aufbakanleitung / advent-of-code-2020

https://adventofcode.com/2020

Advent of Code: Python solutions

https://adventofcode.com/2020

Lessons learned:

• assignment 2 - You can use the bitwise operator ^ to do a xor in python:

if bool(a) ^ bool(b):

• assignment 2 - Immediately assign split values to their variables for efficiency.

n1, n2, letter, x, passw = re.split('[-:\s]', line)

• assignment 3 - If you're using the x coordinate to calculate the y coordinate by, don't forget to divide y over x's step size xn to make them independent again (x*yn/xn). Though it's probably better never to make the two depend on each other in the first place.

for x in range(0, len(trees), xn):
    y = int(x*yn/xn) % len(trees[0])

• assignment 4 - This is a way to convert a list of lists with value pairs to a list of dictionaries, using list comprehension.

    dict_list = [dict([(value_pair.split(":")) for value_pair in list]) for list in llist]

• assignment 4 - If you remove values from the list you're looping over the length will change and values will be skipped. Instead make a copy() of it and remove the values from there.

list = [1,4,7,7,2,3,4,5]; copied_list = list.copy(); test = [3,4]  # Make three lists
[copied_list.remove(item) for item in list if item in test]        # Remove items in test from copied_list

• assignment 6 - This is how you sort a string.

sorted_string = ''.join(sorted(unsorted_string))

• assignment 7 - In Regex look before a certain string with the ?= positive lookahead and find all occurences with re.findall().

re.findall(r'(\d+)*\s*(\w+\s\w+)(?=\sbag)', line)

• assignment 7 - Recursion is very powerful, looks very clean, but it's really difficult to grasp.

def find_bags(color): 
    teller = 0
    for bag in bags_dict[color]:
        teller += bags_dict[color][bag] + find_bags(bag) * bags_dict[color][bag]  # Recursively sum the contents of the bag
    return teller