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2023 d1: Add walkthrough
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‎2023/README.md

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Advent of Code 2023 walkthrough
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===============================
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**Note**: in the hope of speeding up the process of writing walkthroughs each
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day, this year I am *not* going to give a brief summary of the "part 1" problem
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statement at the beginning of each day. Instead, I will jump right at the
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solution. The official problem statements are linked throughout the document for
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reference.
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Table of Contents
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-----------------
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- [Day 1 - Trebuchet?!][d01]
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<!--
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- [Day 2 - ][d02]
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- [Day 3 - ][d03]
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- [Day 4 - ][d04]
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- [Day 5 - ][d05]
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- [Day 6 - ][d06]
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- [Day 7 - ][d07]
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- [Day 8 - ][d08]
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- [Day 9 - ][d09]
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- [Day 10 - ][d10]
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- [Day 11 - ][d11]
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- [Day 12 - ][d12]
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- [Day 13 - ][d13]
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- [Day 14 - ][d14]
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- [Day 15 - ][d15]
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- [Day 16 - ][d16]
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- [Day 17 - ][d17]
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- [Day 18 - ][d18]
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- [Day 19 - ][d19]
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- [Day 20 - ][d20]
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- [Day 21 - ][d21]
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- [Day 22 - ][d22]
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- [Day 23 - ][d23]
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- [Day 24 - ][d24]
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- [Day 25 - ][d25]
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-->
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Day 1 - Trebuchet?!
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-------------------
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[Problem statement][d01-problem] — [Complete solution][d01-solution] — [Back to top][top]
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### Part 1
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Task seems easy enough. How do you find out if a character is a digit? Simply
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check [`char.isdigit()`][py-str-isdigit]. We can do this for each character of
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each line of input, first iterating forward to find the first, and then
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iterating backwards (using `[::-1]`) to find the last. The digits we find will
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need to be converted to `int`, and the first one will need to also be multiplied
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by `10`.
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```python
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fin = open(...)
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total = 0
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for line in fin:
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for char in line:
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if char.isdigit():
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total += 10 * int(char)
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break
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for char in line[::-1]:
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if char.isdigit():
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total += int(char)
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break
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```
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We can simplify this with the help of the [`filter()`][py-builtin-filter]
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built-in function: just filter out any character that satisfies `str.isdigit()`.
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To only extrac the first such character from the iterator returned by `filter()`
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we can simply use [`next()`][py-builtin-next].
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```python
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for line in fin:
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digit1 = next(filter(str.isdigit, line))
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digit2 = next(filter(str.isdigit, line[::-1]))
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total += 10 * int(digit1) + int(digit2)
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print('Part 1:', total)
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```
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### Part 2
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Things get more complex and this is probably the "hardest" day 1 problem I have
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seen so far. We need to also consider English *words* when checking each line of
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input. The first and last digits to appear either as a digit or as an english
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word need to be found.
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There isn't much to do except checking each spelled out English digit for each
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line. We can simplify things by building a `dict` to use as a lookup table:
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```python
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DIGITS = {
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'zero' : 0,
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'one' : 1,
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'two' : 2,
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'three': 3,
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'four' : 4,
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'five' : 5,
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'six' : 6,
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'seven': 7,
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'eight': 8,
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'nine' : 9,
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}
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```
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Now the check is a bit more annoying, so let's create a function for it: it will
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take a string and will check whether the first character is a digit (and in that
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case return it) or whether the string starts with a spelled-out English digit
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(and in that case convert and return it). We'll return `0` in case of no match
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for simplicity.
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```python
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def check_digit(string):
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if string[0].isdigit():
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return int(string[0])
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for d in DIGITS:
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if string.startswith(d):
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return DIGITS[d]
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return 0
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```
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The second loop above can again be simplified with the use of `filter()` +
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`next()`, but since this time we are not guaranteed to find an English word in
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`string`, we need to pass a second argument to `next()` for the default value to
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return in case `filter()` does not match anything.
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```python
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def check_digit(char, string):
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if string[0].isdigit():
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return int(string[0])
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d = next(filter(string.startswith, DIGITS), None)
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return DIGITS.get(d, 0)
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```
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We can now integrate the above function in the loop we wrote for part 1, using a
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second variable for the total. This time, we'll have to iterate over each
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possible substring manually, first forward and then backwards. We can easily use
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[`range()`][py-builtin-range] for that.
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```python
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total1 = total2 = 0
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for line in fin:
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# Part 1
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total1 += 10 * int(next(filter(str.isdigit, line)))
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total1 += int(next(filter(str.isdigit, line[::-1])))
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# Part 2
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for i in range(len(line)):
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digit1 = check_digit(char, line[i:])
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if digit1:
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break
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for i in range(len(line) - 1, -1, -1):
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digit2 = check_digit(line[i], line[i:])
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if digit2:
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break
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total2 += 10 * digit1 + digit2
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print('Part 1:', total1)
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print('Part 2:', total2)
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```
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There is technically a way to "simplify" this even more, again with the use of
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`filter()` + `next()`, but it does not really help anything. However, here it
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is, just for the fun of it:
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```python
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for line in fin:
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total2 += 10 * next(filter(None, map(check_digit, (line[i:] for i in range(len(line))))))
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total2 += next(filter(None, map(check_digit, (line[i:] for i in range(len(line) -1, -1, -1)))))
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```
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First two starts of the year done. Welcome to Advent of Code 2024!
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---
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*Copyright &copy; 2023 Marco Bonelli. This document is licensed under the [Creative Commons BY-NC-SA 4.0](https://creativecommons.org/licenses/by-nc-sa/4.0/) license.*
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![License icon](https://licensebuttons.net/l/by-nc-sa/4.0/88x31.png)
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[top]: #advent-of-code-2023-walkthrough
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[d01]: #day-1---trebuchet
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[d02]: #day-2---
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[d03]: #day-3---
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[d04]: #day-4---
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[d05]: #day-5---
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[d06]: #day-6---
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[d07]: #day-7---
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[d08]: #day-8---
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[d09]: #day-9---
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[d10]: #day-10---
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[d11]: #day-11---
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[d12]: #day-12---
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[d13]: #day-13---
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[d14]: #day-14---
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[d15]: #day-15---
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[d16]: #day-16---
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[d18]: #day-18---
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[d19]: #day-19---
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[d20]: #day-20---
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[d21]: #day-21---
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[d22]: #day-22---
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[d24]: #day-24---
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[d25]: #day-25---
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[d01-problem]: https://adventofcode.com/2023/day/1
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[d02-problem]: https://adventofcode.com/2023/day/2
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[d03-problem]: https://adventofcode.com/2023/day/3
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[d04-problem]: https://adventofcode.com/2023/day/4
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[d05-problem]: https://adventofcode.com/2023/day/5
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[d06-problem]: https://adventofcode.com/2023/day/6
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[d07-problem]: https://adventofcode.com/2023/day/7
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[d08-problem]: https://adventofcode.com/2023/day/8
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[d09-problem]: https://adventofcode.com/2023/day/9
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[d10-problem]: https://adventofcode.com/2023/day/10
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[d11-problem]: https://adventofcode.com/2023/day/11
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[d12-problem]: https://adventofcode.com/2023/day/12
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[d13-problem]: https://adventofcode.com/2023/day/13
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[d14-problem]: https://adventofcode.com/2023/day/14
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[d15-problem]: https://adventofcode.com/2023/day/15
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[d16-problem]: https://adventofcode.com/2023/day/16
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[d18-problem]: https://adventofcode.com/2023/day/18
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[d19-problem]: https://adventofcode.com/2023/day/19
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[d20-problem]: https://adventofcode.com/2023/day/20
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[d21-problem]: https://adventofcode.com/2023/day/21
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[d22-problem]: https://adventofcode.com/2023/day/22
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[d24-problem]: https://adventofcode.com/2023/day/24
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[d25-problem]: https://adventofcode.com/2023/day/25
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[d01-solution]: solutions/day01.py
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[d02-solution]: solutions/day02.py
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[d03-solution]: solutions/day03.py
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[d04-solution]: solutions/day04.py
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[d05-solution]: solutions/day05.py
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[d06-solution]: solutions/day06.py
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[d07-solution]: solutions/day07.py
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[d08-solution]: solutions/day08.py
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[d09-solution]: solutions/day09.py
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[d10-solution]: solutions/day10.py
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[d11-solution]: solutions/day11.py
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[d12-solution]: solutions/day12.py
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[d13-solution]: solutions/day13.py
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[d14-solution]: solutions/day14.py
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[d15-solution]: solutions/day15.py
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[d16-solution]: solutions/day16.py
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[d18-solution]: solutions/day18.py
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[d19-solution]: solutions/day19.py
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[d20-solution]: solutions/day20.py
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[d21-solution]: solutions/day21.py
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[d22-solution]: solutions/day22.py
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[d24-solution]: solutions/day24.py
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[d25-solution]: solutions/day25.py
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[py-builtin-filter]: https://docs.python.org/3/library/functions.html#filter
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[py-builtin-next]: https://docs.python.org/3/library/functions.html#next
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[py-builtin-range]: https://docs.python.org/3/library/functions.html#range
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[py-str-isdigit]: https://docs.python.org/3/library/stdtypes.html#str.isdigic

‎README.md

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### Quick links
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- **[How to run my solutions on your inputs][how-to-run]**
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- **AoC 2022: [walkthrough][2022-wal], [clean][2022-sol] / [original][2022-ori] solutions, complete [calendar][2022-cal] and [leaderboard][2022-lea]**
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- **AoC 2023: [walkthrough][2023-wal], [clean][2023-sol] / [original][2023-ori] solutions**
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- AoC 2022: [walkthrough][2022-wal], [clean][2022-sol] / [original][2022-ori] solutions, complete [calendar][2022-cal] and [leaderboard][2022-lea]
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- AoC 2021: [walkthrough][2021-wal], [clean][2021-sol] / [original][2021-ori] solutions, complete [calendar][2021-cal] and [leaderboard][2021-lea]
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- AoC 2020: [walkthrough][2020-wal], [clean][2020-sol] / [original][2020-ori] solutions, complete [calendar][2020-cal] and [leaderboard][2020-lea]
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- AoC 2019: [walkthrough][2019-wal], [clean][2019-sol] / [original][2019-ori] solutions, complete [calendar][2019-cal] and [leaderboard][2019-lea]
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*Copyright &copy; 2018-2023 Marco Bonelli.*
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[2023-wal]: 2023/README.md
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[2023-sol]: 2023/solutions
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[2023-ori]: 2023/original_solutions
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[2022-wal]: 2022/README.md
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[2022-sol]: 2022/solutions
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[2022-ori]: 2022/original_solutions

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