--- title: each_cons, and layers 🎂 - Day 09 - Advent of Code 2023 slug: each_cons-and-layers-day-09-advent-of-code-2023 published_at: 2023-12-09 15:09:59 +0000 updated_at: 2026-03-04 20:15:21 +0000 summary: description: Learn how to solve Day 9 of Advent of Code 2023 - Mirage Maintenance - in Ruby! In this episode, we'll cover: Parsing the input data into statistics Breaking down the statistics layer by layer to differences Building back up each layer to extrapolate the next value Collecting layers into an array of arrays Predicting future and past values by adding to array ends Refactoring with destructuring for conciseness The solution demonstrates some neat iteration techniques in Ruby: each_cons to yield consecutive pairs unshift to add to array front map, select, inject to transform arrays We walk through the thought process, test examples, and refine the logic until we have scripts that generate the right answers for both parts of this puzzle. If you're participating in Advent of Code or just want to level up your Ruby skills, join me for this festive programming challenge! Advent of Code: https://adventofcode.com/ My Solutions: https://gist.github.com/cjavdev/d15a2a4ffed6c840c2fb28a093e9f927/ Playlist https://www.youtube.com/playlist?list=PLS6F722u-R6KYlGyUv65EFpGKl2Esmurr #adventofcode #ruby tags: [cjav_dev, Learn to code, Beginner ruby, Advent of code, Advent of code 2023, Advent of code ruby, Aoc ruby, Aoc 2023, Vim, Advent of code vim, Advent of code explainer, Advent of code challenge, Code challenge, Advent of code tutorial, Web development tutorial] views: 165 author: CJ Avilla url: https://www.cjav.dev/videos/each_cons-and-layers-day-09-advent-of-code-2023 youtube_url: https://www.youtube.com/watch?v=bEJ0HSKzBCo youtube_id: bEJ0HSKzBCo embed_url: https://www.youtube.com/embed/bEJ0HSKzBCo thumbnail_url: https://i.ytimg.com/vi/bEJ0HSKzBCo/hqdefault.jpg type: video --- # each_cons, and layers 🎂 - Day 09 - Advent of Code 2023 *Published: December 09, 2023* *Views: 165* ## Watch [Watch on YouTube](https://www.youtube.com/watch?v=bEJ0HSKzBCo) [![each_cons, and layers 🎂 - Day 09 - Advent of Code 2023](https://i.ytimg.com/vi/bEJ0HSKzBCo/hqdefault.jpg)](https://www.youtube.com/watch?v=bEJ0HSKzBCo) ## Description Learn how to solve Day 9 of Advent of Code 2023 - Mirage Maintenance - in Ruby! In this episode, we'll cover: Parsing the input data into statistics Breaking down the statistics layer by layer to differences Building back up each layer to extrapolate the next value Collecting layers into an array of arrays Predicting future and past values by adding to array ends Refactoring with destructuring for conciseness The solution demonstrates some neat iteration techniques in Ruby: each_cons to yield consecutive pairs unshift to add to array front map, select, inject to transform arrays We walk through the thought process, test examples, and refine the logic until we have scripts that generate the right answers for both parts of this puzzle. If you're participating in Advent of Code or just want to level up your Ruby skills, join me for this festive programming challenge! Advent of Code: https://adventofcode.com/ My Solutions: https://gist.github.com/cjavdev/d15a2a4ffed6c840c2fb28a093e9f927/ Playlist https://www.youtube.com/playlist?list=PLS6F722u-R6KYlGyUv65EFpGKl2Esmurr #adventofcode #ruby ## Transcript what's up welcome back in this episode you'll see how to solve day nine of the Advent of code for 2023 this one's called Mirage maintenance we're still traveling through the desert island with some Elves and uh in this scenario we are going to get some output from an oasis and sand instability sensor which I don't know it's giving us some output here which has some readings and we want to predict a future reading so there's some instructions for how we're going to do this the way that we work through each of these statistics so like each row here represents a different statistic and we want to try to guess the next number in the row and the way that we do that is for each one we're going to break it down until we get all the differences are equal to zero so we extrapolate by saying what is the difference between the first and second element oh it's three and what's the difference between the second and third element that's also three third and fourth three and then once we have all the differences from the first set of Statistics we can then say what are the differences of those and once those are all zeros we know that we've reached the bottom and then we want to build back up by adding a zero to the bottom row and then the previous Row's last element becomes uh something that we can derive from the difference so then we can say oh the next element in this layer is a three because the difference of uh 3 + 0 is going to be three so then we have from 3 to three the now we know the difference between 15 and the next element is going to be three so we do 15 plus 3 and we get the next elements we're backing into a last element here of each of these rows and our answer for part one is to go through each metric figure out what its extrapolated next value is sum those up and call it good all right so let's grab all of this input here we're going to say input is equal to our new input array and what we can do is we'll say data is equal to input. each line and I think that's it for now so now we want to split these into statistics or stats or something so we'll map each line and split it and then map those to integer so let's see what stats is we run this Ruby main. RB we got a syntax error where's our error input is assigned but not used oh we have double equals here what the double equals that's not going to work okay so now we have these arrays of 0 3 69 1215 0 3 69 1215 that's our parsing our input now what we want to do is for maybe we can just grab one stat and pull pull it off and we want to say uh we want to like work through this stat itself looking at each pair and figuring out what the difference is and that's going to give us the next layer so we can do stat. each cons so that's going to give us the each consecutive two elements and that will yield an A and A B so that's going to be like the left element and the right element and we want to take B minus a and then we can actually pop up map onto the end of this and that should give us back the next stat so if we P next stat here we get back all threes so from the first statistic our next layer is all threes and then if we were to do the same thing again so maybe if we just copied this and pasted it for a second and then print that out you'll see that we get all zeros and also another thing that you might notice is that each layer has one fewer element right like the first row starts with six elements then the next row or the next layer is five elements then four and once we get to four elements and we have all zeros that's when we can stop we need to break up this iteration and do something like until stat is all zeros we want to do this yeah this is using right word assignment I've started to like popping on right word assignment after the end statement just because it like I don't know it it feels like it's moving the process forward instead of having something up here but for now I'll just keep it like this and okay so now if we print out stat at the end we get all zeros so if we print stat at each intermediate step here what does that look like okay that's going to give us threes and then zeros now so we're correctly like breaking down to the zeros but how do we build back up so in order to build back up we need to keep track of what each layer looks like and so the way I solve this is by creating a list of layers and the first thing in that list is the first Stat or like the like the biggest record now what we want to do is like at each layer we want to uh add onto it the current stat as we're breaking it down so we're going to say layers do unshift stat this will make it so that we are pushing on those new stats to the front of the list of layers so that the layers end up looking our original is at the end and then we have I don't know derivative the first derivative and then we have this second derivative and then eventually we have all zeros so it'll look like this that way when we iterate later we can go go layer by layer until we get to the end instead of having to like iterate through the layers in Reverse so this just helps us for later so now we have zeros threes and then our final element for stat that's looking good so this is like part one is like breaking it down figuring out a generalized solution to break it down to all the layers and then the next step is to go through each of the layers we want to go starting with the bottom layer we want to add a zero to this one and then move up to the next layer and so on popping on our predicted next value based on what the previous value was so again we can use each cons because we want to look at each consecutive layer and we're going to do previous and next maybe as the names for our layers and we can break out previous yeah so we're going to have previous do last will give us the last element in the previous array in this case that's going to give us kind of like the Delta and the Delta tells us how much we need to increase the last element in the next array and add onto it so we'll say something like next. push because we're adding something onto the end of the next array as we're iterating through each pair and the element that we're going to add is Delta plus next. last okay so then we should be able to print out layers here and see that we've added on elements okay so we add we correctly added on a three here and we correctly added on an 18 here so this is the key this is the key to the puzzle is like we want to find all of these last elements on the last array which was our original this is the number that we actually just predicted right we predicted that the statistic would go 0 3 6 9 12 15 18 and we need to do that for all of the stats so now that we have the list of layers what we can do is wrap this all up into a function we'll call it predict and this is going to take in some stat and then it will return layers. last we just want we want to take in the statistic and return the statistic but with the new element at the end so that should help us here so now we should be able to say predict stats. first and predict stats St first why isn't that work oh here okay so now we get we went from the original to now having the final one that's great because now we can say stats. map stat and that should give us the list of all of the different stats which this is all right so this list of stats gives us 18 28 68 those are the numbers we need to add up so now we can say map last. suum and now we get 114 and that matches the puzzle example4 awesome now let's grab our puzzle input we will drop it at the end and now we're going to switch our data to be data. readlines and run it again and we get this huge number 1 90121 boom that's our puzzle input or a puzzle answer for part one so that's it so we're like breaking down the layers we're building back up to predict the next layer and that is part one in part two we want to extrapolate backwards so instead of adding on to the end we're going to add on to the beginning and so this was this is actually like a relatively simple change rather than pushing onto the end here this is where we're like adding on to our predicted value instead we want to unshift and push something on to the front and in this case the Delta is based on the first element in the in the previous array so instead of Delta here I'm going to use pr. last and let's also use like pr. first minus next. first I think that that should maybe work let's see what we get for our uh map Let's test it against the example input so this should expand the layers out in both directions so let's actually let's let's P predict stats. first again and just see what we're going to get back here okay so now we have a -3 at the beginning and that looks correct So based on our test input yeah doing this for the example we would get -3 uh Zer and five so it would give us -2 as like the total so if we print this out we got -22 Okay so we've got something wrong here we want to take the next oh this is out of order this is this needs to be next. first minus previous stuff first right okay there we go -3 0 and five and if we sum those up we get two okay awesome so now we can test with our input and we get 95 which was our answer for part two that's our puzzle answer for part two awesome so that is a working solution so we used yeah we built up these layers and collected up a list of lists and each list of lists let us build onto the ends of either one of the things that I thought about for making this like even cleaner is breaking or using a d structuring destructuring here array destructuring to grab like the P1 and then ignore the middle and then grab P2 this is like our Deltas so now we can say subtract P1 and um we can actually do like next. last plus P2 I think that should give us the same answer 905 so that's looking good and then this we could probably unshift the entire thing all at once and simplify this map into underscore 2 minus underscore one and using the named arguments or whatever the named block arguments is yeah this one just a lot of messing around with arrays I was thank ful that it worked and fit in memory because I think there's probably also a solution where you can you don't have to go as wide as all these numbers necessarily right if it if you end up with some set of zeros at the bottom that is bigger than three then you can trim the edges if you need to but yeah glad we didn't have to do that in this exercise all right that's it thanks so much for watching if you're following along with the series hit that subscribe button and we'll see you in the next one cheers --- [Back to all videos](https://www.cjav.dev/videos) --- ## Hey Agent, thanks for visiting! 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