---
title: Game play logic - Implement Chess with Ruby for beginners
slug: game-play-logic-implement-chess-with-ruby-for-beginners
published_at: 2021-06-01 17:00:00 +0000
updated_at: 2026-03-04 20:14:10 +0000
summary: 
description: In this series of videos, you'll learn how to implement a simple terminal based Chess game with Ruby. In this episode, we start implementing the Game class and the logic for the play loop. We create a Player class to read the start and end positions from a human user as comma separated values. We'll create a new Error type for raising a custom exception.  We talk about:  * Dependency injection * Passing a class as an argument * Custom error classes (InvalidMoveError)   Playlist: https://www.youtube.com/playlist?list=PLS6F722u-R6LZxh7mvEUvZKshcH6KSNhr Code: https://replit.com/@CJAvilla/chess#main.rb
tags: [Ruby exception handling, dependency injection, Passing classes as arguments in Ruby, OOD, OOP, Chess in Ruby for beginners, Chess in Ruby, Chess with Ruby, Ruby chess game, ruby tutorial, ruby tutorial for beginners]
views: 640
author: CJ Avilla
url: https://www.cjav.dev/videos/game-play-logic-implement-chess-with-ruby-for-beginners
youtube_url: https://www.youtube.com/watch?v=a3MSZm6GfRQ
youtube_id: a3MSZm6GfRQ
embed_url: https://www.youtube.com/embed/a3MSZm6GfRQ
thumbnail_url: https://i.ytimg.com/vi/a3MSZm6GfRQ/hqdefault.jpg
type: video
---

# Game play logic - Implement Chess with Ruby for beginners

*Published: June 01, 2021*
*Views: 640*

## Watch

[Watch on YouTube](https://www.youtube.com/watch?v=a3MSZm6GfRQ)

[![Game play logic - Implement Chess with Ruby for beginners](https://i.ytimg.com/vi/a3MSZm6GfRQ/hqdefault.jpg)](https://www.youtube.com/watch?v=a3MSZm6GfRQ)

## Description

In this series of videos, you'll learn how to implement a simple terminal based Chess game with Ruby. In this episode, we start implementing the Game class and the logic for the play loop. We create a Player class to read the start and end positions from a human user as comma separated values. We'll create a new Error type for raising a custom exception.

We talk about:

* Dependency injection
* Passing a class as an argument
* Custom error classes (InvalidMoveError)


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

## Transcript

hey friend welcome back uh again we are building out chess in this episode we're going to work on is adding logic for taking turns so we're going to build out a game class and we'll talk about uh swapping between two different players we might build a player class and we're going to take turns entering in uh moves and then re-rendering the board and doing some fun stuff there so let's add let's add a new file called game.rb and in our game class we're gonna have a new method play and let's see what do we want play to actually do so this is going to be some sort of like this is where our game loop is so we want to like um prompt like prompt it like the current player to enter a starting position then we want to prompt the player to enter an ending position then we want to like move the piece okay so we need some concept of a current player so when the game is first initialized i think we want to pass in or sort of like initialize we probably want to set up uh in the in the initial initialization of a game here um i think we want to like we need reference to a board we need reference to a board renderer we need reference to players we need we need a couple of different things now again um we have a couple of options here so one thing we could do is just say like at player one is player dot new at player two is player dot new and maybe these take in the color white and black and then we could sort of just toggle between those two so maybe the current current player is player one um and then we can say like maybe we have a method here swap player and the swap player is just going to like switch who player 1 is so then we can say like at um or yeah let's see so let's do adder reader for player one and player two and then we can do an adder assessor for current player and here we can say self.current player is equal to um the person who is not the current player so then we want to say we kind of want to say like if at current or like if current player is equal to player 1 then we want to return player two otherwise we want to return player one and that should like swap who who the current player is um and then yeah we need the con some sort of concept of a player so we need to like create a new class called player and it is going to be i guess it's going to be initialized with the color and um yeah so i guess we can just print out the color like it's so-and-so's turn like it's player one's turn or something like that so then in the game maybe here we can like to start out we can say like puts it's let's see it's current player dot color uh it's their turn and then let's see let's see what that looks like so we need to go back into into main here and we need to import both player and game and then we need to test this out so i'm actually just going to delete all these and then say like g is equal to game dot new and then g dot play and then we'll see what happens we can run this it's blank's turn okay so when we initialized the player we passed in the color and we have a reader for the color and let's see yeah player.new we passed in the colors it's so-and-so's colors game current player did i spell something wrong here no why isn't it printing something out interesting um so let's p current player and see if there's actually something there because we should have gotten back the string representation of the symbol black and white okay so we see the player and then what if we puts current player.color so it should be player one we should see black it is black's turn so why is it not showing us anything maybe we need to do 2s here or something it is a symbol but it should still show when we print it out like that uh player yeah player.new so the other thing too that's on my mind is that in the initialize method anytime that you see um okay so that is definitely not working so what is going on with that so the current player is this player and the color is not being set so in the player maybe i misspelled color in here nope did i spell initialize wrong maybe that has bitten me tons of times misspelling initialize uh i see why in python they call it init it's like underscore underscore init underscore underscore which does like the same thing as it here in ruby okay so now now we're at a point where we kind of really need to like figure out what's going on and why the color isn't being passed in um and so we could like put color here uh oh i'm saying at color but i'm actually storing it at color equals color alright that that makes way more sense all right that was confusing i don't know why that took so long but um printing out things for the win right okay so it should say it is black's turn let's see okay fantastic it is black's turn great all right um then what we want to do is say like um puts select a piece and then we want to receive some input and i guess the input we could we could receive the input as like comma separated values but the input is going to come in as strings no matter what and then we're going to need to like parse it into um we'll need to parse it into integer values and i'm thinking that like we don't actually want to know too much about how we get the move while we're inside the game class like i think the player should be responsible for knowing how to receive a move from the player the reason is that like technically a player we could we could implement a human player class or a computer player class or a dog player class or whatever and each of those might have different ways of finding which move they want and really all we care about is the move so what i want to say here is is maybe like start pause is equal to current player dot get pause or something like that and then that would retrieve like get pause in inside of the player class we can implement get pause as the as a method that asks the player for some input so let's make a method here get pause and we're going to say gets dot chomp that should get a string from uh from the standard in and then remove the new lines from it and so we're gonna say like uh raw position is equal to that and then what we wanna do is like um i think we wanna say like x and y is equal to raw pause dot split on comma and then we can so after we split them on comma we need to turn them from strings into integers so then i think we can say dot map and this will be applied to each of the elements and then we can say like the part and we want the part.2i and let's see what we get back so then i i guess we're going to return x and y as our locations or row and column or whatever it doesn't really matter a ton here so actually in fact i think we could just return this maybe and in even going deeper we could probably just return that so we're getting a string from the terminal then we're going to remove any new lines from the end or any white space from the end then we're going to split on comma and then we are going to map it to integers and return the value of that entire thing as our get position so let's run it again and from the game do we have it printing this out so let's uh let's print out start pause and let's run this so we should it should see like it's black's turn select a piece then it should wait for input we're going to enter in some input okay so select a piece so now we're going to enter in like one two and we hit enter we get back the array one two that's exactly what we want okay select a piece maybe we say select a piece to move and then we say puts like select a position to move to and then we're gonna again prompt for the end position this time so we're gonna say like okay we get the the end pause and we'll print that out also p and pause and let's take a look okay select a piece to move one one and then select a position to move two four four okay so now what we need to do is like actually figure out okay it's black's turn they are right when they're selecting a piece to move we should check to see like is that piece actually their piece and if it is then and only then do we still like ask them to select an end position otherwise we want to like have them again select a piece to move so i think we want to sort of like loop do we want to keep doing this until they select a valid piece so here what we need to do is say like ah okay so now we're at the point where we actually need a board we need a board class so um the question becomes like should we all right so back to what i was saying before like because we are using the name of a class here inside of the initialize method we know that we're creating a really hard and strong dependency between game and player right like the player is not being passed in and so when you use this game class you can only use this type of player there's no way to change from this class of player to a human player or a computer player or whatever because we are using the the class name here so um you might have like the instinct to say like oh let's just do at board as board dot start chess and we're off to the races right our game has reference to a board it has reference to two players but it's actually like a little bit better practice to do what's called dependency injection so right now what we're doing is we're like hard coding the dependencies directly into the class right so we're depend we're we're writing these class names because we're writing the class names it makes it more difficult to change how this works in the future so instead of instead of hard coding these class names let's actually pass in let's pass in let's pass in the board and then we could either pass in the class of the player or we could pass in the actual instances of the players so let's this one's going to be bored instead of instead of hard-coding it here and then we can let's also just do player one and player two and we'll have those passed in from whoever is the user of the game class that way if we wanted to make player 2 a human player and player 1 computer or human versus human or whatever we could do that from when we're initializing and newing up an instance of the game so here we're going to say this is player 1 and this is player 2. that makes it a little bit more flexible hopefully that's obvious so this is like known as dependency injection and it makes it much easier to to work with your classes they become much more flexible and so down here now we need to pass in a board so b is four dot start chess and we can pass in um the board first and then we need player one and we need player two um i think we can just say like player.new black and player.new white the other thing we could have done is like set the color inside of the game but for now this is probably this is probably cool all right so what do we've got what have we got going on now all right so we're initializing our game class we're passing in the board and our players and then we're hitting play and then in the boar or i'm sorry in the game uh in the game class we are playing and we're looping over this and saying select a piece to move but the whole reason we were talking about all of that was that we actually need to know what the piece is at that starting position so let's grab the piece from the board so now that we have the board we should be able to say let's also make that a reader board so now that we have the board we can say the board at the starting position is the board at the starting position's color the same as the current player.color um and if it is then we want to break otherwise we want to otherwise what we want to do is like say reselect a piece but first before we do that we want to print out a message just like a little error message that says that's not your piece so puts um uh did not select a and then we can say the current player.color piece so it'll say like did not select a black piece or did not select a white piece when that happens and then we should yeah it should sort of like walk us through that the other thing we want to do probably is like every time before we we play we want to like render the board out so in addition to this board in like this board being injected we probably also want to inject like the renderer um so maybe we just call this like renderer and we can also store that at render is renderer and again we'll make it an adder reader and down here we can say renderer dot render actually yeah um because we need reference this needs reference to the board i think um yeah we just made this an adder reader let's make it add our assessor so that we can set it on the board class and then we'll come back over here and we will say renderer is like this and then we'll set its board so render or dot board is equal to board um okay so notice how i'm continuing to just keep all the instance variable definition or declarations and setting in the initialize method we're not actually using instance variables anywhere else um that's to keep things flexible okay we run this again now we should see initialize wrong number of arguments three okay right because we didn't pass in the renderer and in fact we could probably pass well yeah i was thinking about maybe just passing in the class name for the renderer which is common so yeah board renderer text maybe and then we can actually like new it up in here so let's let's i wanted yeah let's just talk about um renderer class so we can pass a class around as an argument so here we're not actually initializing an instance we're just passing the class name right and because we're passing the class name we can use that class just like we would any other class inside of here so we can actually say the renderer class whoops render class dot new and pass in board that makes it a little bit cleaner and now we could just easily swap out what the renderer class is we could have done the same thing with the players but uh whatever that's fine the risk we run with accepting the player the actual instance of the player is that we don't have control over which color the player is but i think that's okay okay so here we go it's black's turn select a piece to move so if i pick an invalid position so if i say like 3 3 then let's see what did we get line 21 in our loop so we're inside of our loop and line 24 inside of game block in play 24 is it the same color as the current player what is our error here undefined method color for nil class okay so this is where we're trying to check the color of the board position now all right now it's time to actually dig into the nil object pattern um because actually we'll come back and refactor again all right so board at start pause is not nil and its color is our color so because we have to do this check here this is super annoying right and this is actually a really common error in ruby is undefined method blah for nil class it's because you're trying to call a method we were trying to call color when we reached into the board and we got back nothing because we were passing 3 3 which is like over here right there's no piece there and i was expecting it to say did not select a black piece but instead it crashed because we have a nil a nil exception for nil class and so the way that you get around it is generally by going everywhere and putting like uh is this thing not nil and if it's not nil then do whatever which is what we just did but there is a a tool a handy tool for this that we'll come back to in another another episode i want to just do like kind of like a big refactoring with null with the null object pattern in one sweep so all right so let's um pick a piece okay did not select a black piece fantastic so now if we pick like one one one one okay then we are then we're prompted to select a position to move to this is where we need to loop again and say like okay um you know try to move to that end position and then if it doesn't work then keep trying so we need like i think we need like potentially another loop but let's just say 2 2 and then it gives us back it's just printing out 2 2. um so let's see so we're prompting for the start position then here we're prompting for the end position and i think we do want a loop here and what we want to do is actually loop and we want to try to move the piece on the board so we want to say like board dot move move piece i think from start pause to end pause um and it might fail um this thing might raise an exception so i think we need to we probably need to wrap this in a uh in a begin rescue um but let's let's just let's just try it out because we we should fail if we're trying to move the piece to an invalid spot so let's rerun this okay select a piece to move uh one one and we're going to move it to two two and it failed okay here we go undefined local variable or method start position oh great yes okay so i am creating this local variable start position here and i'm then trying to use it down here the the reason why i'm seeing this exception undefined meth or undefined method or local variable start position is because this is block scoped so this start position variable is only available inside of this block right because i created this local variable inside of this do end block it's not available inside of the method yet and because of that because it's inside of this block i do not have access to it down here where i'm inside of another block so i need to i need to lift the scope of this variable up one level by and i can do that by just saying like start position is equal to nil so we'll sort of like initialize this local variable so that then when i set it here i'm actually just updating this local variable that is local now to the function play and should then also be available down here so that's a really good um yeah good thing to think about in terms of like block scoping in ruby so okay select a piece to move one one and we want to move to two 2 and great end position 2 2 is not in available moves 2 1 3 1. ok so what i think we want to do here is add begin a begin rescue block and generally it's bad practice to just put rescue because that will rescue all possible exceptions that are going to happen inside of here so instead it's a little bit better to pass the specific type of error that we expect to be raised here and right now from our board class on line 59 we are just saying runtime error so what i think we want to do is go to our board class we're going to go back to our board class and rather than just raising these exceptions with string values what we want to do is raise a new error type which is like invalid move or something like that so if we go up here and we create a new file and we call it invalid move uh error or something invalid move error dot rb then we can create a new class invalid move error and this is going to inherit from like argument error there's a whole bunch of different exception types that you could um that you could return and for now we won't pass anything in but i think argument error should accept the string as as a first argument we can actually look that up ruby argument error let's see or should we inherit from runtime error yeah argument error dot new and then you use this is this is basically what we want so the error is that the the value that was passed in is in is invalid but this gives us a very specific error type that we can then handle inside of our game class so we want to we're going to rescue this type of exception is the invalid move error so when we are in the board and we see that there was an invalid thing rather than just raising this string value for runtime error we're going to raise a invalid move error.new and we're going to pass that string value in so that we are actually raising a very specific error type okay um all right let's see i think that should run and work a little bit better um if we go back to our game so this we want we're going to continue looping but if we don't if we are if we actually are able to move the piece then we don't want to keep looping so then we're going to say just like break here and if it was an invalid move we can say um like puts unable to move to that location um actually we can just print out the error message so here we have reference to the exception so we can just say p e dot message i think and that should like or puts e.message should print out the message for the exception that we just created the invalid move error exception that we just created all right let's see okay pick a piece to move one one and we're going to move it to two two and that should fail on an initialized constant invalid move error okay so we need to import this thing try again okay one one and we're moving to two two great end position two two is not in available moves two one or three one select a position to move two so then we can say like oh okay let's actually just go to two one and then we didn't see okay so then we're then we're ending our we're ending our executions because we don't actually have a loop yet for our game but this is the logic that we have for actually like retrieving the move so i think rather than having this all inside of play like the play function now is what it's almost so it starts in line 17 and it ends on 43. so this is a massive massive function now it's way too big way too big so what i think we want to do is just print out like um maybe let's make a method take turn and then we'll call it or yeah just take turn like that and then we can remove all this and put it into uh take turn and we could even like break out the loops into their own methods or something but for now this should be this should be fine uh okay select a piece to move um didn't okay great all right i think this is a good start all right so we've got our play method here and we're doing take turn and then after the turn is successful so we assume that these loops are just going to keep going until valid moves are selected and they kind of like work through picking the right move after we take a turn we want to swap the player and then we want to print again and then we want to loop so i think we could we could just call play or something to make it sort of recursive or we could just say um while while the game is not over do this stuff and then we'll need to figure out what does it mean for the game to be over which for now we can just return false and we can come back and like figure out how we decide whether the game is over because it'll be either one or it'll be check um or stalemate or something i don't know um but this should give us a loop so while the game is not over which it's not um then we're going to print out the board we're going to say whose turn it is we're going to take the turn and get get the input to move the piece we're actually going to try to move it we're going to move the piece and then we're going to print the board or we're going to swap who the current player is we're going to print the board again and keep going um actually does this this does print doesn't it renderer.render i think it does um yeah let's just see board renderer uh yes so we're saying puts so yeah it is printing that okay so we it's black's turn we're gonna select a piece to move one one and we're gonna move it to uh two one okay and that moved the piece fantastic it's white's turn select a piece to move and if we pick uh what is that six six two or six one and then we do like five one all right we've moved the piece forward fantastic all right it's black's turn pick a piece so let's say one one two and let's move it let's move it two forward so one two two two three two all right fantastic we moved it forward great it's white's turn let's move this white piece forward so we're gonna pick what is this five oh gosh five this is like good uh good brain teaser for trying to figure out where um where things are in arrays so five one and we want to move it to four one and then okay so now we can pick this black piece and that should be uh like three two and we wanna move it so that it takes this white piece here and that is at 4 1. amazing so we're taking pieces we are able to take pieces so this is cool this is super cool so one of the things that we might want to change is like rather than printing this out every single time i think it makes sense to probably clear the board and i think we might be able to just call clear um like right before we render i think i don't know um let's see if clear the thing is clear thing okay maybe we have to like actually shell out to clear so clear is a method like i think this might be a bash or something it's like a terminal method that clears out the previous output so let's see if we can actually use this so one one two two or two one okay so it did not it did not clear it but whatever that's fine uh so yeah i wanted to do like this thing right so this is clear but yeah it's fine uh one one up you didn't pick a white move or a white piece select the piece to move oh gosh six zero five zero okay so we're definitely able to move the pieces around and print the board out so that's that's pretty sweet um all right so this clear thing probably isn't gonna work what do we want to do now i think this is probably a good stopping point we were able to sort of like swap which player was the current player we talked about dependency injection we are passing a class around and then using the initialize or newing up or calling the constructor function on that class passing in the board here so this is kind of a fun little thing that we learned about we've got a ternary operator here that's going to flip who the current player is that's fun and yeah we talked about initializing and working with custom error classes so this was a this was a pretty jam-packed um a good jam-packed episode i think yeah i think we'll leave it there and then in the next episode we can start talking about potentially adding logic to check who's the winner and some other um some other gameplay things to uh to flesh out our chess game but i think like in practice right if you were just using the the board as it is right now if you kind of just followed the rules then technically um you should be able to um work with a second player and figure out where um like where the king is and whether or not you're in check just kind of by looking at the board so this is almost we're at the point now where it's as if you had a physical board in front of you right where you could technically move the pieces in only their valid positions which so that makes it a little bit um superior to having a physical board but we're not actually checking whether you're in check or checkmate or any of those things yet so uh so let's go try to figure that out next all right thanks so much for watching we'll see in the next one

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