---
title: Monkey Math evaluating expressions - Advent of Code 2022 day 21 with Ruby
slug: monkey-math-evaluating-expressions-advent-of-code-2022-day-21-with-ruby
published_at: 2022-12-21 18:00:31 +0000
updated_at: 2026-03-04 20:15:16 +0000
summary: In this video, you will learn how to solve Day 21 of the Advent of Code for 2022. This one is called Monkey Math, and we will be using Ruby. We will solve an equation by iterating over our input and replacing expressions with values. Then, in part two, we will solve for X by replacing the Human value with X and printing out a long equation. By the end of the video, you will have a better understanding of how to solve equations with Ruby.
description: Monkey Math evaluating expressions - Advent of Code 2022 day 21 with Ruby  Challenge: https://adventofcode.com/2022/day/21 Solution: https://gist.github.com/cjavdev/49230e5c547ece89efb07dbb0c5d8ced  Website used for solving the big equation: https://www.dcode.fr/equation-solver  #ruby #adventofcode
tags: [cjav_dev, web development tutorials, web development for beginners, vim, ruby, advent of code, advent of code 2022, advent of code 2022 day 21, advent of code ruby, advent of code 2022 ruby, advent of code day 21 with ruby, advent of code 2022 day 21 with ruby, ruby solution, ruby tutorial, monkey math, evaluate expressions, send math operator ruby]
views: 338
author: CJ Avilla
url: https://www.cjav.dev/videos/monkey-math-evaluating-expressions-advent-of-code-2022-day-21-with-ruby
youtube_url: https://www.youtube.com/watch?v=xJqkse6gojo
youtube_id: xJqkse6gojo
embed_url: https://www.youtube.com/embed/xJqkse6gojo
thumbnail_url: https://i.ytimg.com/vi/xJqkse6gojo/hqdefault.jpg
type: video
---

# Monkey Math evaluating expressions - Advent of Code 2022 day 21 with Ruby

*Published: December 21, 2022*
*Views: 338*

## Watch

[Watch on YouTube](https://www.youtube.com/watch?v=xJqkse6gojo)

[![Monkey Math evaluating expressions - Advent of Code 2022 day 21 with Ruby](https://i.ytimg.com/vi/xJqkse6gojo/hqdefault.jpg)](https://www.youtube.com/watch?v=xJqkse6gojo)

## Description

Monkey Math evaluating expressions - Advent of Code 2022 day 21 with Ruby

Challenge: https://adventofcode.com/2022/day/21
Solution: https://gist.github.com/cjavdev/49230e5c547ece89efb07dbb0c5d8ced

Website used for solving the big equation: https://www.dcode.fr/equation-solver

#ruby #adventofcode

## Transcript

what's up welcome back in this episode
 you'll see how to solve day 21 of the
 Advent of code for 2022. this one's
 called Monkey Math we're using Ruby
 today the monkeys are back and we're
 worried that they're gonna steal our
 stuff and play monkey in the middle
 again this time we got to do some math
 so this is our input here the key on the
 left this is the name of the monkey and
 on the right is some expression or some
 value and so the monkeys are yelling out
 they're either yelling out a number or
 they're yelling out two other monkeys
 names and then some operation so it's
 saying like this root monkey is saying
 pppw plus sjmn and pppw is the name of
 another monkey and sjmn is the name of
 yet another monkey and we want to add up
 the results of those two but both of
 those also depend on other monkeys so
 we're gonna grab our input here we'll
 head over to the terminal and we'll open
 up
 day 21 and we're going to end add this
 at the end as always we're also going to
 add our Preamble as always we're going
 to say if ARG v.empty we're going to set
 some data equal to data.readlines of
 Chomp true and then otherwise we're
 going to grab our data off of a file
 that will read as input the way that I
 would solve this is is separate the
 amount into a couple different
 collections one collection is the
 monkeys that are already solved so dbpl
 here we already know that that value is
 five so we don't really need to do
 anything with dbpl other than know that
 this is a key and this is a value into a
 dictionary so we'll call that the solved
 monkeys
 and then for the other monkeys here like
 root where there's an expression let's
 just call that expressions or something
 and what we want to do is we want to
 iterate over each of these lines we're
 going to parse the line and then we're
 going to figure out should this go into
 solved or should it go into Expressions
 so say data to each line
 and then we're going to say if the line
 or then we want to split so we're going
 to say name and expression is going to
 be split on the colon and space
 so if there's a colon space then on the
 left we're going to get the name and on
 the right we're going to get either the
 expression or some value and so here we
 can say something like if the expression
 includes a space
 after we've done something with it so if
 this on the right includes a space that
 means it has like something space plus
 space something then we're going to use
 that as an expression otherwise we're
 going to just put the value directly
 into solved and we're going to convert
 it to an integer so that we can use it
 as a number now before we just dump this
 expression in as is just as the string
 let's also split that so we'll say
 expression.split and we're going to get
 back this triple so the thing on the
 left some operand and then the thing on
 the right so let's actually call it left
 operand and right that's what we'll get
 at when we split on space
 so someone left a comment on a previous
 video saying could just call split you
 don't have to pass in Split space that
 is like the default as you get back all
 the words so thanks for that comment
 lots of people have been dropping in
 helpful stuff so thanks again for for
 everyone who's been like participating
 now in the list of Expressions we're
 going to have the key being the name
 pointing out an array and in the array
 we'll have the left side we'll have the
 right side and then we're going to have
 some operand now rather than just
 keeping the string for the operand I'm
 going to convert this to a symbol that
 way we can use send to evaluate the
 operand as a method on the left with the
 right as the argument so we're going to
 see that later but for now this should
 be a good start so let's print out
 solved and we'll print out expressions
 and look at what we get
 I do this literally every single time I
 don't know how to remember that it's
 include instead of includes okay so on
 the top we have our solved all of our
 solved monkeys so this is D dbpl and its
 value is five and then we have our list
 of Expressions so root is PP PW plus
 sjmn now what we want to do is we're
 going to iterate over all of the
 expressions and try to solve those
 so the goal is that at the end we want
 to figure out what the root monkey is
 going to yell so the root monkey is this
 first monkey and we need to like solve
 for what that root monkey is going to
 yell we're going to go through and
 replace all of the expressions with the
 values from the solved case until we end
 up with a some sort of equation here on
 the right of root that we can actually
 solve so here let's iterate over the
 expression so we're going to say
 Expressions dot each do name and
 expression and then we're going to get
 the left and right off of the expression
 if the left is a string so if the
 actually if the left is not an integer
 and if we have solved the left
 so solve dot has key of the left
 then we're going to say that the left is
 equal to the the solved value from the
 left
 and that's going to give us some new
 thing that we're going to then
 ultimately put back into the array so
 Expressions at
 name is going to be equal to left up
 right but now this left is going to be
 replaced by the solution value so we
 need to do this also for the rights the
 right hand side so we'll grab that drop
 it in and we're going to just replace
 all of the left width and that should
 work okay now what we need to do is say
 if left is an integer and right is an
 integer then we want to like actually
 evaluate the expression and put that
 into the solution so solved at name is
 now going to be set equal to the left
 and we're going to send that operation
 with the right so this is basically the
 same as solved at name is equal to left
 plus right or whatever the operand is so
 when we have a left and right value
 around an arithmetic expression in Ruby
 that is the same as calling a method on
 that so we can actually open up IRB here
 let's look at how math Works in Ruby so
 if we have a is 1 and B is 2 and we say
 a plus b this is actually calling the
 plus method the plus method on a passing
 in an argument called B so you can
 actually do like a DOT
 plus and call it with a B like this and
 plus is a method on a because a
 implements the plus method so you can
 actually do a DOT send and pass in a
 symbol for the name of the method which
 is plus and then we can put in the
 argument B and we get back three so if
 we wanted to add one to the number one
 thousand or ten thousand we could pass
 that in as an argument to plus so here
 we're calling send we're saying left dot
 send the operand and we're passing in
 the right hand side of the expression
 and we're going to get back some value
 that's then saved inside of solved
 but before we move on from there we
 actually want to remove that name from
 Expressions because at this point this
 means that we have solved that monkey's
 things so let's run this and see what we
 get okay can't add a key new key to a
 hash during iteration uh oh okay so
 we're getting this error can't add new
 key to the hash during iteration because
 we're deleting the key and then we're
 trying to re-add it here so this is
 going to be we want to put this inside
 of an else statement so if we were able
 to solve it we want to start off and
 solved otherwise we want to update the
 current value at that point all right so
 what do we get back after running that
 statement now our solved has grown and
 we have more more inside of our solution
 but we still have some values inside of
 our expression that don't have that have
 not been completely evaluated so we
 actually want to do this a couple of
 times so what we could do is say while
 expression things dot any
 we want to keep
 iterating over all the expressions and
 evaluating them and we would get back
 okay so now we should we have nothing in
 expressions and instead our our solved
 is populated and the root in this case
 has the value 152 and that's actually
 the answer so what we can do is remove
 some of these print statements and then
 we can just put solved at root and that
 should be our answer so if we run it
 again we get 152 if we run it against
 our we got to add our puzzle input here
 we'll grab our puzzle input drop it into
 a file called input run this again
 against day 21 with input and this is
 some humongous number that is the answer
 for part one okay so that's the answer
 for part one that's the number that the
 root value is going to yell given our
 input okay so this was this was how we
 solved part one we built up our
 Expressions we built up a solution and
 then we kept iterating until we backed
 into to an answer for for the root okay
 let's take a look at part two so due to
 some kind of a monkey elephant human
 mistranslation you seem to have
 misunderstood a few key details first
 you got the job wrong for the root
 Monkey instead of applying the
 expression for root we need to apply the
 equal sign also h-u-m-n that's you
 that's that's a human and so in the
 example what we need to do is ignore
 what we got as the input for human and
 we need to figure out what the value is
 going to be for human we'll say this is
 part one and in part two as we're going
 through and iterating over our input
 instead of evaluating human we need to
 say something like if the name is Human
 then we want to skip it otherwise we
 want to go through and add everything
 else so my plan here is to replace human
 with the value X so we're going to solve
 as much as we can
 and once we have solved and kind of like
 gotten our Expressions backdated until
 as far as we can I'm going to put X in
 place of whatever value human was and
 then we're going to print out a long
 equation and then we'll have to solve
 for x okay so we're going to start off
 by not including human at all in the
 solution here now instead of being able
 to just iterate until the expressions
 are empty
 we can't do this anymore because even
 after we iterate several times there's
 still going to be some Expressions where
 we're trying to solve for x so what we
 want to do now is just let's just
 iterate a bunch of times so we'll say R
 is equal to zero R plus equals one and
 then we'll just say while R is less than
 15 or something and 15 rounds should be
 plenty to make our way through the
 potential Expressions all right so at
 the end of all of that let's just print
 out expressions and see what we get
 and we no longer have solved for root so
 let's just do that so if we look at
 these Expressions here all of them
 should have probably a monkey name and
 then an operand and then an integer
 value after 15 rounds so this is looking
 pretty good some of these numbers you'll
 notice are very very big and this one is
 a subtraction and okay so this is this
 is looking correct now what we want to
 do is after we have all of our
 Expressions now we need to go through
 and do some In-Place replacement of each
 of these names so what I'm going to do
 is find the expression that maps to each
 of these values in these arrays and I'm
 going to replace that with the entire
 expression this is of how
 how you might imagine
 executing a program might work right and
 here here's how we're going to do this
 so first we're going to just continue
 using this while loop as our way to
 solve as many Expressions as possible
 then I'm going to run another loop here
 let's start with one iteration so we'll
 say Expressions that each do key and
 then the value in this case is going to
 be
 either it will either be an array with
 three elements like this or it will be
 an array where it has it's got arrays
 inside of it and we're going to fill
 those or replace those
 strings so now we want to say something
 like V equals V Dot map so we're going
 to iterate over all of the values inside
 of
 here
 and we want to go get the expression for
 the strings so I'm just going to make
 another a little helper method that says
 get and we are going to pass in the name
 and the expression or we're going to
 pass in the name and yeah other
 expressions and we'll call get with v
 here and pass in Expressions so we need
 to do a couple things so if n
 is a string then we want to get
 Expressions at n
 otherwise we want to get n let's see if
 this let's see how this let's see how
 this does so now we want to set
 Expressions at k equal to V and also if
 there was and if Expressions have that
 key then we'll return let's actually set
 x equal to
 Expressions at n and then we're going to
 return X but before we do that we're
 going to delete that value n from the
 Expressions that way we're like
 shrinking the size of this of the
 possible Expressions as we work through
 it okay so after one iteration are we
 getting any better I think we are see
 how this one was like I don't know a
 little bit longer than this one so now
 if we run this I don't know three times
 do so we'll run through and try to solve
 this several times
 so that's getting even better there's
 still a few in here notice that like now
 the value for dhdf is an array of arrays
 and in this case we actually want to
 replace this cwrj with the expression
 from c w r j and so what we need to do
 is we also need to consider the case if
 if n is an array and if so we want to
 recurse so for each of the elements
 inside of n we want to go up and get
 those expressions
 so that should give us some value that
 we are going to map over Okay so let's
 let's run this okay we're getting much
 closer now so now we have wvlt and this
 is like a really big sort of expression
 and root still has a couple of things it
 needs to expand so let's run this I
 don't know 10 times or something that
 should get a that should be plenty all
 right so now the only key that's left in
 the Expressions is root and that gives
 us this giant thing on the right hand
 side that we need to evaluate we need to
 do something like two times whatever
 this giant number is minus and then
 whatever the evaluation of this giant
 expression is and then instead of Plus
 at the end we need to switch root so
 that it's using equals so as we're
 parsing what we can do here is we can
 just set like op is equal to
 the equal sign dot 2 SIM if the name is
 equal to root and if we run this again
 now you'll notice that we have an equal
 sign
 on so that we have this is what we're
 trying to make it equal to and on the
 left we've got this huge thing notice
 that here we still haven't replaced
 human with with any value because human
 is just going to be our x value what we
 can do in our
 iteration
 is let's see so if n is equal to Human
 then we're going to return X okay let's
 see if that worked all right so now we
 just get X here so the the last step in
 this process is to spit out a big
 equation and then we'll use that with
 some some
 tools online to figure out what the
 actual value is so let's run this
 against normal input and this is like
 what we want to print out is something
 like this this equation on the left well
 let's make a method here called I don't
 know print EQ this will again be a
 recursive thing and it's going to check
 if equation is an array then we're going
 to print the open paren
 otherwise we're going to print the close
 paren otherwise we're going to print
 eq.2s and then here at the bottom we'll
 just call
 print EQ for expressions
 and let's see what we get all right so
 this is our equation okay so now we want
 to do is go to I don't know solve for x
 decoder thing
 and there's this decode.fr website look
 at this thing this is so cool we were
 just watching the National Treasure the
 National Treasure movie and this looks
 just like it so I want the exact value
 and I wonder if I can just say solve
 equation
 boom okay so this is this is it right
 here x equals 301 and that is for the
 example answer 301 so that's what we got
 for root for the test use case now if we
 run it against our input we get back
 this massive equation so I'm just going
 to copy that again and we're going to
 lean on this National Treasure style
 website for solving our equation and
 here on the left we get this giant
 number which is going to be can we copy
 that boom all right that is the puzzle
 answer for part two
 all right that's a wrap for day 21 of
 the Advent of code I hope you're gearing
 up for the holiday winding down anything
 you've got going on at work at school
 and you're probably starting to travel
 getting ready to go see some family
 spend some time with your friends and
 hopefully you just unwind a little bit
 as the year comes to a close thanks so
 much for watching and we'll see in the
 next one

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