Post: Advent of Code day 2

posts/adventofcode/2017/02-corruption-checksum.md

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+---
+title: "Corruption Checksum — Python — #adventofcode Day 2"
+description: "In which I detect corrupt spreadsheets."
+slug: day-02
+date: 2017-12-02T09:15:46+00:00
+tags:
+- Technology
+- Learning
+- Advent of Code
+- Python
+series: aoc2017
+---
+
+
+!!! commentary
+    Today I went back to plain Python, and I didn't do formal tests because only one test case was given for each part of the problem. I just got stuck in.
+    
+    I did write part 2 out in as nested `for` loops as an intermediate step to working out the generator expression. I think that expanded version may have been more readable.
+    
+    Having got that far, I couldn't then work out how to finally eliminate the need for an auxiliary function entirely without either sorting the same elements multiple times or sorting each row as it's read.
+
+First we read in the input, split it and convert it to numbers. `fileinput.input()` returns an iterator over the lines in all the files passed as command-line arguments, or over standard input if no files are given.
+
+```python
+from fileinput import input
+sheet = [[int(x) for x in l.split()] for l in input()]
+```
+
+Part 1 of the challenge calls for finding the difference between the largest and smallest number in each row, and then summing those differences:
+
+```python
+print(sum(max(x) - min(x) for x in sheet))
+```
+
+Part 2 is a bit more involved: for each row we have to find the unique pair of elements that divide into each other without remainder, then sum the result of those divisions. We can make it a little easier by sorting each row; then we can take each number in turn and compare it only with the numbers *after* it (which are guaranteed to be larger). Doing this ensures we only make each comparison once.
+
+```python
+def rowsum_div(row):
+    row = sorted(row)
+    return sum(y // x for i, x in enumerate(row) for y in row[i+1:] if y % x == 0)
+
+print(sum(map(rowsum_div, sheet)))
+```
+
+We can make this code shorter (if not easier to read) by sorting each row as it's read:
+
+```python
+sheet = [sorted(int(x) for x in l.split()) for l in input()]
+```
+
+Then we can just use the first and last elements in each row for part 1, as we know those are the smallest and largest respectively in the sorted row:
+
+```python
+print(sum(x[-1] - x[0] for x in sheet))
+```
+
+Part 2 then becomes a sum over a single generator expression:
+
+```python
+print(sum(y // x for row in sheet
+          for i, x in enumerate(row)
+          for y in row[i+1:]
+          if y % x == 0))
+```
+
+Very satisfying!