---
title: Pawn moves - Implementing Chess with Ruby for beginners
slug: pawn-moves-implementing-chess-with-ruby-for-beginners
published_at: 2021-05-30 17:00:08 +0000
updated_at: 2026-03-04 20:14:10 +0000
summary: 
description: In this series, we&#39;re discussing several object oriented design concepts while building a simple terminal based Chess game with Ruby. In this episode, we continue work on our Pawn class -- a surprisingly tricky piece to get right given it&#39;s small size ;) We add support for deriving the list of available_moves for a given piece on the board at a specific location and talk about some limitations.  Playlist: https://www.youtube.com/playlist?list=PLS6F722u-R6LZxh7mvEUvZKshcH6KSNhr Code: https://replit.com/@CJAvilla/chess#main.rb
tags: []
views: 399
author: CJ Avilla
url: https://www.cjav.dev/videos/pawn-moves-implementing-chess-with-ruby-for-beginners
youtube_url: https://www.youtube.com/watch?v=K5VCekNAd0c
youtube_id: K5VCekNAd0c
embed_url: https://www.youtube.com/embed/K5VCekNAd0c
thumbnail_url: https://i.ytimg.com/vi/K5VCekNAd0c/hqdefault.jpg
type: video
---

# Pawn moves - Implementing Chess with Ruby for beginners

*Published: May 30, 2021*
*Views: 399*

## Watch

[Watch on YouTube](https://www.youtube.com/watch?v=K5VCekNAd0c)

[![Pawn moves - Implementing Chess with Ruby for beginners](https://i.ytimg.com/vi/K5VCekNAd0c/hqdefault.jpg)](https://www.youtube.com/watch?v=K5VCekNAd0c)

## Description

In this series, we&#39;re discussing several object oriented design concepts while building a simple terminal based Chess game with Ruby. In this episode, we continue work on our Pawn class -- a surprisingly tricky piece to get right given it&#39;s small size ;) We add support for deriving the list of available_moves for a given piece on the board at a specific location and talk about some limitations.

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

## Transcript

hey welcome back thanks again for joining me as uh you and i build chess this time what we&#39;re going to do is work on building out the the valid moves for pawn which turns out to be a little trickier than all of the other pieces so um upon we&#39;ve we initially included this steppable module so in previous uh in previous episodes we talked about inheritance and we talked about including modules to add functionality to a piece um and it turns out that pawns don&#39;t actually work like a lot of other pieces and so um we actually need to write a pretty pretty uh interesting method here for for pawn so let&#39;s do that something&#39;s going on with replit here where i need to probably reload so great so i&#39;m going to remove steppable because we don&#39;t actually want to use that instead what we need to do is we need a couple things we need to know like for a pawn a pawn is like the only piece that can only move forward it can&#39;t move backwards like all the other pieces i think can move both you know forward or backwards in certain directions and so for a pawn we need to know if it&#39;s going forwards or not so what we need to do is like say forward let&#39;s let&#39;s make a method called forward direction and if the color is i think if the color is black then we want it to be like negative one otherwise we want to be one so um yeah because our board when our board prints out here so we have a board let&#39;s actually like re-render this board again um and take a look at what we get oh actually we need board dot um b equals board dot start chess okay stop and then re-run it when we print out the board we need to figure out like if the if we have the black pieces on the top or the bottom and then uh figure out sort of which index they are so that they can only move in the correct direction here um ah right we okay so we&#39;ve sort of um we we modified the uh the initialization of pieces but we didn&#39;t update our board so let&#39;s go do that too so we&#39;re gonna do a little bit of catch-up here um so we&#39;re gonna pass in i forget what we need to pass now i think we need to pass the we need to pass in the board and the location and then the color board location and then color so we need to pass in the board so self and then the location in this case um we can say yep just uh 1 c and then the color and then this one will be self 6c and then the color there&#39;s definitely a better way to do this i&#39;m just not i&#39;m just not remembering uh exactly the tricks to make this happen so are we initializing our board correctly now with start board or let&#39;s see start chess and then we should be rendering out rendering out the board okay great alright so black is on top white is on the bottom that means that when black is moving forwards it should be going in this direction which is increasing in the number in the the index of the this row and then white should be negative one so let&#39;s make sure that we got that right so back into pawn so if it&#39;s black it should be increasing so i did get this wrong okay so that should give us our forward direction for the pawn and then the next thing we need to do is we have to implement um we want to implement a method that has the same name as these slideable so we want to implement available moves inside of the pawn here and the available moves is going to be some sort of like combination right so if um so we kind of like a pawn can either move forward like the default is like move forward one right um but there there are also other moves where if you&#39;re on the uh the start line um you can move forward two and then if there is a an opponent that is diagonal to you then you can also move that diagonal direction if enemy diagonal you can move that direction so i think those are kind of the three different cases that a pawn can move around so what we want to do rather i don&#39;t think we&#39;re even going to actually use move directions because we need to sort of build up the available moves based on all these different cases so we&#39;re going to say moves here is an array and then we&#39;ll say moves here at the bottom and so what we want to do is say like if the position in front in the position one forward so that&#39;s going to be um yeah let&#39;s just do this this forward case the forward case first so if the position on the board one step forward is empty so that if the board at one forward is empty then we can moves add one forward so we haven&#39;t defined one forward yet sometimes i&#39;ll just like do this sort of fantasy driven development where i&#39;ll like try to write out what i want first and then i&#39;ll go back and like figure out how to like actually make the variable what it should be and so in this case one forward is going to be the let&#39;s see so the direction the the x is always going to be the same so it&#39;s going to be our current position um which is going to be like uh let&#39;s see like our current row and like yeah current r and current c is going to be equal to location we have this location because that is it&#39;s a instance method on piece which is where we store the location where we&#39;re at on the board so we have access to that location we also have access to the board here so we can we can work with either the location or the board while we&#39;re inside of this pawn class because it inherits from peace so this will give us our current location on the board and now the one forward step is going to be it&#39;s going to be the same it&#39;s going to be the row plus one right so the current row plus forward dir um that&#39;s going to be like one step forward and then it&#39;s gonna be the same column so current c i think this is like one step forward that that should be the one step forward and then we&#39;re checking like is the board at one forward empty and if it is then we add it to one forward and when you&#39;re a pawn you can&#39;t actually take an enemy that&#39;s right in front of you i don&#39;t think i&#39;m pretty sure you can only go diagonally so this i believe is our one step forward so this should work fine we can also do two forward so let&#39;s do two forward here two forward is going to be current the current row plus uh forward direction times two and then the same current column and then this is actually going to be really similar right like if if two forward is empty then we want to add two forward to our list of moves okay so this is going to be our two forward notice that we&#39;re not updating the current um the current row or the current column in this method um okay so then we need to check if there&#39;s an enemy diagonal to us so how do we do that well we need to um let&#39;s say so we&#39;re gonna have we&#39;re gonna have a diagonal potentially up into the right and we&#39;re gonna have a diagonal that&#39;s potentially up into the left so if we just say what do we wanna say here we wanna say like um uh diag uh left maybe is going to be the current r plus uh yeah plus the forward direction right and then current c plus the forward direction uh or no actually this this can just be plus one because um that should be actually that should be like whatever relative to one direction and then the other is going to be diag right um and the the row is going to be the same but the column is going to be then minus minus 1. okay and then we want to check like if um enemy at diag left then we want to moves push in diag left uh and and then if there&#39;s an enemy at diag right then we&#39;re gonna push in two moves diag right i think this might work i don&#39;t know the one case that we didn&#39;t already address is whether or not um we&#39;re on the board still so i think we need to also check to make sure that we&#39;re on the board and i think that like one way that we could do that is just sort of like filter these out to see if we&#39;re on the board um maybe we can do that here just say like dot select uh um the move where board dot in bounds i can&#39;t remember if we call the inbounds or in range um inbounds yep so if it&#39;s inbounds then uh of the the move so we only want to select we only want to select the moves that are inbounds okay i think i don&#39;t know let&#39;s let&#39;s check it out let&#39;s see if this works so at this point um because pawn is getting so tricky um i do think that we need like i in practice i would write some automated tests for this uh because we are getting we&#39;re like starting to get into territory where like um things are going to get messy here pretty quickly so let&#39;s see the board at uh 1 1 that should be a pawn dot available uh available moves so we&#39;ll put print out the available moves for the pawn at one one so that should be this one and i think we should only have two available moves we should only be able to move forward and move forward too okay undefined method square class for bor interesting so pawn line 12 so on pawn line 12 if bored at one forward the board should totally have that method the board class has okay go back to board we should have the getter method right right here we have that on oh what what happened to our board oh yeah here class board right so this should this should totally work this should give us back uh okay no method error undefined method square root okay interesting let&#39;s see um oh okay okay i passed in self and because we are in a factory function so this is in a class method so this is actually the the class board instead of the instance of the board so these all need to be changed to board okay so that tripped me up because i thought typically typically self ref refers to the instance but we are now in a class method so we were talking about the class so let&#39;s try this again we were passing in that class and expecting that um the board class itself had the square bracket operator but it does not it has an instance of it okay um available moves undefined method empty for nil class um okay so let&#39;s go back to pawn line 12 and take a look here empty um right so that&#39;s asking yeah so i thought we had a method on the board empty uh yeah oh so it&#39;s just empty we don&#39;t actually like grab the location so we would call dot empty like this and then pass in one forward um okay so that that should work fine um okay there&#39;s this is another case where the null object pattern comes in handy and again i keep i keep teasing this but we&#39;ll come back to it we will come back to it and check it out in a bit all right so we are executing something i&#39;m not sure why it takes so long okay uh great let&#39;s print that out with a p instead of a puts i think that might actually be right two one and three one um yes right two one and three one i think that might be right which would be exciting because then we would have the available moves for every piece okay so two one so we are at one one and then we can go to two one and we can also go to three one great so let&#39;s let&#39;s uh let&#39;s update our board here and let&#39;s just put a new pawn at a position that has our let&#39;s put let&#39;s put a white pawn at at 2 1 or at 2 2 2 2 is equal to pawn dot new and we&#39;re going to pass in the board we&#39;re going to pass in 2 2 and we&#39;re going to pass in white so we&#39;re going to put a white pawn right here at 0 0 1 1 2 2 we&#39;re going to put a white pawn right here and then the available moves for pawn 1 1 should include 2 2 now because we&#39;re going to have an enemy there so we&#39;ll just test that out and while that&#39;s going i think we also want one at 2 0. to test the final case okay ah so that did not well okay let&#39;s see uh run oh we&#39;re checking the available moves before putting the pawn in place so we need to move that down here okay stop run all right so we&#39;re creating our board we&#39;re creating the renderer and then we&#39;re passing in or we&#39;re creating two white pawns that are gonna show up uh in front of as possible enemies of the pawn at one one so when we check one one we should see great great excellent okay so this pawn here can either move forward it can move too forward it can take this one or it can take this one i think there might be rules in chess where like if there&#39;s something if there&#39;s a move where you can take a piece then you have to take the piece or something but whatever this is fine now let&#39;s also check the available moves for the piece that&#39;s right in front of this enemy because it should have zero moves because it cannot move forward so that&#39;s going to be the pawn at one three or no one two so the pawn at one two should have no available moves i think three two three two that seems incorrect right because there&#39;s a piece in front of it so it thinks it can move to this is one two three it thinks it can move to this position it cannot so we have a bug let&#39;s go back to our pawn class um it can only move two forward if this one was not empty so like if the board was not empty in front of us then i think we want to say just return return moves or something [Music] oh the other thing is we didn&#39;t check if we&#39;re at the start line uh oh shoot okay um yes so if we&#39;re at the start line then we can we can move forward too it has to be empty at four at two forward and it has to be empty at one forward and we have to be at the start line at start okay so then we need to define what it means to be at the start def at start so if we are if uh yeah the color is black and the uh the row or column let&#39;s see all right so let&#39;s um let&#39;s also make a convenient method for the current r in currency because we keep breaking out the location into current r and currency so let&#39;s just make that a method on piece here so we&#39;ll say def current r is at location.fir or location dot def current c is location.last and then we can come back into our piece into our pawn class and we can just remove this line and then we want to see like if the current uh row so if it&#39;s black then in the current row is 1 then we&#39;re at the start or if the color is white and the current row is six so that should tell us whether or not we&#39;re at the start and if we are at the start okay so yeah so we&#39;re going to add two forward if we&#39;re at the start and the board is empty at one forward and the board is empty at two forward this this should give us uh this should get us really close so that&#39;s looking good um i&#39;m gonna remove this uh okay all right cool so let&#39;s run this again and we should get no i think we should get no available moves back right okay cool so available moves is empty that&#39;s what we expected so everything is looking good i think that available moves should now work for every piece um and again in practice i would probably write some uh some sort of automated tests around this but i think that&#39;s looking pretty good so i think we&#39;ve got this to do done we&#39;ve got make a bail of available moves work for a pawn um yeah i guess in the next episode we can add some logic for actually moving around the board um and taking a piece okay so we&#39;re gonna we&#39;ll come back in the next episode and talk about how we might move a piece on the board to a new position thanks so much for watching we&#39;ll see in the next one you

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