Advent of Code 2022 - Bash & Python

Hey there! This is my attempt at solving the Advent of Code 2022 puzzles in Bash and Python.

I retired Bash on day 11 part 2 because it was too slow 🐢, and switched to Python for the rest of the puzzles. 🐍

• OSX Monterey 12.6
• Bash 5.2.2(1)-release (x86_64-apple-darwin21.6.0)
• Python 3.8.2

Handy bash commands

# Remove newlines
tr -d '\n'

# Replace newlines with spaces
awk -v RS= '{$1=$1}1'

# Perform arithmetic (e.g. sum) on numbers separated by a space
sed 's/ /+/g' | bc

# Parse nth word in a line
cut -d' ' -f2 # 2nd word
cut -d' ' -f3 # 3rd word

Things I learned (Bash)

• The ‘#‘ symbol can be used to count the length of the string without using any command, e.g. line="test123"; echo ${#line}
• Shell expansion:
  • $(command) - command substitution
  • $((expression)) - arithmetic expansion
  • ${variable} - variable substitution
  • <(command) - process substitution
• ${variable:start:length} - Substring, where start is the index of the first character and length is the number of characters to extract. Use ${variable:start} to extract from start to the end of the string.
• comm - compare two sorted files line by line and write to standard output: the lines that are common, plus the lines that are unique. IMPORTANT: The files must be sorted beforehand!
• uniq - remove duplicate lines
• fold - spread the content into multiple lines, with a specified width, e.g. fold -w1 for one character per line.
• Always prepend files with #!/usr/bin/env bash instead of using zsh directly, because zsh doesn't support many of the features we need. For example, zsh doesn't work well with the comm command, gives us a syntax error.
• <<< is the syntax for here strings -- it feeds the string to the left side's stdin

read first second <<< "hello world"
echo $second $first

• Variables are global by default, even if they are declared inside a function. To make them local, use the local keyword.
• Bash has arrays, and you can implement stacks and queues. See day 5 for a Stack example.
• IFS (Internal Field Separator) is important! It determines how Bash recognizes Word boundaries. Useful when you care about parsing whitespace.
• Set multiple variables at once with read: read -r var1 var2 <<< "hello world", or read -r a b c < <(cut -d' ' -f2,4,6 <<< $line)
• == is an alias for = and it performs a string (lexical) comparison, eq performs a numeric comparison.
• Array keys are accessed with ${!array[@]} and values with ${array[@]}. See day 7.
• Use tac to reverse a file, last line first.
• Bash supports multidimensional arrays:
  • Declare an array with declare -A array_name
  • Set an element with array_name[key1,key2]=value
  • Get all keys with for key in "${!array_name[@]}"; do echo $key; done
  • Get all values with for value in "${array_name[@]}"; do echo $value; done
  • Get length with echo ${#array_name[@]}
• Text interpolation only works with double quotes, not single quotes.
• In Bash, you can break out of multiple loops! By default, break only breaks out of the innermost loop. To break out of all loops, use break 2 or break 3 etc.
• You can use BASH_REMATCH to capture the matched string in a regex. If the latest [[]]-expression matched the string, the matched part of the string is stored in the BASH_REMATCH array.
• Remove the first part of a string with string=${string#substring}. Remove the last part with string=${string%substring}.

Things I learned (Python)

• Use zip_longest from itertools to iterate over multiple lists of different lengths.
• Python's equivalent of the ...rest operator is *rest.
• I can use my own custom sort function with sorted by passing in a key function, with the help of functools' cmp_to_key function. Example: sorted([1, 2, 3], key=cmp_to_key(lambda a, b: a - b)) (See day 13)