2024: Day 2

Setup

The original challenge

My thoughts

• Wrote my first Python function 👶
• “Read mode” using open()
• I keep typing <- instead of =
• It’s len() not length()
• I forgot the : in the for loop definition

Part 1

import numpy as np

# Open the file in read mode
with open("input") as file:
    # Read all lines and store them in a list
    input = [list(map(int, line.strip().split())) for line in file]

For each file I want to

• calculate the difference
• check is it’s “safe”
  • diff is either all positive or all negative
  • Only allowed in “small steps” of size 1, 2 or 3

def check_safe(file):
    """
    This function takes a file and returns if it is "safe" or not 
    """
    diff = np.diff(file)
    increasing = np.all(diff > 0)
    decreasing = np.all(diff < 0)
    small_steps = np.all(np.isin(abs(diff), np.array([1, 2, 3])))

    safe = (increasing or decreasing) and small_steps

    return safe

sum(list(map(check_safe, input)))

486

Part 2

I spent a while trying to avoid just brute forcing by looping over removing all possible exception. I found defining those edge cases a bit harder and just looped over the removal instead.

def check_safe_tolerant(file):
    """
    This is a tolerant check which allows one file to be removed. Returns if it is "safe" or not 
    """
    if check_safe(file):
        return True
    else:
        is_safe=np.zeros(len(file))
        for i in range(len(file)):
            is_safe[i] = check_safe(np.delete(file, i))
        return any(is_safe)

sum(list(map(check_safe_tolerant, input)))

540