Hello,
I am trying to solve the day 7 using Rust and this is what I came up with so far:
use std::fs;
fn calculate(answer: &i32, numbers: &mut Vec<i32>) -> bool {
if numbers.len() >= 2 {
let tmp1 = numbers[0];
let tmp2 = numbers[1];
numbers.remove(0);
numbers.remove(0);
numbers.insert(0, tmp1 * tmp2);
if calculate(answer, numbers) == true {
return true;
} else {
numbers.remove(0);
numbers.insert(0, tmp1 + tmp2);
if calculate(answer, numbers) == true {
return true;
} else {
return false;
}
}
} else {
if *answer == numbers[0] {
println!("> {} true", numbers[0]);
return true;
} else {
println!("> {} false", numbers[0]);
return false;
}
}
}
fn main() {
let contents = fs::read_to_string("sample.txt")
.expect("Should have been able to read the file");
for line in contents.lines() {
let tmp = line.split(":").collect::<Vec<&str>>();
let answer = tmp[0].to_string().parse::<i32>().unwrap();
println!("{:?}", answer);
let numbers_str = tmp[1].split(" ");
let mut numbers: Vec<i32> = Vec::new();
for num in numbers_str {
if num.len() == 0 {
continue;
}
numbers.push(num.parse::<i32>().unwrap());
}
println!("{:?}", numbers);
if calculate(&answer, &mut numbers) == true {
println!("it's true");
}
}
}
I don’t know why the recursion is not working. any help would be appreciated.
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I am not sure what is the real bug in here but I do know one thing.
you should not work forwards in the list of numbers to see if it equates.
Instead you should work from the last number back to the first one. This is because if the each number tested cannot divide without a remainder or be subtracted to 0 or above then the list of numbers is entirely invalid.This is a state solver type problem. You are basically bruteforcing each possible state and seeing if one is valid. This is computationally expensive.
take a look at the example:
3267: 81 40 27
81 + 40 * 27and81 * 40 + 27both equal3267when evaluated left-to-right
So working backwards from the final state3267to the first state in the list81:
3267 / 27 = 121 - 40 = 81and so it is valid!
3267 - 27 = 3240 / 40 = 81Also valid!but here are also examples that show being invalid:
192: 17 8 14
192 / 14 = 13.7Invalid, skip checking the rest to save time!
192 - 14 = 178 / 8 = 22.25invalid! needs to be the same as the first number in the list and cannot have a remainder!
192 - 14 = 178 - 8 = 170 != 17invalid! needs to be the same as the first number in the list!
this exhausts all paths that can be created!wow, that’s a nice idea. I will try to solve this problem with this method if mine doesn’t work.
Are you getting any error or is there an input that isn’t working as expected? I don’t know Rust but I copied the logic of “calculate” into Python and it worked for my puzzle input, so I don’t think the logic is incorrect. Are you sure the file is being parsed correctly?
last input isn’t working, for the last input answer should be true but I am getting false. here is the output:
rustc t1.rs && ./t1 190 [10, 19] > 190 true it's true 3267 [81, 40, 27] > 87480 false > 3267 true it's true 83 [17, 5] > 85 false > 22 false 156 [15, 6] > 90 false > 21 false 7290 [6, 8, 6, 15] > 4320 false > 303 false > 54 false > 14 false 161011 [16, 10, 13] > 2080 false > 173 false > 26 false 192 [17, 8, 14] > 1904 false > 150 false > 25 false 21037 [9, 7, 18, 13] > 14742 false > 1147 false > 81 false > 16 false 292 [11, 6, 16, 20] > 21120 false > 1076 false > 82 false > 17 falseIn case this helps too, here’s the Python version of your code (with print statements removed):
def calculate(answer: int, numbers: list[int]) -> bool: if len(numbers) >= 2: tmp1 = numbers[0] tmp2 = numbers[1] numbers = numbers[2:] numbers.insert(0, tmp1*tmp2) if calculate(answer, numbers): return True else: numbers = numbers[1:] numbers.insert(0, tmp1+tmp2) return calculate(answer, numbers) else: return answer == numbers[0]Thanks I will try to run the python version and see if it works. I think the problem is that I am passing list of numbers by reference and in python it’s passed by value, this is the reason the python version works while mine doesn’t.
82 = 11 × 6 + 16
Your algorithm should then multiply 82 by 20 to get 1640, but it doesn’t. It stops one number short.17 = 11 + 6
Your algorithm is now stopping two numbers short.Looking above at the output for 292, your algorithm gives the correct answer but only checks 4 numbers. There are 8 possible combinations (2 choices between each number = 2×2×2 possible combinations), it’s not checking all of them. In fact it’s stopping short just like above.
81 = 9 × 7 + 16
Stopping short of multiplying 81 by 13.The first two numbers in each case are equivalent to what my Python version checks, but after that your algorithm no longer checks all the numbers. The array is shorter than it should be, and becomes shorter as the function is called recursively.
I’m going to assume that Python keeps multiple versions of the array in memory and uses a different one each time calculate is called, while your Rust implementation isn’t and every call to calculate is using the same array and thats why it’s getting shorter. I don’t normally use languages where I have to manage memory so that’s just a guess!
If it helps, the output for 292: 11 6 16 20 in Python is
14742 1147 1053 94 3744 301 442 47If you’re not getting that output then somethings wrong.
I’m going to assume that Python keeps multiple versions of the array in memory
yeah you are right, I am passing the vector of numbers by reference, that’s why my solution doesn’t work.