---
title: Available moves for a Piece - Implementing Chess with Ruby for beginners
slug: available-moves-for-a-piece-implementing-chess-with-ruby-for-beginners
published_at: 2021-05-29 17:00:03 +0000
updated_at: 2026-03-04 20:14:11 +0000
summary: 
description: In this series you'll learn object oriented basics while building a basic terminal based Chess game with Ruby.  In this episode, we'll extract some of the common logic that is the same across all piece classes into a parent Piece class and setup a basic inheritance tree. Then we build Slideable and Stepable modules to share the logic for discovering available moves for all pieces that fit into those categories.  We cover:  * inheritance * modules  Playlist: https://www.youtube.com/playlist?list=PLS6F722u-R6LZxh7mvEUvZKshcH6KSNhr Code: https://replit.com/@CJAvilla/chess#main.rb
tags: [ruby inheritance, ruby modules]
views: 585
author: CJ Avilla
url: https://www.cjav.dev/videos/available-moves-for-a-piece-implementing-chess-with-ruby-for-beginners
youtube_url: https://www.youtube.com/watch?v=2LXc7QYwcRk
youtube_id: 2LXc7QYwcRk
embed_url: https://www.youtube.com/embed/2LXc7QYwcRk
thumbnail_url: https://i.ytimg.com/vi/2LXc7QYwcRk/hqdefault.jpg
type: video
---

# Available moves for a Piece - Implementing Chess with Ruby for beginners

*Published: May 29, 2021*
*Views: 585*

## Watch

[Watch on YouTube](https://www.youtube.com/watch?v=2LXc7QYwcRk)

[![Available moves for a Piece - Implementing Chess with Ruby for beginners](https://i.ytimg.com/vi/2LXc7QYwcRk/hqdefault.jpg)](https://www.youtube.com/watch?v=2LXc7QYwcRk)

## Description

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

In this episode, we'll extract some of the common logic that is the same across all piece classes into a parent Piece class and setup a basic inheritance tree. Then we build Slideable and Stepable modules to share the logic for discovering available moves for all pieces that fit into those categories.

We cover:

* inheritance
* modules

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

## Transcript

hey you're back and that means you must continue wanting to uh play um play chess or learn how to play chess or learn not even learn how to play it learn how to make it with ruby um so what we're what we did last time we set up our board we sort of got something rendering with a really basic class that's called board renderer text that's for like rendering it out in text what we're going to do now is actually figure out how we can decide whether or not a piece can move so what i wanted to do was just talk about like if a piece um what are all the different possible moves that a piece can make given its position and so before when we started this we initialized all of our pieces with a color right and so we have a bishop and we have a king and a knight and a pawn and all of these have one single initialized method that right now is just taking in the color similarly we have an adder reader these these lines here at the top are actually the same across every single piece and so what i wanted to do was talk about inheritance today and so it turns out that if you have a bunch of classes that are all sort of of the same type and they all share similar logic but need to differ based on um they need to differ in some subtle way uh then you might want to use inheritance now typically you want to prefer composition over inheritance meaning that you it's better to like pass in things to change or to use like standard logic in this case though we want to set up all of our pieces the same exact way so what i want to do is create a new file called peace.rb and this is going to be the parent class this is going to be like the top level piece that all other pieces will inherit from so we're going to create a class again this is sort of a blueprint and this will be the class that all other pieces inherit from so all pieces will now just have instead of having this this kind of like code inside of the logic for the class the individual class instead we're going to do is we're going to inherit from peace and the way we do inheritance in ruby is right after the class name we put this less than sign and then the name of the parent class so now we have a class pawn that inherits from peace now knight is going to be different than pawn because knight is going to have different a different symbol for its string value we could actually simplify this potentially a little bit and just say like def sim or something like that that had the symbol and then we could like change the color and print it out in a different way but for now we'll just keep it like this so that each method defines its own 2s and its own move directions which we will we'll continue using and then instead we'll go to each of these pieces and we'll make sure that they inherit from piece and now we can safely remove that code that's at the top there so we want a king to also inherit from piece and we can remove this code that's at the top here a knight is also going to inherit from peace and we're going to remove this stuff from the top a piece nope pawn good queen similarly it's going to inherit from peace what this makes it easier to do also is that if we uh as we go down the road if we want to initialize all of these pieces the same way we only have to change it in one spot and we're actually gonna we're gonna do that here in a moment all right so then rook also needs to inherit from piece not double uh just one okay i think we're back to a good spot where each piece now just has its two-string method and it has this move directions method now in piece what i think we want to do is start to figure out how to decide what are all the different available and possible and total moves that a given piece can make so we have the directions that a piece can move so for instance a king can move in all eight directions around itself but that's technically not really true if the king is in the starting position and it has um and it has ally pieces all around it so one thing that we need to do i think is from the piece class is in addition to the color i think we also want to pass in reference to the board and to the position of the piece so let's start out by passing in the board and so we'll say every piece now has reference to the board and we can make that also an adder reader okay and then similarly what we want to do is ch we want a method that checks to see if a piece at another position is an enemy piece and then or if like the position is empty so let's say like um on the piece class maybe we can ask like uh if a piece is an enemy so if a piece at a given position is an enemy so let's say we'll pass in the location here and we can say is the piece at that location an enemy so we want to say board at location so is the piece of that location nil or is the board at that location um is the color of that piece equal to my color this should tell us whether or not a piece is an enemy piece now this is where it gets really helpful in our main function to be able to initialize a board without actually setting up all the pieces on it so here what we can do is we can say board.new and then we can set up just a couple pieces so we can say board at zero zero is going to be dot new and we'll pass in the reference to the board and then we'll also pass in the color black and then we'll put a piece right next to it on the board at 0 and we'll say that this one is white okay now what we should be able to do is ask if um if this pawn so the pawn that's at this position so piece is oops so the the piece that's at position zero zero so this pawn right here if we ask it piece dot enemy of uh zero one we expect this is going to give us back true so we'll comment this out because the piece at that position is white it's not black and so it should be an enemy so we're going to say run acquire relative bishop uninitialized constant piece so we haven't actually imported the piece the other thing i wanted to do was like instead of having to like import all of these eventually importing just the piece yep so let's let's import pieces so let's make a new class here called pieces dot rb and it will be purely responsible for um importing all the pieces so we'll go back to main no uh let's go to board and then we'll cut this and put it in pieces and then we'll just require all the pieces this way and we want to require the piece first require peace just so that they're loaded in the right order so now in maine we should be able to just require pieces and then the board i think this might work let's see false puts enemy at that position is false why is it false peace dot enemy so peace dot enemy is the location nil or does the location ah it does it not equal our color i think we actually just want it to be not nil we want it to be not nil and not our color because if it's a nil location then it's not an enemy technically it is just an empty spot and true okay so that is an enemy and then if we go back to our main and we say is the piece an enemy at position zero zero and it should not be because that's our self so we should see true and then false here true and false and we do okay so that is great we are able to check to see if a piece at another location is an enemy um all right the next piece or the next uh the next thing that we want to do here is figure out how we can um how we can move the pieces around so when when we're looking back at this board and let's actually put the let's put this at 0 1 and let's check to see how this how let's actually do 0 0 and let's put a rook and what we want to start figuring out is the available moves for a piece so let's say dot available moves which doesn't exist yet so let's go implement this method on the rook class so let's go to rook so we have our move directions and now we want to figure out how to implement available moves which i think is nope this is not responding very quickly i don't know what's up okay so the available moves are going to be we want to iterate over the move directions that we can go in and we want to see like um for a rook in particular like a rook can slide so a rook can slide up to a piece of its own color or it can slide up to and including one more square if it's an enemy piece to like take that piece right so what we want to do is we want to iterate over the directions that we can move in so move durs.each do i guess this is going to be like dx dy or something like that dr dc so that's kind of like the diff in the row and the diff in the column and then we want to check like we sort of want to build up an array of possible sort of positions that we could move to for this piece so i'm just going to say moves is equal to an empty array and this is what we're going to return from this method also is a list of moves now what we want to do is say like okay now that we have the directions we need to build based on our current position ah so we do need our position so the current piece doesn't actually have reference to its own position yet we have the board we have the color i had a hunch that we would also need our position so let's say location and let's store this as at location is location and we'll say location okay so when we initialize a piece we need to pass in the board and its location so we need to go back even further to our board class and update this start chest thing to pass in all the right instance methods but for now we can just keep messing around with with this here and where did we make it we made it the second argument or the second argument so back in main so the second argument here is going to be zero zero so technically we have the the state of where the piece is on the board we are storing that in two spots we're storing it on the piece itself and also on the board so it's kind of like duplicating that but it'll be helpful inside of the rook class when we try to decide what what our available moves are let's go back over to rook now that we have position we want to start at our given position and sort of grow in the direction that we want to move or like start sliding in that direction to see where we hit some boundary so if it's almost like it like we actually need to loop here so loop so what are we going to do we're going to say like until we hit a piece um we want to like add the location of or add the location plus dr dc um to moves okay so we have we have the move direction and we have our current position so if our current position was if our current position is 0 0 then we want and our move direction here our first one is 0 1 then what we want to do is we want to add 0 0 plus 0 1 so then we're going to end up with 0 1 and if this position ends up being empty or an enemy then we want to add it into the list of moves so how do we how do we get this we're going to need to destructure like our current r and current c so the current row and current column is destructured from self.location and what we want to do that's where our starting point is going to be is current r and currency so we're going to say for every direction that we can move in we're going to say current r plus dr comma current uh c plus dc that's going to be like the new possible location in that one direction right um and we want to keep incrementing in that direction until we um yeah until we end up okay so i think we actually want to grab this location every time we are considering a new direction because we want to like reset back at the current location so we're at the current location and now we want to start doing our loop so loop or like while or something loop let's just do loop do and then end okay and so what we're going to do is we're going to say that the current r current c is equal to um or actually we can just do like current r plus equals dr current c plus equals dc so that should give us like an updated position and then we want to say like if um if this is empty on the board so if board empty at this location or um well yeah let's just start with empty so if the board is empty at that location we want to add that location to moves and we want to keep going so moves we want to shovel in location so this shovel operator is the same as saying moves dot push and uh or to push loc and that adds that adds the this this array to this um this array of arrays so it moves we're going to shovel in location okay so we're going to keep looping and we're going to keep going in this direction if the board is empty at that location we're going to add it to moves if if we're not on in range of the board we should break so if um so let's say let's see here so we want to break if so break is how we get out of this loop here so loop do is just going to keep looping forever it's kind of like while true so break and when we call break here it should only break up to it should only break out of this loop and bring us back to this each so we want to break if if the location is off the board so if board.inrange location right because we have we wrote that method in the first episode in range so we are in bounds we have as this method down here okay so if um if we're not in bounds if the location is not in bounds then we want to break the loop what are some other cases that we want might want to break the loop well if if the if the board is empty we just want to shovel on there if um the board dot enemy if it's an enemy at that location or i guess we implemented enemy on piste directly so we can say if it's an enemy at that location then we do want to add that to our list of moves but we also want to break after we do so so that we don't keep going and like attacking all the enemies um so then we want to break here so this is a little bit confusing but basically what we're doing is we're like we're saying that for each direction that we can go in for all these four directions what we want to do is take our current location so reset back to where we are now and then we want to iterate or sort of slide this is the part that's actually sliding in a certain direction and uh we want to take our current row and our current column and we want to increase them by our direction so in some cases we're actually going to be going backwards right so if we do plus equals with the minus 1 that's going to be going down so we're going to increase them by that direction and that will give us a new location and if the loot if the new location is not in bounds then we're done and we want to break out if the location is empty we want to add it to moves so let's let's do that as like a post fix thing here now whatever we'll keep it as keep it fixed okay and then otherwise if there's an enemy at that location then we do want to add that one location to moves but then we want to break out and go consider another direction and then if we get to the end of all the directions then we should have a list of moves that we should be able to spit out here so let's go take a look at rook's available moves i don't know maybe this will work we'll see so if we run this uh it fails um undefined method empty oh gosh we didn't write an empty method on the board so let's make a method empty at location and we'll just say like if uh grid at um row column seal to location if the grid at that row and column is nil then it's empty we don't have a null we don't have a null object yet or a null object pattern yet so for now it's just going to show as uh as empty oh and we're not we're probably not oh we didn't print it out yeah so let's do p yeah so let's actually clear this and see if that'll help us all right run all right so is this the actual possible moves if we have a rook that's at zero zero we can move up one we can have two three four five six seven and then we can also move um to the right all the way to the end good this is great so now if we put the rook in a different spot let's say we put the rook on like two two so we say two two and then we grab the rook at that position then we should get different numbers here uh okay great so 2 3 2 4 2 5 2 6 2 7 and then also backwards 2 two zero okay i think this is totally working and so this available moves method here that we've written will work for every piece that can slide this will work for every piece that can slide so what are the pieces that can slide we've got rook bishop and queen i believe are all pieces that can slide so what we want to do is rather than keep this logic in in the rook class directly we're going to break this out and if we wanted to like include some sort of logic in just a handful of classes we don't really want to move this into piece because the available moves for say a king like a king is not a sliding piece so a king should not be able to just loop over all the possible directions and keep moving in that direction and so what i wanted to do now was introduce the concept of a module so a module in ruby is uh is similar to a class where you cannot actually instantiate an instance of it instead it is just a bag of logic or like a bag of functions that you want to like attach to something or some like set of logic that you want to include or extend a class with so um in ruby it's conventional to name your modules um with the able or like ibbl suffix so in this case let's call it slidable dot rb slideable and instead of calling it with a class we say module slide a bowl and in here this is where we're going to move our method for available moves so let's put this into slideable okay so now slideable has the actual available moves and if we go back to rook we can clean this up and say the way that we like move the slidable modules code so the way that we make this method available inside of rook is we say include slidable so this is how we extend or add logic to the rook class with instance methods if we wanted to add them as class methods we can use extend i might be misremembering that let's see module extend versus include in ruby yeah okay so it's that's right okay so extend does import them as class methods include includes them as uh instance methods honestly i've almost always used include in rails there's this thing called an active record concern or a concern in rails and concerns have ways that you can add things as both class and instance methods with like the special class methods thing but for now just know that like it's common to include something if you want to add the methods from that module as instance methods all right very cool so now what we can do is just copy this slideable thing and add it to queen and boom we've got a slideable queen same thing with bishop right so now we've got a slidable bishop all right so that is how we're going to implement the slidable module now let's talk about some pieces that only step so maybe let's talk about the king for instance so uh we want to name the method exactly the same and we wanted to like act or behave pretty similarly so let's actually copy the entire thing and let's actually just off the bat we'll create a new method and we'll call it stepable dot rb and this will be a module step able and it is all about stepping so instead of looping here i think what we want to do is just remove the loop and is that all we need to do let's see all right so let's think through this so we have all the move directions for a king it's it's all eight directions around and we're gonna have we're gonna start at the current location and then we are going to create a new location that has the change in direction and that's our new location and if it's not in bounds we want to break oh we don't really want to break we want to do we don't want to break but it has to be inbounds if we're going to add it so let's do i think we can do next which should go to the next part of the loop so we do next if the board is not in bounds if the board if it's an enemy at that location then we do want to add it and if it's empty i'm sorry if it's empty we do want to add it if it's an enemy we do want to add it we don't need this break anymore and we can actually simplify this just a little bit because i think we can say if it's empty or it's an enemy then we want to add it to the location and i think we might even be able to just say like if it's inbounds if it's inbounds and it's either empty or an enemy then we add it to the location uh let's let's okay so the um the conditional there was getting a little complicated and this is actually a little bit more clear when you're reading it so i'm going to keep it simple like that all right let's go check it out so we've got steppable we need to include steppable in king say include step a bull and then let's go back to our main class all right so then here we can say king and the same logic should apply so we should be able to just say go for it and see the pieces uh uninitialized constant bishop slideable ah yes so in our pieces here let's let's import the modules at the very top so we can say slideable and steppable and then let's run slide a bowl slide a bowl okay cool so we've got some results so from 2 2 we should be able to move to 2 3 or 3 3 or 3 2 or 2 1 or 3 1 or 1 3 or 1 1 okay i think one two three four five six seven eight great i think that's good and then if we go to zero zero right like we should not be able to move into the like negative space so i think this should only give us one two three results ah yes so that's another really nice thing about having the null object pattern all right what's next so we've got to make sure that that slideable module is included in what is it it's also a part of pawn it's also part of knight okay so all of our pieces should have a module so we've got bishop king knight pawn queen rook great so now we should be able to at least get the available moves now what's interesting is we need to figure out um in fact like we're we're only finding the possible moves or yeah so this is including which moves are going to work for taking an enemy um and so i think i think that's a good place to stop we do need to come back and work on pond pawn is surprisingly challenging right because a pawn can move diagonally but only if there is a an opponent there and so we need to to update our available moves maybe or even our move directions depending on whether or not there's an enemy in a diagonal position we also need to like be able to move two spaces if we're on our first step versus several spaces otherwise so we'll come back and we'll mess around with pawn in the next episode potentially um otherwise i think we're gonna leave it there thanks so much for watching i really appreciate it and we'll see in the next [Music] one you

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