---
title: Regex, towers, and each_with_index - Advent of Code 2022 - Day 5 with Ruby
slug: regex-towers-and-each_with_index-advent-of-code-2022-day-5-with-ruby
published_at: 2022-12-05 18:00:15 +0000
updated_at: 2026-03-04 20:15:10 +0000
summary: 
description: Solution for Day 5 of the Advent of Code in 2022 with Ruby. In this solution, we'll talk a bit about regex. The hardest part of this problem, in my opinion, is deserializing the input into useful data structures.  Challenge: https://adventofcode.com/2022/day/5 Solution: https://gist.github.com/6519248312ec4d6596ddd320583ba685  #ruby #adventofcode
tags: [cjav_dev, web development tutorials, web development for beginners, vim, ruby, advent of code, advent of code 2022, ruby tutorial, advent of code day 5, advent of code 2022 day 5, advent of code ruby, ruby solution, ruby regex, towers of hanoi, advent of code tutorial, web development tutorial for beginners, ruby tutorial 2022, ruby tutorial for beginners 2022]
views: 371
author: CJ Avilla
url: https://www.cjav.dev/videos/regex-towers-and-each_with_index-advent-of-code-2022-day-5-with-ruby
youtube_url: https://www.youtube.com/watch?v=NNd7se8NlrU
youtube_id: NNd7se8NlrU
embed_url: https://www.youtube.com/embed/NNd7se8NlrU
thumbnail_url: https://i.ytimg.com/vi/NNd7se8NlrU/hqdefault.jpg
type: video
---

# Regex, towers, and each_with_index - Advent of Code 2022 - Day 5 with Ruby

*Published: December 05, 2022*
*Views: 371*

## Watch

[Watch on YouTube](https://www.youtube.com/watch?v=NNd7se8NlrU)

[![Regex, towers, and each_with_index - Advent of Code 2022 - Day 5 with Ruby](https://i.ytimg.com/vi/NNd7se8NlrU/hqdefault.jpg)](https://www.youtube.com/watch?v=NNd7se8NlrU)

## Description

Solution for Day 5 of the Advent of Code in 2022 with Ruby. In this solution, we'll talk a bit about regex. The hardest part of this problem, in my opinion, is deserializing the input into useful data structures.

Challenge: https://adventofcode.com/2022/day/5
Solution: https://gist.github.com/6519248312ec4d6596ddd320583ba685

#ruby #adventofcode

## Transcript

foreign what's up in this episode you'll learn how to solve day five for the Advent of code in 2022 with Ruby day five is called Supply stacks and what we're doing is working with a giant cargo crane to move some stuff around so uh this is our input this is our puzzle input for today and we're going to sort of like move these elements from one stack to another and uh this reminds me a little bit of this project or this problem called Tower of Hanoi towers of Hanoi and this is kind of a game where you can move pieces from one Tower to another now this is a little different because there is no such like size of disc but it's similar in that we have some instructions that we're going to receive and we want to move things from one Tower to another so in this first problem what we're going to do is like if we take a look at our input our test input sample input here you'll notice that the first instruction says we want to move one element from Tower two to Tower one so we want to move this D from Tower 2 to Tower 1 and then we'll end up in this second configuration now in the second step it says we want to move three crates from stack one over to stack three but you'll notice that we have to move the crates one at a time so first we move the D over and we put that on top of the P then we move the n and put that on top of the D and finally we move the Z and that ends up on the very top of this third Tower finally we're going to move both of the elements from stack two over to stack one again one at a time so move the C first then we move the m and it ends up on top of it so for me looking at this problem there's two there's two parts of it one is the deserialization like how do we uh you know read this in as text and build up a data structure that will work for us and then number two how do we go through and sort of like execute these instructions in the right order and move them over and to the right Towers Etc so what we're going to do is grab all this open up our day five and at the bottom here again we'll drop in our test input and now what I want to do is um let's just let's just start off uh the smart way and instead of having to debug a ton we're going to require our spec auto run and I'm just going to say rspec dot describe and what we want to do is have a couple different tests one is going to be like it you know um it reads the towers as expected so we just want some sort of test that's going to help us out here and so the first thing I want to do is sort of like read in this data so I'm going to say before all I'm going to read in like at data and or like the at the towers and at instructions is going to be something like data.read dot split on two new lines okay so this is going to read in all this data down here and it's going to split on two new lines so the first chunk is going to end up in towers and the second chunk is going to end up in instructions so here because we're using um this like two variable assignment we're going to split or we're going to like deconstruct the array that returns from this split operation into these two towers are these two data things and then we can use that and say something like Towers is uh read towers of at Towers and then we're gonna do something like expect uh Towers dot length two equal three because we're expecting that there's going to be three ultimate Towers at the end and we're gonna expect that Towers like the first Towers um length is going to equal two because that's going to be just n and Z we can also make expectations about what's in there so just to like really help us out with like what um how this is being broken down so we're going to expect the the first element in this first Tower is z and the second element is n okay all right so this should get us this should get us going so we can run this Ruby day 505 okay undefined method retower so let's go Implement read Towers it's going to take in some data I don't know some Towers yeah let's call it data and now we're going to say like data Dot split so this Tower thing we want to split it on new line to get each of the different rows so we're going to say data.split on new line and that should give us some rows that's going to be different than what we actually want though okay expected three but we got four okay the other thing we can do when we're using rspec is if we want some other um information about the error we can add a comma after this equals and just say Towers just so we get like the actual raw data that we can take a look at so here we see this is what we're actually getting back so this is pretty good so we have a couple different elements and each element right now is sort of like a layer in the stack it's not a tower it's a layer and so we want to sort of just throw away this one two three we don't actually care about the labels what we're going to do is use the the longest row in a layer as the reference to like how long the row is going to be so now that we have these split on new line we also want to map these like we want to map over these and split them again and in the case where of D or like this first layer like this D is in the second tower so we need some way of knowing that like okay if there's four spaces we need to ignore that but we need to consider that being like a tower or like an entry so we can keep track of which Tower D is in so what I want to do is say I'm going to map the line and I'm going to split it on um a four space white space mark and let's see what we get here okay great so now we have the first element is broken up correctly so now we have an empty string which references sort of a space above the first Tower where nothing actually exists so this is kind of like representing this empty space and then we have D so that's good if we look at the second row though we're still not splitting this one up so in addition to splitting on four white spaces let's also split on a single white space and we end up like this so this is looking really good right we've got um empty the empty space we have D we have n c those are split up and then zmpa split up and then we still need to discard sort of this last one to ignore this last row what I think I want to do is just reverse the entire array and then start iterating over it so that we're starting from like the base level and working our way up and then at that same time we can sort of exclude the labels so let's say reverse and then we'll say dot each with index and each row or level and its index is going to give us something so now we can say next if I is equal to zero just like skip over if if I is equal to zero so each with index is going to yield the um the elements in the array it'll also give us this I which is kind of like the just the index that we can keep track of as we go so we're going to skip over if the index is zero so that we ignore the labels and then otherwise what we want to do is now we want to take those levels and we want them each or we want to like iterate over those so now we want to say something like level dot each do element and for each element we want to skip the element if it's an empty string right so we're going to say next if L is an empty string otherwise we need to start sticking these things into a tower and again because we're starting at the base like we're starting them in reverse order um we should be able to like put these into some new array so let's create a new array called towers that we will return uh and this Towers will keep track of all the different Towers so um yeah so we kind of also need to know which Tower we're entering into so we want to use each with it index again here so we'll use uh J so if Towers at J is empty then we want to set it equal to a new array otherwise we're going to shovel the element into the array and let's see what we get okay so I think or our test output change and I think we're like getting close to the end here right like now we're getting this but we should yeah let's just look at what towers looks like at that point too okay so now our towers are set up like this so we have an array with z and n and we have an array with z and N that's good and then MCD MCD and finally we have P so they're all looking great the only thing that's wrong now is that we still have like the square brackets around these we got to do some uh something to remove those so let's right here we can just I don't know I think this is a good spot we'll just say level.map and we'll say for each element L.G sub and we will just um remove if it has a square brackets and if we run this boom passing test fantastic okay very cool uh okay so now we are able to read our towers and they should be in a format that makes sense and is useful for us the other step or like the next step here is that we need to like make sure that it reads the instructions so it reads the instructions okay so we're going to say instructions is read instructions and we want to expect that the instructions length is four and we also want to I don't know we can make we can actually just say like uh we expect it to equal something like um I think we just want like an array of arrays where it's like one two and one three one and three and then we'll we'll know like how to implement the move method based on that so let's say one two one uh three one three and one two two one one two I wonder when GitHub copilot will be able to listen to like what you're saying and then figure out stuff uh okay so let's uh run our tests our instruction thing should fail because that method doesn't exist so let's write a new method here it's actually uh yeah so def well whatever um read instructions data so we're gonna say again data.split on new line um and then we want to let's see so for the instructions I think we want to we could just use regular expressions with capture groups so this is another feature of regular Expressions where we can say like oh if it starts with move whatever from to um yeah so we want to map it and we'll say that for each line we want to match against some thing and instead of putting one here we can use a capture group and we'll say n and N is going to be a in some number value and we'll use the same pattern for each of the numbers so this is going to be from and that's also a number and then this is going to be 2 and that's also a number and that should match okay so let's just see what we get for read instructions so we'll just P instructions here and then run our test and we got okay so we get all these match data objects and you'll notice that when we get a match data object with a capture group there's these sort of like um symbols here it's which we can sort of treat as a hash the match data class has a square bracket operator where we can pass in the name of the Matched thing and we'll get back the value so this will give us like one two one so that's fantastic so let's um yeah so then what we can do is say like we want to map over the matches and convert them to integer so we got n from N2 to I and instead of getting back a hash we want to just get back an array so we'll just say like give us give us back an array with all of these three things inside the array and that should be good so let's see what we get here oh we have a passing test fantastic okay so that's how we read instructions we're just going to read all the instructions with sort of regular Expressions now we need a way to actually modify a tower so we we're reading in the towers reading in the instructions now we need to apply the instructions incrementally to the towers so let's let's write another another test down here we're going to say like it moves elements or something and so first we're gonna read the towers and then we are going to read the instructions and maybe we have like a new method called like move and it's going to take in the towers and let's actually try like a single instruction at first so it's it's going to take in the towers and then it's going to move some number um of elements from some Tower to some Tower so this is going to be the first the first arguments right move one from two to one so move one element from Tower two to Tower one and then we can make expectations about the the resulting Tower so now we can say we expect Towers uh zero that two equal um so after we make that move it should be z and d or something like that like Z n D I believe so let's run this undefined method move we haven't implemented that yet so let's say def move towers and then we have some number and we have from and two and we actually don't want to do pop we just want to do pop one so pop is going to remove an element from the end of this Tower and this shovel operator is going to put it into the end of this Tower so we are operating on the tops so this is kind of like our crane operator but we want to do this n dot times is like how many times we want to move something um okay we expected znd but we got ZN so um all right is this actually running towers.2 Towers we're moving from Tower two to Tower one ah right because this is going to be off by one so this is gonna be we want to just like follow the instructions and um the index into the array is not actually one based it's like zero based so that should get us that should get us moving all right let's also make some expectations that it was properly removed from the other Tower so we're gonna say this Tower two should have M and C so Tower two should have M and C okay and Tower three should still just have p all right we'll run our example again okay still passing all right that's fantastic all right so that moves a sort of like a single element or like moves an element and then what if we move all of the elements so it works for the full test case okay so then in this case what we want to do is we want to sort of like move all let's let's make a new method called move all we'll give it towers and the instructions and then we'll expect that um yeah we'll we'll write some expectations about it but um after we do then what should we get at the very end we should get sort of this format so expect Tower zero to equal just C expect Tower one expect our uh two so Tower two should be p d and z p d n uh Z it might seem like a lot to go and Implement all of these tests but I promise that it actually saves you time uh I mean I don't know there's like these people who solve the admin of code problems super super quickly I don't know how they do it all right we gotta go Implement our move all method and this is going to be something like yeah move uh for each instruction and to okay so we're going to call move for each of those instructions all right fantastic um cool all right run this passing test okay so now what I want to do is let's say that we're only going to do this R spec auto run thing like if arc V is empty and then otherwise we're going to read in from the file so we're going to say uh like Towers instructions is file dot read RV split on that read the towers read the instructions move all from the towers to the instructions and then there was one last sort of piece where we want to print out the like the last element of the Tower so Towers dot map uh last dot join I don't know let's see if that works and now we are ready to go grab our example input so we need to grab our puzzle input here I'll grab all of this create a new file called input paste it in and then we'll say Ruby day five input and this is our output rlf NTB blah blah blah which is the puzzle answer for part one fantastic all right so we're moving towers around we're following instructions and it's all working we could definitely refactor I mean we're 20 minutes deep into this thing so not all of these work so quickly all right part two uh it turns out that the crate mover or the crane called crate mover 9000 was actually a little dirty and it's not crate mover 9000 it's a crate mover 9001 and instead of moving one crate at a time it actually moves multiple so when it's moving its crates if you're moving uh when as we're following the instructions instead of just moving one crate at a time it's gonna move that number of crates at a time so we actually just have to make one small adjustment here because inside of our move method this is where it matters so the reason we did we used the number n here and we said n times we want to pop off the top and put on the bottom or move it from the top of one Tower to the top of the other Tower and then it would kind of like do that n times what that did was made sure that we were moving one at a time end times but now what we want to do is we're going to modify this for part for part two and instead of moving in times we're going to move n elements so we can use pop with an argument and we can pass it the number that we want to pop off and I think here we want to do plus equals because we don't want to shovel an array in as an element of the other thing we want to just move um we want to concatenate the existing tower that is um going to receive the elements we want to concatenate whatever's there if there's zero elements if there's five elements whatever we want to concatenate what's already there with the elements in order from the other Tower so we're going to pop in off of the other Tower we're going to concatenate those on with plus equals and so now the from Tower will have n fewer the two Tower will be concatenated with the rest and we can pop in our input here and day five input and we get mhq tlb whatever whatever oh boom that's our answer for part two okay so it was just an Ever So slight change in this move method all right let's let's talk about just a couple of different things so the trickiest part I think of this solution was kind of deserializing the towers maybe there's also a lot of regular expression stuff going on here so if you are unfamiliar with regular expressions in Ruby I would like highly recommend checking out rubular rubular is like this really fun ex regular expression editor so for instance when we were talking about building the regular expression for this move from one to two here you can have like move and then you can put your capture group in here and and that's going to be D plus so now you can see that the match groups are n and you can also see like okay this is going to match on move one and then you can say like okay from and then uh two and this is also going to be another integer so now you're seeing the match groups show up and you're also seeing kind of like your um your different test cases so now you can say like oh what if it you know was different numbers or whatever like that'll show you kind of like the different matches as you go along so rubular super handy uh the other thing that was kind of interesting here I guess we just we use reverse and this is the first time in this year that we've used each with index which again yields the index there um anything else fun and funky about this solution uh one other thing that uh like can be kind of useful is instead of saying like instructions that each do with n from and two split up here we could just say like instruction and then I think we can Splat that here instruction I don't know it's like a super small little adjustment but um we get the same answer there so that is working uh I guess the other thing is that we finally got like our spec behaving nicely with auto run inside of the same file where we also have sort of this stuff at the end here um yeah so it is it is not the same like pure Ruby pipeline uh there is a way to rewrite all of this to continue being a pipeline but I thought it would be easier to understand if we broke it up into a few different methods and also really easy to like kind of surgically come in and make changes for part one and part two so hopefully that was useful thanks again so much for watching and hanging out hope you're enjoying Advent of code for 2022 and uh yeah we'll see you next time cheers [Music]

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