--- title: Distress signal, pattern matching, eval, method - Advent of Code 2022 Day 13 with Ruby slug: distress-signal-pattern-matching-eval-method-advent-of-code-2022-day-13-with-ruby published_at: 2022-12-13 19:00:16 +0000 updated_at: 2026-03-04 20:15:11 +0000 summary: description: Distress signal, pattern matching, eval, method - Advent of Code 2022 Day 13 with Ruby Challenge: https://adventofcode.com/2022/day/13 Solution: https://gist.github.com/37763ea6c5998f3178b201170c01d72e #ruby #adventofcode tags: [cjav_dev, web development tutorials, web development for beginners, vim, ruby, advent of code, adventofcode, aoc, advent of code 2022, advent of code 2022 day 13, advent of code 2022 day 13 ruby, advent of code day 13 with ruby, ruby solution, ruby tutorial, ruby pattern matching, ruby eval, ruby method, ruby method instead of block, how to pass a block argument ruby] views: 191 author: CJ Avilla url: https://www.cjav.dev/videos/distress-signal-pattern-matching-eval-method-advent-of-code-2022-day-13-with-ruby youtube_url: https://www.youtube.com/watch?v=k2MojjRowik youtube_id: k2MojjRowik embed_url: https://www.youtube.com/embed/k2MojjRowik thumbnail_url: https://i.ytimg.com/vi/k2MojjRowik/hqdefault.jpg type: video --- # Distress signal, pattern matching, eval, method - Advent of Code 2022 Day 13 with Ruby *Published: December 13, 2022* *Views: 191* ## Watch [Watch on YouTube](https://www.youtube.com/watch?v=k2MojjRowik) [![Distress signal, pattern matching, eval, method - Advent of Code 2022 Day 13 with Ruby](https://i.ytimg.com/vi/k2MojjRowik/hqdefault.jpg)](https://www.youtube.com/watch?v=k2MojjRowik) ## Description Distress signal, pattern matching, eval, method - Advent of Code 2022 Day 13 with Ruby Challenge: https://adventofcode.com/2022/day/13 Solution: https://gist.github.com/37763ea6c5998f3178b201170c01d72e #ruby #adventofcode ## Transcript hey what's up welcome back in this episode you'll see how to solve day 13 of the Advent of code for 2022 with Ruby this one is called distress signal so we're still trying to get back in contact with the elves and this one is going to be working through handling some packets determining whether or not they are in the correct order so this is some of our example input here we'll start off by just grabbing all of these different arrays of integers and dropping them into our terminal all right so again at the end we will drop in our example input and as we're going through parsing these packets it's going to tell us that a packet data lists of lists and integers and each list starts with a square bracket and thankfully in Ruby this is exactly what lists of lists look like and so we're going to actually just use eval to convert these directly into Ruby arrays so as we're going through and comparing the two values we're going to have this like left and right concept and then there's a bunch of rules that we want to compare and for the first part of our exercise we just want to tell whether or not the pairs are in the right order and the way that we do that is by going through these set of rules so first let's get our input into some nice formats we'll say data is data dot yeah dot read then we can say data.split in this case we want to split on two new lines so that we can get the pairs of packets so for now we're just considering each pair of packets and we're going to tell whether or not this is in order and then after we've got our new lines we want to split those on their individual new lines now we have an array of like two different arrays and then we want to map into their actual values so let's let's look at what we've actually got here we'll print out this result after parsing that far and you'll see that we have this array of arrays this is our first pair of packets and then we have our second pair of packets and so on and so forth so what we want to do now is go through each of these pairs and evaluate the packet using eval so that we get back a ruby array so what we're going to do now is we're going to say dot map and ins as we're mapping over each Tuple of packets we're going to have we're going to have to like so this 0 1 is going to be a pair then we want to map over each of those and we want to call eval on it so one way would be to just say like eval you know a or something like have this one taken a and then eval a and that should give us back our result that we want so that's that's actually evaluating that into Ruby but I wanted to talk about how you can pass a method instead of a block here so we've seen this thing where we could say like you know maybe dot map and colon Chomp and this will use the chomp method on each of the elements that we're mapping over so this is calling the chomp method on the on each individual element but if we have some function like eval here that's available on kernel that we want to call on each of the elements instead of just passing in map colon eval we can't we can't do that because eval is not a method at least as far as I know is not a method of string and so in order to Eve to evaluate this we can pass the Ampersand and then method which is another method it's a method for looking up methods and then eval as as the argument there so this is a cool little shortcut to passing functions as arguments into other methods that are expecting blocks all right so now that we have all of our packets let's start to determine whether or not they are in a valid order so let's make a new method here valid that will take in our left and right I'm just going to use l and R because we're going to use these a lot and the first rule says that if both values are integers so what I I am going to do is use our pattern matching again so we're going to say case for left and right if they are both integers then we want to do something so if they're both integers the lower integer should come first so if the left integer is lower than the right integer the inputs are in the right order oh the other thing that I wanted to do is say like as we're comparing the values in the lists the first value is called the left and the second value is called the right so we're not actually going to evaluate left and right directly we're going to evaluate like the head of L and the tale of L as separate things so I wanna this is a way that we're gonna kind of like look at the first element of each of the packets individually so we're going to look at L head and our head as individual elements one at a time so if they're both integers if the left integer is lower than the right integer then the inputs are in the correct order so we'll say return true if L head is less than R head now if the left integer is higher than the right integer the inputs are not in the right order so we're going to return false if L head is greater than R head okay otherwise the inputs are the same and we're going to continue checking in the next part of the input so at the bottom here we want to finally call something like valid for L tail and rtail now this is going to get a little bit messy so that's just one of our rules and we have two or like three rule sets that we want to Define so what I want to do at this point as always is say if ARG v.empty we're going to require R spec auto run we're going to write some tests so we're going to say our spec dot describe it works for the example input okay and then I'm just going to grab all of the example input and we'll split this up into like lots of different test cases so we'll drop that all in there okay so then here I want to say a oops a equals this b equals this expect valid of A and B to be true and this walk through gives us that pair one is in the right order pair two is in the right order so we should have like true true false false and then the last one is false and at the beginning of all these we need a equals a equals okay so now that we have a test we can run this and our spect respect oh gosh that's funny okay our spec dot describe okay no matching pattern error all right so this is good so now we want to go in and look at our pattern our patterns again so this in the second rule we see if both are both values are lists so if we're in Array array Town compare the first value of each list then the second and so on if the left runs out of items first the inputs are in the right order so what this is saying is we need to again like recursively call this valid method for each of the items in L head and our head and so we're going to say result is equal to valid of L head and R head and then it goes on to say if the values if the left list runs out of items first they're in the right order so I'm just going to add like this this stop condition since we're starting to add recursive calls in here I'm going to add a stop condition that just says return return true if l dot empty okay and then we're going to say look at the next part of this if the right list runs out of items first then the inputs are not in the right order return false if Arda empty okay if the lists are the same length and no comparison makes a decision about the order continue checking so if the lists are the same length and we go through and we compare all of the items and nothing makes a decision meaning like maybe they're both lists of one one and we check the first one we check the second one and then both L and R are empty then we're gonna we're gonna hit this case but it's actually not true because we need to go on to like the second group of elements for that packet and so here I'm actually going to return nil if L is empty and R is empty that way if we compare the head and it ends up being the same then we need to move on to the tail and so in this case if result is nil so something like result.nil so if the result is nil then we want to continue checking the tail otherwise result so here we're going to have two early returns this one we're going to run the same recursive validity function on the left and on the right and then otherwise we're going to get this result so let's see if we changed our test output okay so no matching pattern we got another no magic pattern error for an array and an integer so that's going to bring us to our last rule so if exactly one value is an integer convert the integer to a list which contains that integer as its only value so here we're going to say something like if we're in Array integer town then we want the result to be something like the array being the left head and the integer being the r head we're going to stick that inside of an array and recursively call valid and we want to do the same thing if we end up with an integer and array we're just going to do the opposite so we'll send the left head inside of an array in the right head just like that all right now let's run our tests and see what we get okay expected false but we got true for which use case Okay so we're getting an error here or we're getting a failure where it says it should be false but it got true so line 63 so line 63 is this one okay so in this case uh why is this failing so oh you know what these are mixed up a that should be a and that should be B okay that's easy fix okay all right so now what we need to do is it says what are the indices of the pairs that are already in the right order so the first pair has an index one the second pair has index two and so on so they're not zero index they're one indexed and the above examples the pairs are that are in the right order are one two four and six so the sum of these is 13. now that we're able to map over our input and we have a working valid function now let's map each of our pairs into whether or not they are valid so we'll say that each with index because we're going to need the indices right dot inject zero so we're going to add up the indices if they're valid so here we're going to get this is our sum and then we're going to get a and b right and we want to increase our sum if it's valid if if a and b are valid we want to increase by the index plus one otherwise we're going to add zero and I think this yeah this one needs to be in quotes and let's just see what we get all right we got 13 back that is the test answer so let's run it against our own input so we're going to open day 13. input we'll copy our puzzle input here and we got 13 again that's because we need to put okay so if arcv is empty bottom all right so we get 5675 5675 is our answer that's part one okay so we now know at least where we have some of the packets in the right order we're ready for part two okay so in part two now we need to put all of the packets in the right order so in part one we were just looking at pairs of packets and in part two we need to figure out the entire order of all of the packets we received and we also have these special distress signal protocol divider packets so we're going to look at every single packet in our example list and then we're also going to have to add in these two divider packets and then we're going to sort the entire list of all of the different packets so that they are correctly in order and that they include our divider packets and after we've done that we're going to find the indices of the divider packets and that is going to be our decoder key so we're going to take like the indices of this 2 and the 6 and then we'll multiply them together and that gives us what our decoder key is all right so let's jump into part two so let's comment this out and we have this is so for part two because we are not looking at pairs anymore we're actually going to change how we're decoding this so we're going to split on just a single new line and then we're going to reject if any of those lines are blank basically so we're going to say reject if it's if it's empty so let's run this against our just basic test input and we should get back okay so now we have these arrays of arrays that where each packet is sort of a string so now we want to like again run map and evaluate each of those let's see what we get back here now we need to go through each pair of these and sort them so now what we can do is we can use the sort method which takes in a block where we can pass in A and B and we can say if a and b are valid then we want to return negative one otherwise we want to return one and that will like automatically sort for us so let's see if that works all right so now these are in sorted order except we still haven't figured out how to like inject two and six so what I think we can do is just after we've evaluated we can say dot push and we'll push on these decoder packets or like these yeah I don't know whatever these fancy packets are that need to be added in there as extras and now if we run this again we should get something that looks correct so I see the 2 is in there and the six is in there and they seem to be in the right orders because fours are coming before it and sevens are coming after it so I think that is correct now we want to find the indices of two and six let's just store this off as like with decoder packets and then we'll just I guess we can say a is with decoder packets dot find index of two and we'll do the same thing for B and then we'll just p a times B so if we run this we get back 117 oh it should have been 140. this is also off by one so we'll just or it's supposed to be one indexed so we'll add one to each of those and we get 140. okay so that should work so let's run it against our puzzle input and we get back 20 383 20 383 was the puzzle answer so that's part two okay so there was a couple things going on here so let's take a look at so I think the most interesting for this session is probably using this this method method to grab a method off of that's just like available in scope and then applying that to each of the elements using map so that's cool and then also if you haven't seen this sort method that you can take in A and B so that's going to kind of like look at pairs of elements as it's running its sort algorithm so you could imagine like a bubble sort right where you where you just kind of like go through each pair and say like is this one bigger than the other one if it's not then leave it if it is then swap it and then you just like do that several passes until they're all sorted obviously it's going to work a little bit smarter than that I can't I don't actually know which sorting algorithm it uses under the hood but really all we need to do is pass it some function that will value that is negative or positive I think this will work with any numbers it doesn't have to be negative one it could be like negative four and ten or something and that'll continue working as long as the one of them is negative and one of them is positive I believe this should work this also can work with the comparator so I've seen this before with you know you can just do like a spaceship B and then depending on whether or not the underlying objects that are stored in variable A and B respond to this spaceship operator that will that'll work I think this is actually the default so when you call sort without passing in a block it will execute the comparator function which again gives you back negative one zero or one depending on what how you've implemented this and that'll sort all of your elements so in this case we're just like hard coding a a check to see you know whether our pair of packets is in a valid order and if not then we're going to return you know negative one otherwise we're going to turn one and that'll get us in the right sorted order and again we're using yeah pattern matching this time with types in order to go through and write all of our rules for validity that was a fun one thanks again so much for watching really appreciate your time and attention hopefully this was useful and we'll see you in the next one cheers --- [Back to all videos](https://www.cjav.dev/videos) --- ## Hey Agent, 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