---
title: transpose, flatten, described_class, bingo solver! - Advent of Code 2021 - Day 4 with Ruby
slug: transpose-flatten-described_class-bingo-solver-advent-of-code-2021-day-4-with-ruby
published_at: 2021-12-11 00:00:05 +0000
updated_at: 2026-03-04 20:13:45 +0000
summary: 
description: transpose, flatten, described_class, bingo solver! - Advent of Code 2021 - Day 4 with Ruby  00:00 Part 1 22:00 Part 2
tags: [cjav_dev, web development tutorials, web development for beginners, vim, ruby, rails, advent of code, advent of code 2021, day4, bingo, transpose, flatten, described_class]
views: 253
author: CJ Avilla
url: https://www.cjav.dev/videos/transpose-flatten-described_class-bingo-solver-advent-of-code-2021-day-4-with-ruby
youtube_url: https://www.youtube.com/watch?v=JeMIC7tYxFY
youtube_id: JeMIC7tYxFY
embed_url: https://www.youtube.com/embed/JeMIC7tYxFY
thumbnail_url: https://i.ytimg.com/vi/JeMIC7tYxFY/hqdefault.jpg
type: video
---

# transpose, flatten, described_class, bingo solver! - Advent of Code 2021 - Day 4 with Ruby

*Published: December 11, 2021*
*Views: 253*

## Watch

[Watch on YouTube](https://www.youtube.com/watch?v=JeMIC7tYxFY)

[![transpose, flatten, described_class, bingo solver! - Advent of Code 2021 - Day 4 with Ruby](https://i.ytimg.com/vi/JeMIC7tYxFY/hqdefault.jpg)](https://www.youtube.com/watch?v=JeMIC7tYxFY)

## Description

transpose, flatten, described_class, bingo solver! - Advent of Code 2021 - Day 4 with Ruby

00:00 Part 1
22:00 Part 2

## Transcript

hey what&#39;s up welcome back in this episode we&#39;re gonna solve day four of the advent of code for 2021 it is essentially building a bingo game where we get a list of the numbers that are called in order and then we get a series of different boards and we have to go through and mark off the boards as we go so we&#39;ll jump into our code i&#39;ve already got sort of some of the rspec stuff set up so r spec dot describe and we&#39;re going to have a board class and it is going to just tell us whether or not we have bingo on the board so i guess we want to describe the bingo method and we can make that end in a question mark because it&#39;ll tell us true or false whether or not the board has bingo so it um return or like is false for an empty board and we&#39;ll just create a new board b is board dot new and we want to pass every board is going to be five by five um so i think we&#39;re just going to do like yeah how do we want to do this um like nil i guess array.new5 and then every time we want to create a new array of five nils and that is what we want to pass into board dot new so let&#39;s see so that should give us like our blank board expect be to or like not to be bingo and then we can run it and it&#39;s going to say we don&#39;t have a board class so uh uninitialize constant board oh and also when you&#39;re using rspec.describe here for a class name there was a comment from cinon that said instead of writing the class name every time you can use described class which rspec sets for us to prevent typos so here instead of bored i guess i can use described class dot new which is a little funky because we&#39;re calling dot new on this thing there&#39;s also like a concept of a subject or let or whatever so we&#39;ll play around with described class and just see how that works for us so uninitialize constant boards we want to go back over to our bingo here and say class board and i guess yeah let&#39;s let&#39;s split like that and then we can run our tests again wrong number of arguments one given zero expected definite and we want to take in our um grid or something at grid is grid um and we can have a reader for our grid it&#39;s like whatever maybe we want to just keep that internal maybe we don&#39;t but whatever and then now it&#39;s telling us what is the error here expected not to be bingo expected blah blah blah to respond to bingo so it doesn&#39;t have the bingo method yet so we need to add def bingo we&#39;re just changing the file enough to get a a different output and our test is actually passing now so we need to write a new test it is true for a bingoed board so we can create another board here and in this case what we want to do is instead of passing in this array or i guess we want to say like grid is equal to array dot uh grid is equal to this thing so yeah so i&#39;ve been using vim for a long time um or like since 2005 16 years or so on and off so i used vi for a bit then i used visual studio for a while so let&#39;s mark off grid at zero i and we&#39;ll do this for 5 times 5 times we&#39;re going to say i and grid at i equals x and um that should make it come off as bingo to be bingo um okay so we&#39;ve got inside of our board now we have like a bunch of x&#39;s so we want to check and see if we have x&#39;s so if a board is is bingo the um either one of the rows will all be marked off as x or one of the columns will all be marked off as x so one thing we can do here is actually we could call this rows row or grid and then we could have a new method columns which is at rows dot transpose which should work and then okay so for bingo we want to see at rows.any are there any rows where all the where the entire row um is like where all of the cells i guess we want to say okay so row where all of the elements in the row cell where the cell is equal to x i think that&#39;s right um or if any of the columns um are bingo so let&#39;s see all right so we&#39;ve got we&#39;ve got a passing test so let&#39;s try let&#39;s like comment out columns and uh it still passes so this is going to be for a bingo board with a row and then we also want one that works for a column so it&#39;s true for a bingo board with a column and here what we can do is we can just swap our i for for that and that should actually fail right now yeah because it needs this and cool so now we have all three tests passing so we&#39;ve got we&#39;ve got the concept of bingo or like a board can be bingo meaning like a board can have five in a row either a row or a column all right let&#39;s do the next step here which is to um we want to take in some list of numbers and some like list of boards and then we want to like start marking the numbers on that board i guess we also need some way to like mark a number right so like describe uh mark do and um and this is just convention putting the octothorpe pound sign hashtag thingy at the beginning of the name of a method so marks a number uh okay so then if we have um let&#39;s say that we create this grid and then at grid at zero zero is equal to five and then if we say we pass in the grid and if we see b dot mark five we would expect that b dot rows at 0 0 to equal x so we wanted to like mark off like by calling mark 5 we wanted to find the 5 and replace it with an x so let&#39;s see if this works notify method mark def mark okay expected that got that so now we want to say something like um rows.each um with index do row x and then row.each with index do column y and then we say like if the column or this is kind of like the cell if the cell equals the number that was passed in then we want to mark at rows at x and y and we want to set that equal to x i think that should work let&#39;s see all right cool so all of our tests are still passing we&#39;ve got the ability to mark a board and yeah i think that looks pretty good okay so now we want to start working on the actual bingo class so r spec dot describe bingo do and and we want to say something like it um plays the numbers or something or like plays a number i don&#39;t know i&#39;m not sure what this is going to look like so bingo is bingo.new and we want to pass in i guess like our input is going to be a bunch of numbers and a bunch of boards or something so i don&#39;t know if we want to like pass in an actual board or if we want to pass in um like just like an array of boards and then an array of yeah i think we need an array of boards and we need an array of numbers so oh right and then here we got described class instead of that uh and then here what we want is like i guess we kind of want to create um board is board dot new we can&#39;t use described class for this part because board is not the described class bingo is the described class so here we&#39;re creating a new board one two three four five and then six seven eight nine ten okay so we want to create a new instance of bingo passing in the first board and really we&#39;re going to pass a list of boards in and then we want to pass in some list of numbers that&#39;s going to get executed so one two three four five maybe and then we want to expect um bingo dot winning board to equal board i don&#39;t know like we haven&#39;t even defined the bingo class yet so let&#39;s do that class bingo uh all right what does that look like wrong number of arguments so it&#39;s going to take in boards and numbers boards is equal to boards numbers is equal to numbers and then we need a winning board method and we also need i guess maybe we need a method like play bingo dot play which actually like executes um each of the numbers so like numbers dot each do n and then boards that each do board and for each number we want to mark the board or dot mark n um and then if board.bingo then we want to set winning board is equal to board and return or something and then here we&#39;ll just return at winning board i guess i don&#39;t know um boards numbers all right we wrote a lot of code and we didn&#39;t really uh write too many tests for this play method but we&#39;re sort of expecting that it&#39;s going to mark stuff off uh okay so expected board got nil uh okay so if board.bingo winning board is bored and return uh winning did i spell that wrong winning board winning board huh all right so then we&#39;re gonna yeah let&#39;s drop in a buy bug and we will put it here now when you&#39;re when you&#39;re trying to figure out like where to actually drop in the buy bug um you can set break points inside of blocks but it&#39;s kind of tricky and you don&#39;t want to like step into the each method and so i usually try to like put the buy bug call inside of the block so we have board it looks like that n is whoops puts n is one so now we should um see board and that should be marked off with an x in the one position and it is um board.bingo should be false and it is so we continue continue continue continue board um okay so we&#39;re about to mark off um five so now we say next or dot bingo should be true and it is and then we should set winning board to board and then return at winning board is board and then we return bingo dot winning board is that board and board okay wait what okay so that totally worked why didn&#39;t that work the first time all right so yeah it&#39;s totally working i don&#39;t know why that test failed maybe it just didn&#39;t save or something um all right so we&#39;ve got the concept of a winning board now if we go back to the exercise here it wants us to report it by summing up the unmarked numbers and multiply that by the number that was just called so i guess we want to say like a last call or something like last number is n that way we have the last number and here we can just say def like i don&#39;t know score or something i don&#39;t know and then we want to have some expectation down here so like it works for uh well okay so hmm which board won this board one and yeah well let&#39;s just let&#39;s just kind of like keep it e well okay yeah so oh this is fun this is fun so we want to know the sum of the numbers between 6 and 25 and i think that&#39;s going to be so if you&#39;re if you sum all the numbers from 0 to n the value is n times n plus 1 divided by 2. n times n plus 1 all divided by 2. and so for this case i think we can take um like i think what we want to do is say we expect the score our bingo dot score to equal um something and that something is going to be the sum of all those numbers times the last number that was called and i think the last so the last number that was called is 5 times and then we want to have like 25 times 26 divided by 2 divided by 2 and that whole thing minus well what is 5 times 4 times 3 okay so 5 times 6 divided by 2 is 15 minus 15. uh it&#39;s actually we can just do this 25 times 26 is 650 divided by 2 is 325. so 325 325 minus 15 discrete math for the win so then that&#39;s 3 3 10 was it times 5 is 1 1 5 1 okay that&#39;s that&#39;s wrong um okay so uh let&#39;s do this again so i think this is supposed to equal one five five zero like i think that&#39;s the key for this board i don&#39;t know we&#39;ll see so all right so the score i think is going to be um winning actually we&#39;ll just return winning board dot score and in fact let&#39;s just let&#39;s just call yeah let&#39;s just let&#39;s just call it on the winning board itself or we&#39;ll define the method on the board itself and yeah we&#39;ll just delegate down to it def score is oh the score for the board is going to be the sum of the remaining numbers so we can say like at rows.flatten dot reject um if it is in x dot inject plus so we&#39;ll like okay so what we&#39;re doing here is flatten is saying take that 2d array and make it a 1d array so squish everything into 1d that gives you all the elements then reject or remove all of the the elements that were marked as x and then we&#39;re injecting a plus meaning like add those up okay so that should be the score the winning board score times last number and so that should give us 1550 and it does okay so i guess that might get us pretty close so if file is equal to dollar zero then we want to read in our input so we&#39;re going to add a new example input here and this is going to be equal to all of this stuff so this is the example input we want to compare the example input to our ultimate real input and i guess what do we want to do here how do we want to actually read this in we want the first line that&#39;s for sure that&#39;s going to be our numbers and then everything else is going to be the board lines so i guess we could say like numbers comma board lines is equal to file.readlines of argv.first and numbers is equal to numbers.split on comma dot map are like dot chomp dot split on comma dot map two i p numbers let&#39;s see if this works ruby day 4 bingo dot rb day 4 example input we get back all the numbers that&#39;s great okay and then we want to take these boards and iterate over each of those board lines so board lines i each do line if or like next if line dot chomp is equal to nothing otherwise we want to collect up uh we want to collect up the things into rows so like rows is equal to an empty array so we want to say rows dot push line dot split on space and map to i so that should give us a bunch of rows and then if rows dot length is equal to 5 we&#39;re going to create a new board and the board is going to be board is board.new rows and then we&#39;re going to set rows equals to empty array and we will say boards actually we can just say boards.push okay and then we&#39;ll set that back to an empty array after we&#39;re done iterating over all the lines we can create a new bingo bingo.new for the boards and the numbers and then we say bingo dot play and we say bingo puts bingo dot score and four thousand five hundred and twelve is that the example answer 4512 cool so then we get our puzzle input which is this giant thing here and we jump in here input drop in all of those answers and then we run this again but instead of example input we&#39;re running on input and 2496 2496 was our answer so that is working so that&#39;s cool that&#39;s the end of part one for day four advent of code solving a bingo puzzle super fun part two on the other hand it might be wise to try a different strategy let the giant squid win okay so to figure out which board will win last i guess what we want to do is instead of returning right here we want to say like if board is bingo and winning board is nil then set the winning board in the last number but don&#39;t return because we want to play through all of the different numbers to see which winner is the last winner so we want to say something like uh do we need to know the last number two the last once it wins what would its final score be okay so we need to keep track of um what is the last board that is going to win bingo and so what we can do is say like if boards dot um reject b beat up bingo dot count is equal to one um non-winning boards is equal to this uh i guess we could also make a method for that so like uh losing boards is boards without any that have already won so we&#39;re saying like only give us the boards that have not won or i mean maybe to make this a little clearer we could do select select the ones that are not bingo um and then we can say something like um if losing boards.length is equal to one then we say at losing board is losing blue zing boards dot losing boards dot first and then what we need to do is uh keep track of what the last number is okay so after we mark the losing board we have to say something like if losing if the losing board is not nil uh okay maybe it doesn&#39;t really matter where this is at so if the losing board is not nil and losing board bingo then we say at losing number is equal to n so like let&#39;s say we&#39;ve exhausted all the boards and we have one losing board left that losing board is not necessarily bingoed yet so we have to go through all the numbers we want to keep track of it as soon as we get to the last one we know that&#39;s the losingest but we want to keep going through all the numbers and continue marking them off until we find the one um that has that so maybe like def losing score is at losing board dot score times losing number and maybe we can just say whoops um losing score and uh oh undefined method plus for nil class hmm did we okay so what do we want okay so the once it wins what would its final score be so okay so what we want to do is we want to see are we um getting into either of these also this losing boards thing this is going to be super slow but whatever uh and okay we&#39;re running we&#39;re running we&#39;re running we&#39;re running actually you know what we can do this on our example input it should be faster example input okay so we got to our losing board losing boards is one board and it has not been marked off uh for 20 yet and so we&#39;re setting the losing board um at losing board dot bingo it should be false it is false but then we should get to a number okay i guess we only need to set that once uh oh right that&#39;s probably what that&#39;s probably what happened um we only want to set that if there is we only want to set this if there&#39;s no losing board so if at losing board is nil and the length is one then we want to set that otherwise we don&#39;t yeah we don&#39;t want to because it will result in something uh okay undefined method times for nil class so what is being nil p at losing actually yeah buy bug bye bug there&#39;s also like a new debugger losing board dot score whoa the score is nothing because we filled the entire board okay losing board.score or zero at losing number should be 20 okay uh oh this needs to be also like a return after we find the losing number we should return uh okay 1924 was that the answer here 1924 okay cool so now we should be able to run it on our own input and wow it&#39;s slow wow super slow two twenty five thousand nine twenty five twenty five thousand nine twenty five that is the answer okay so yeah uh the whole crux of the game was in this play method where we iterated over each of the numbers to try to like play each number and when we played each number we iterated over each board and if the board was bingo uh we marked it and found like our winning board otherwise we tried to find our losing board and our losing number cool so that wraps day four for the advent of code 2021 thanks so much for watching and we&#39;ll see you in the next one [Music] you

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