Advent of Code 2021 – Day 1: Sonar Sweep

Link to the Advent of Code Problem: https://adventofcode.com/2021/day/1

Language Used: Python

Efficiency: Part 1: O(n), Part 2: O(n)

Prework

Most of the Advent of Code data is served as a text file with newline separated data. This is our data loader, which we will use variants of for different days in the challenge:

stringData = open('day1_data.txt').read().split('\n')

data = []
for line in stringData:
    data.append(int(line))

This will take the day1_data.txt file, and load our input into an list called data, as integers. This is important to do ahead of time for this day, as string and integer comparison can give differing results.

Part 1 – Iterative Comparison:

This is a fairly straightforward problem. We should simply iterate over the data, tracking the last value obtained, while incrementing a counter every time the new value exceeds the last value obtained.

# Part 1

lastMeasurement = 0
increaseCounter = 0

for measurement in data:
    if measurement > lastMeasurement:
        increaseCounter += 1
    lastMeasurement = measurement

print(increaseCounter - 1)

We have to print increaseCounter - 1 instead of just increaseCounter due to the comparison of the first value to a 0 value. We could also do this:

# Part 1

lastMeasurement = data[0]
increaseCounter = 0

for measurement in data[1:]:
    if measurement > lastMeasurement:
        increaseCounter += 1
    lastMeasurement = measurement

print(increaseCounter)

This code should run fairly quickly. When ran with our time checker, usually this would run in milliseconds.

Part 2 – Sliding Window Sum Comparison:

The second part of this challenge is basically the same problem, with a catch! We need to track a sliding window of values. An easy way to do this is with slices. We will also have to consider a cursor, since we need to reference where we are within the data list.

With a cursor and the use of slices, we can express a sliding window as sum(data[cursor:cursor+3]). This will cut a section of the list out, from position cursor to cursor+3, and then get the sum() of the slice… this is our window!

It will also be easier in this case to use a while loop, to better control the limits of the cursor. We can simply do this by using len(). Since slices that attempt to reference past their limits only return what is available, we don’t need to pad this. For reference:

Python 3.9.13
>>> a = [1,2,3,4]
>>> a[0:44]
[1, 2, 3, 4]

In the example above, I attempted to get a slice larger than the list, and only the available data was returned. Since our data set does not contain negative numbers, having a window with less than 3 items in it will never increase our counter.

Here’s our solution:

# Part 2

oldWindow = 0
increaseCounter = 0

cursor = 0

while cursor < len(data):
    if sum(data[cursor:cursor+3]) > oldWindow:
        increaseCounter += 1
    oldWindow = sum(data[cursor:cursor+3])
    cursor += 1

print(increaseCounter - 1)

Conclusion

Thanks for reading! Look out for the next entries!