I kind of like recognize that this is ending in WR this one's starting with HCS so we've I think we've like figured out the compartments in the backpacks correctly now what we want to do is figure out which elements in the left or in the first compartment are the same as the elements in the second compartment for this we can use set intersection and so now I think what we want to do is just map each of those onto the first and in fact can we do that here can we just say left and right is equal to this and then when we get that no that okay so this is what we're going to do we're going to now say dot map and this is going to give us we're going to yeah so we'll we'll use array destructuring in the arguments so here we'll say left and right and then we want to say left intersect okay let's see if this works boom all right that's it now we have the items that are the the items that are like the same in the left section in the right section so if we again look at the first compartment here the lowercase p is in the first compartment and if we look at the second compartment lowercase p is in the second compartment and so that is like our first answer and then if we look at our examples here they tell us that okay in the first rucksack we're looking for lowercase p so that works let's look at the others Big L big p VTS Big L big p VTS good okay so now we've got the right letters now we can just say flatten and now what we want to do is figure out what are those priorities so the priorities here to help prioritize the item Arrangement every item type can be converted to a priority lowercase items a through lowercase z have priorities 1 to 26. so again we can use like ASCII their ASCII character code thing and so again we want to map this onto something and this is going to be like yeah so this is going to be underscore one if if I guess we can use regex here so if it's in a through z then we want it to be um so if it's a capital letter that's going to be the values 27 to 52. so then we want to grab the ORD for the capital letter and there is this ASCII chart here look how old school this is It's So pixelated and gnarly but if we look in here we should find character a capital A has the ASCII value of 65. so if we want the uppercase letter the uppercase a should have the value 27 so I don't know 65 minus 27 so 38 so I think we need to subtract 38 from this so minus 38. I think that gives us what we want otherwise we want to go all the way back for lowercase A through Z so let's look again so lowercase a is 97 and in the lower case so lowercase should give us one so we want to subtract 96. I think this gives us what we want let's just see in this case we get 16 38 42 okay so let's see if that matches over here 16 38 42 22 20 19 22 29. okay so this is exactly what the example suggests and then we want to sum we want to sum these up so we can probably I don't know we can make this another one-liner and then dot sum I love these pipeline things you can do in Ruby it just makes it so clean 157 is the example input so what we want to do is grab our example output so we're going to copy all of the rucksacks from the day 3 output and we'll open up day three input and now what we want to do is open up our implementation here and we'll say that data is actually file.redolines of org V Dot first and then we're going to say Ruby I guess we have it over here Ruby day three and then day three input and we get 7997 which matches my puzzle output great that's part one so in part two we need to finish so we finished identifying all the misplaced items we like put them all in the right places but now there's some like concept of badges so each of the elves are divided into groups of three so when we're looking at our input here like each of these chunks of three lines is going to be the rucksacks for three elves that are in a group of three they're and for each sort of threesome there's going to be a value that is going to be the same in all three of the rucksacks and so we want to figure out what is the value in this first threesome that is the same and then we want to look at the second threesome and see what is the same value in there so in the first group the only item that appears in all three rucksacks is a lowercase R so we see there's lowercase R in the first line there is a lowercase R in the second line and a lowercase R in the third line so it's very similar to the first problem right but instead of comparing the two compartments now we need to compare across three things so what we want to do here is we're going to open up day three and rather than just splitting on we'll just actually comment this comment this out so this is going to be part one and then for part two what we want to do is it's going to be super similar right so we're going to use the same approach where we're going to do like this intersection logic thingy but instead of slicing the compartments in half here what I think we want to do is we need to figure out how to group them by threes and yeah so here let's use each slice again but instead of using each slice to kind of like split it in half we're going to use each slice to give us each sort of group of three and here we'll just say 2A just so we can see what the result looks like and we'll just run this against our our example input okay so here we get an array of arrays the first one is the first three rucksacks for the first threesome of Elves and the second three is the second three elves and now what we want to do is get the characters for each of the elves rucksacks and then do the intersection of those so what I think we can do here is say map each of the groups and then for each group we want to map and get the characters thank you GitHub copilot we'll run this again and see what we get okay so now we have an array of arrays and that looks good okay so now what we can do is say map and do the same kind of like comparison between left and right but instead of using left and right now we'll just use one against two against three and that should give us like the intersection of one two and three and here we have lowercase R and uppercase Z so that's like exactly what we wanted again and I think the priority the priority for this second problem is going to be exactly the same so then we find the the priority for the items we sum them up and we should get some answer so we're going to go through here and run this again and okay oh because we need to we still need to flatten and then we can run this again boom we get 70 that is the test answer let's go up here enable our file version and now we can say day three with the input and we get the value 2545 and that is my answer so if you're going through this if you're following along at home and you're grabbing the input for your puzzles your answers are going to be different than my answers all of these are just like custom generated there's a really cool talk on YouTube that you can head over and watch it is all about how the Advent of code is put on it's really really interesting this guy Eric he goes through and collects and builds out all of these problems and it's just such a fun time in December every year so I would yeah if you get a chance Reach Out say thanks to Eric and yeah again if you're gonna if you have your own puzzle you're gonna get a different answer so you can't just type in my answers but hopefully you can follow the same patterns and I'm also hopeful that you're learning a couple things here and there the things that were interesting about this episode was using each slice and we used it twice once was to like split or rucksack in the two different compartments and the second was to group like threesomes of our elves into into groups that we could then use to figure out which badges were in the compartments for those elves here we're also using regex and I didn't really talk about this too much but this this right here is saying okay with whatever letter is passed in as the first argument to the block to map we're going to use the I don't know this someone called this something really funny I thought it was like it looks like a cigarette with like smoke coming out of it or something and so we're comparing that against the regular Expression A through Z and that is inside of these square brackets so this is basically checking to see is one a capital letter A through Z and if it is we're going to subtract 38 from that because we want the value to be 27 28 29 30 etc for a through z if it's Capital here this is going to be like this is a ternary operator so we're saying like evaluate the condition on the left and if it's true so if it's a capital letter we're going to subtract 38 from the ASCII value for the character otherwise if it's not true so if it's not a capital letter if we've got a lowercase letter we want to subtract 96 from the ASCII value that brings us all the way back down to one for lowercase a so I don't know if this is how other folks are approaching this but this makes it relatively straightforward also if I'm using like way too many like fancy features of Ruby let me know down in the comments sometimes I don't know for this year I I think it's pretty fun to just go through and build these pipelines that are just like you know step after step after step after step and then just spits out a value at the very end Ruby is just amazing what a beautiful language thanks so much for watching really appreciate your time and attention hope you're having fun with the Advent of code and we'll talk to you next time

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