---
title: Check and checkmate - Implementing Chess with Ruby for beginners
slug: check-and-checkmate-implementing-chess-with-ruby-for-beginners
published_at: 2021-06-02 17:00:03 +0000
updated_at: 2026-03-04 20:14:10 +0000
summary: 
description: In this series you'll learn object oriented basics while building a simple terminal based Chess game with Ruby.  In this episode, we implement the logic to see if a player is in check or in checkmate.  Playlist: https://www.youtube.com/watch?v=Zc0ZuH3Qpxw&list=PLS6F722u-R6LZxh7mvEUvZKshcH6KSNhr Code: https://replit.com/@CJAvilla/chess#main.rb
tags: []
views: 543
author: CJ Avilla
url: https://www.cjav.dev/videos/check-and-checkmate-implementing-chess-with-ruby-for-beginners
youtube_url: https://www.youtube.com/watch?v=4tpxeDJTQqk
youtube_id: 4tpxeDJTQqk
embed_url: https://www.youtube.com/embed/4tpxeDJTQqk
thumbnail_url: https://i.ytimg.com/vi/4tpxeDJTQqk/hqdefault.jpg
type: video
---

# Check and checkmate - Implementing Chess with Ruby for beginners

*Published: June 02, 2021*
*Views: 543*

## Watch

[Watch on YouTube](https://www.youtube.com/watch?v=4tpxeDJTQqk)

[![Check and checkmate - Implementing Chess with Ruby for beginners](https://i.ytimg.com/vi/4tpxeDJTQqk/hqdefault.jpg)](https://www.youtube.com/watch?v=4tpxeDJTQqk)

## Description

In this series you'll learn object oriented basics while building a simple terminal based Chess game with Ruby.

In this episode, we implement the logic to see if a player is in check or in checkmate.

Playlist: https://www.youtube.com/watch?v=Zc0ZuH3Qpxw&list=PLS6F722u-R6LZxh7mvEUvZKshcH6KSNhr
Code: https://replit.com/@CJAvilla/chess#main.rb

## Transcript

welcome back you and i are working on this chess game uh in the last episode we worked on building out the game and sort of like some of the play loop logic where you can pick a piece up and move it around to another part of the board we talked about dependency injection and a couple other really cool things about object-oriented programming what i wanted to do in this episode was talk about seeing if a board is in check so how do we figure that out well in chess right like you are in check if your king is in danger so if your king is in danger that in in our like world right so a piece has some set of moves so we've got steppable and slidable right and if if the available moves of a piece of one color includes the position of the king of another color then the king the person yeah the person who's king is um in the available moves of a piece means it's sort of like in danger or uh could be killed so that would put them in check so i think what we want to do is add a method to the board class here um to see if a piece is in check or if a current or if a player is intact in check so we want to do like in check here and basically the logic for in check is going to be something like um i guess we need to pass in the color to see if like is black in check or is white in check and the way that this will work is we're going to say like loop over all the pieces of the opposite color and um if any piece has in uh available move with the position of the king with color then color is in check so we want to loop over all the pieces of the opposite color so how do we do that i think it would be uh handy if we had some way to get all of the pieces from our internal representation here of the grid so we want to grab we want some way to get all every single piece um and because our grid is a 2d array there's actually a helpful method here in in ruby that we can use so if we if we open up ir irb not irp so if we open up irb let's take a look at um the flatten method on array so our grid here our grid is a 2d array where we have like some elements in the array these these happen to be pieces and we might have multiple yeah we're gonna have multiple arrays like one for each row on the board there is a method on the array class called flatten so if we just looked at the grid here we could say grid.flatten and that would give us back every single element that from the subarrays flattened out and this turns out this works even if you have like sub sub arrays right so if we put like one in the sub sub array here and say grid.flatten then we get back a super flattened out thing i think you can even pass this level so you can just say like flatten one that'll only flatten it down one level so that you don't end up flattening this sort of like sub array that's over on this level if we say flattened two then it should flatten them all so you can sort of use this flatten method and pass it arguments if you want or you can just say flatten it'll give you like the whole list of things so if we said like flatten over here on our grid then we should get back basically the list of all the pieces and a bunch of nils so in order to remove the nils we can sort of just do something to exclude those so maybe let's make a method here pieces and we're going to say grid dot flatten dot reject uh peace dot nil i think this should give us back all the pieces maybe we can go check this out and say we've got our start chess and then what if we like print out if we print out beta pieces we should just get a bunch of pieces back so if we run this and again this is going to be just like a single array of pieces that allow us to work through this ah look okay so we don't want to use p we want to use puts because now that the pieces have reference to the other uh they have reference to the board the board has a reference to the other pieces so we get a pretty nasty output there so puts um beat up pieces look at that okay perfect so this is going to be all of our pieces on the board and that works great and then we could do something like pieces dot select like p where p dot color is uh black or something to get all the black pieces so that should i think that should work fine for finding like all the pieces of a given color um and maybe we want like a convenience method that's like giving me all the black pieces but oftentimes we're going to be working directly with a color as a variable so let's just jump back into game and mess around with this even more so i'm sorry back into board so we've got our pieces so we want to loop all over all the pieces of the opposite color so we want to say like pieces dot select um where the piece dot color does not equal color so that should give us all the pieces of a different color uh dot each do piece and then we want to see if any of those available moves has the position of the king so it's something like uh ps dot available moves that include um the king position and then we need to figure out the how to find the king position um so if any piece has an available move with the king position then we return true so if this return true we want to explicitly use return here to just like break out and return yes we are in check that king is in check um okay yeah i guess that's part of that's a continuation of this comment here and then otherwise if that's not the case then we're going to return false by default but we don't know what king position is yet so we need to figure out what the king's position is so let's see king pause is going to be i guess we want to find from i guess we can use pieces again so we can do pieces dot select where the pieces where the piece's color is equal to the color that was passed in and the pieces or the piece dot class is equal to king or we can do like p dot is uh and pass in king so is uh this is one of the ways that you can check what kind of thing something is so is uh checks the class of the underlying uh instance there's also a couple other things that this does like it'll go up i think this will go up the inheritance chain so if you you can say like is it if it's a piece and technically it's both a king and a piece because the king inherits from peace but um this should give us the the piece itself then we want to say dot location and that'll give us like the location of the king position um okay i think that looks fine maybe um all right so now in order to test this out rather than just like starting up new chess here what i think we want to do is just say board.new and then stick a like a i don't know a white queen and a black king and then uh check to see if it's in check so we want to say like um b at like i don't know 1 1 is going to be a king.new and it's going to have the board and it's going to be at 1 1 and it's going to be black and then we will create a queen and a queen can move everywhere and she will be at 2 2 and this queen is going to be a white queen and then b dot in check for black should be true and b dot in check for white should be false um because well there is no king position for white uh sure let's just grab um let's grab this and then make a white one that's at like six six and we'll say it's a white this is going to be a white king but we are going to need to handle the case where there is no king of the opposite color on the board which is an exceptional case right um okay no undefined method location for array uh okay and this is on board in the board class again on line 57. i'm going to board line 57 uh no location method on select so select is going to filter down the pieces and return an array so i think we might want to actually do find font maybe it's find by let's see i can't remember if it's find or find by because we wanted to return an actual instance find uh let's see okay there we go true and false so that is exactly what we wanted black is in check white is not in check and we got that that we got that working just fine uh the other thing that we wanted to do here though was say like um i guess this is gonna fail if it doesn't find anything because it's nil uh so i mean there's like this optional chaining thing that happens or that we could potentially reach into and use but um let's just say this is the king we'll say this is the king or maybe king right and then if king dot nil and and then we'll say king pause is equal to king dot location so if the king is nil then we want to raise an exception so we're going to say raise this is like probably never going to happen or shouldn't happen right if you are playing a normal game because the game will be over as soon as we get to checkmate which is that the king would uh has no way of moving around so if the king is nil then we want to raise like uh no king no king found uh sure okay so let's run this again just to make sure we have still true and false we didn't break our code all right so we've got we've got in check working and we're finding the king position for this given color and then if it's nil we're raising an exception we're grabbing the king position we're looping over the pieces right and checking to see if the available moves include that king position now the next piece that we might want to do is check mate right so we have in check now let's do checkmate and this is also going to take in a color and the thing about checkmate is that it's impossible to be in checkmate if you're not in check so let's return false if you're not in check for that color otherwise what do we want to do to be in checkmate well i think what we might want to do if every single piece is unable to move because okay so what is what is actually what is check page so checkmate i think is when you are unable to move at all and you are in check and even if you move a piece you're still going to die on the next move right and so um how do we want to implement this i think we might need to do something where we um what do we want to do huh because when we're looking at available moves right the available moves so far does not consider anything about checkmate or being in check so at this point you can move anywhere that this piece could like based on the pieces possible directions uh and whether it's like slidable or steppable it can move anywhere but we are not disallowing making a move into check so i think what we want to do is add another method to piece that checks to make sure that um not only is the piece available but it also like doesn't put you into check so i think we want to add some other method to piece that says like you uh we need some like higher level method that that doesn't let you move into check so let's go into peace and um yeah so we have available moves so what if we just said like deaf valid moves or something like that or like um safe yeah safe moves and this is going to tell us like it's going to look at all the available moves um what is this going to do so it's going to like look at available moves and it almost needs to try every single available move and make sure that it doesn't move you into check so like you kind of need to go over every single available move and temporarily like move to that position see if you're in check and if you if it does move you into check then you cannot that should not be included in the like resulting moves so this is going to be like another array where we're going to return some list of moves at the bottom and we're going to iterate over the available moves so available moves that each do move and we want to see like we need to like try the move and um if in check uh if not in check then like um add the move to moves so what do we how do we want to do this try the move thing well i think what we might want to do is like technically we could like sort of move to that spot we can move the piece there and then move back or we could we could duplicate the entire board so we could get like a copy of the board try the move on the copy and if we're in check then we don't include it um that way we don't have to like worry about moving the piece and then moving a piece back so that that also brings up some interesting stuff with the board so if we wanted to duplicate the board there is actually a method on ruby objects called dupe right so if we can go to like irb if we had some like x is equal to the string test if we say uh x x dot inspect x or let's say x is player dot new no x is uh let's see require relative lib slash player player dot new um black okay so then like i think player player.dupe yeah okay so this will actually create a new player object a new instance so you can tell because the um the memory address is changing here so we're getting a different instance um so dot dupe i think it's just a method on every single instance in in ruby like it just like comes with an object but we can we can rewrite this or let's actually just play around with it and see what happens with the board so if we said require relative lib slash board dot rb or whatever and then we say b is board dot new or board dot start chess um oh gosh let's do this in main over here so if we were to say b is board dot start chess and then b dot dupe right and then we want to print out like b or let's make one of these things dot new with b dot render um let's take a look at what we get so we've we're this is rendering out the our like the do board is equal to this thing um and that's what we want to pass into the renderer now if we move a piece on the duplicated board dupe or dot move piece from 1 1 2 2 one and then we render it again oops then let's also render the um the original board and see if we can tell any differences all right so let's say puts um puts uh dupe board after move and then this is going to be puts original board after move all right let's run this okay so the duplicated board after the move this is what we expected right we're moving the piece from one one to two one and it is it's moving the piece that's right and then the original board after the move ah so the original board we're actually moving we're moving the piece on the original board also so that is not actually duplicating the underlying grid so we're not deep like deep duplicating so in order to deep duplicate we probably need to write our own method here so let's write um let's write our own method like at the very bottom here we'll say def dupe and what do we want the duplication method to do so this should give us back a brand new board so new board is going to be board dot new and then what we want to do is go over all of the pieces in the current board class which we have a method for that pieces dot each do piece we want to return the new board at the end and what we want to do is on this new board we want to set up we want to set it up so that it has the same piece like the positions of the same pieces so i think we can do new board at piece.location equals piece oh but we want to have new instances of the pieces too because those are going to be moved around so we actually need a new piece so new piece is equal to piece dot class dot new and oh gosh what is this taken so p p stock peace.class is going to be like king or pawn or or queen or whatever and then this new method takes in the board it's a new board it takes in the location so ps.location and it takes in the color piece.color wow okay so this is cool this is cool um so this should give us a brand new piece class or like a duplicated piece class and then we want to we don't want to put the piece we want to put the new piece onto the new board at the pieces the old pieces location which i mean technically we can do like new piece dot location just to be safe and then we're going to return a new board so let's re-run our test here and make sure that the original board is not modified when we are calling dupe on it awesome okay so we have a duplicated board and that's where the move happened we have our original board and there was no move that happened here fantastic all right so this is like how we would implement like deep duplication and the reason that we need to do this is that the underlying objects here so like all of this grid this like 2d array is when we're duplicating the board class we're still maintaining reference to the same underlying array and so we need to like create a whole new set of arrays and a whole new board and new pieces and everything all right so now that we're able to duplicate the board if we go back into our board class for checkmate oh no we were going in peace right we're trying to define valid moves so we want to try the move so we want to like duplicate the board so from piece i guess we could say like new board is board.dupe and then we want to say newborn.move newborn.move piece from our current location to the move and then we want to check like if newborn.in check for our color so if we're in check so if we're not in check actually so if we're not in check then we want to uh we want to add to the moves so let's see here we want to say like moves we're going to shovel in the move all right cool um then the move is valid or safe or something whatever okay so this safe moves is gonna be um this is like available moves that don't also move us into check so let's just add a little comment here so like uh like available moves that don't move us into check okay so back in the board class when we're checking if we're in checkmate we want to see if yeah i think we might actually want to use like safe moves here um and we probably want to use that in a bunch of other places too right so when we move the piece we shouldn't actually be able to move it unless it's a safe move and where else we have this available moves okay let's move that change that to safe moves um available moves okay so i don't see it anymore i do nope available moves no okay um cool all right so let's assume that that is looking good um okay why don't we see if we can figure out how to what do we want to do well we want to put um yeah i think we want to like put the game state again into a state where we uh can check to see if we're in checkmate so if we put the king like in the corner and then we put a rook in a queen next to it somewhere so b at like 0 0 is going to be the king and this is going to be the one that's in check so we've put it at 0 0 or this is going to be the one that's in checkmate and then we want to put it we want to put the queen um at like 2 2 or something or like 2 2 0 let's put a rook at 2 0 b and two zero and this is going to be white we also need a king let's put the king on the other side uh all the way across and we'll make that a white king and then we need another rook um so if we have one at two zero and one at zero two i think uh or zero three i don't know like that should make it so that we have one rook that can go this way and one rook that can go this way to kill the king and then let's also put them three zero actually three one i think might also work yeah i think that should work so that the king is in checkmate then so now if we say b dot checkmate for black is that true we definitely should have some automated tests for this we need to rewrite this with uh with ruby okay so block and initialize is too deep block in safe moves okay so all right board on line 28 we have okay when you see this stack level too deep it means that we're calling like functions and we're getting into some sort of like recursive loop that's going like way too deep so if we go into board 28 i made a change here to available moves that we should not have i'm sorry to safe moves that we should not have so okay yeah so this one wait no safe moves if the safe moves include this but we can't use safe moves inside of in check because in check is used when we're checking to see if a move is safe so available moves oh right okay return false if we're not in check otherwise we need to see if the pieces of this color have no safe moves so if um pieces we're in the board dot select the pieces where the piece dot color is the same as that color now we want to check if any of them have how do we want to do this safe moves right so this should give us the pieces let's just say like uh color pieces no let's just say pieces that select yep so that's going to be like all the black pieces or all the white pieces so um now we want to go through all of those pieces and see if any of them have safe moves so colorpieces.any piece ps.safemoves.length or like dot empty should we check to see if they're all empty right if the if the pieces if the pieces safe moves are all empty then we should be in checkmate let's see if that returns true uh stack level two deep again so board 28. board 28 okay and safe moves so this safe moves here ps.safe moves should not be calling all right what is this calling so let's take a look at the stack trace here so all.each block and checkmate safe moves so when we call checkmate here so we're calling checkmate on the board all right and when we're in checkmate on the board we're checking to see are we in check return false if we're in check otherwise give me all the pieces and then if all the pieces safe moves are empty so then when we call safe moves on a piece then we're going to the piece class and safe moves is going over available moves and it's checking to see if that moves us into check and hmm i am not sure why this is not working or why this is causing a stack stack level too deep so where is the recursion happening here so if we call move piece is move piece doing something fancy so in board if we look at move piece ah yes safe moves okay so now we're at a position where we in order to move the piece so move piece is a method that we're using in safe moves when we're trying to make the move on the duplicated board um and because we are using move piece in the duplicated board then it's also using safe moves so we don't actually want to use safe moves in move piece so what i think we want to do is make a another method that does this stuff that actually moves the piece so let's make a method move piece bang which will take in the start position and the end position but it won't actually validate anything start start pause and pause and this is where we're going to actually like move the piece here but we're not going to validate anything at all because we have already sort of validated stuff and then down here we can just call move piece bang with start pause and end pause and then we can just use available moves here and that should work i think yeah oh wait no so when we are figuring out safe moves when we're figuring out safe moves we want to use the move bang method instead of the move okay yeah so move piece bang [Music] this should make it so that it moves without running the validation and the validation is the thing that's checking if it's in check and we don't care about that because we're going to check it here so i think this might be what we want that's super confusing by the way so i hope yeah i hope you're following along so move piece bang undefined local variable or method piece for board okay so if we go to the board on line 105 yeah i didn't copy enough in here did i so board let's see board on line 105. another another like rule of thumb is to try to keep your classes less than 100 lines this this this class in particular is starting to grow a little bit out of control um start piece okay so we actually want we don't want to set this to nil yet because we need reference to it so we get there's actually like a really kind of clever way to do to like swap something for another thing um and so i'm gonna move all these comments up and what we're gonna do is we're gonna say start position comma self end position is equal to nil comma piece no nil comma start position and this i think is like a fancy way of sort of just swapping swapping elements so we're saying like put nil in the start position and put what's in the start position in the end position and this will like evaluate the right hand stuff and like get this ready to go and then plop it in here i think that's what's gonna happen i don't know we'll see so we run it and then did you mean pieces ah because we don't have the piece anymore so now we should just be able to use the end position um i think let's see uh in check no king found oh fantastic okay great so now what what why oh six six six okay when it duplicated the board remember when it's duplicating the board it's using the positions on the pieces and because we had these i didn't like copy those over correctly um that that failed so now it's returning false which i don't believe that's like that's wrong because this black king should be in check um so oh no because this is again this is again wrong so three zero and three uh one okay um gosh oh true fantastic all right great so is white in checkmate white is not in checkmate fantastic awesome so then if we can say like in check black should totally be in check now um and then we run it and true okay great so now we have implemented check uh and checkmate and we also implemented like board duplication and a few other things so hopefully this is hopefully this is useful this was a really juicy episode and really kind of got into the guts of how the board works for moving the pieces around and duplicating the board and figuring out where things are on the board so i think we're actually getting pretty close to having a working game that we could play and it would actually tell us like if the game's over and such we can go implement this in the next episode we're going to come back and check to see who's won who's not one and then as we're taking turns we'll check to see if the player is in check so that should be that should be fun so thanks so much for watching and sticking around and yeah we'll see in the next one [Music] cheers you

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