#489 firstFrost

Using python to find the first frost: cassidoo’s interview question of the week (2026-09-28).

Notes

The interview question of the week (2026-09-28):

Given an array of daily temperatures and a number drop, return an array where each element is how many days you’d wait until it’s at least drop degrees colder than that day. If that never happens, put 0.

Example:
> firstFrost([70, 68, 72, 60, 65, 55], 5)
> [3, 2, 1, 2, 1, 0]

> firstFrost([50, 49, 48], 5)
> [0, 0, 0]

> firstFrost([40, 30, 45, 20], 10)
> [1, 2, 1, 0]

Thinking about the Problem

For a given day, things are quite simple - scan ahead until the difference exceeds the drop.

The challenge is perhaps finding a smart way to do this without having to enumerate the array for each day separately. Can’t immediately think how, so it is perhaps best to just start with a naïve implementation and see if that prompts any ideas.

Initial Solution

Starting with a naïve solution:

scan the daily temperatures
for each entry, scan ahead counting the days until the temperature drop exceeds the limit
record the result and move on to the next day

def firstFrost(daily_temps, drop):
    result = []
    for i, temp in enumerate(daily_temps):
        days_to_frost = 0
        for j in range(i + 1, len(daily_temps)):
            if daily_temps[j] <= temp - drop:
                days_to_frost = j - i
                break
        result.append(days_to_frost)
    return result

And that seems to work ok:

$ ./challenge.py "[70, 68, 72, 60, 65, 55]" 5
# Given:
# * Daily temperatures: [70, 68, 72, 60, 65, 55]
# * Drop: 5
# Days to wait for frost:
[3, 2, 1, 2, 1, 0]
$ ./challenge.py "[50, 49, 48]" 5 2> /dev/null
[0, 0, 0]
$ ./challenge.py "[40, 30, 45, 20]" 10 2> /dev/null
[1, 2, 1, 0]

Tests

I’ve setup some validation in test_challenge.py:

$ ./test_challenge.py
...
----------------------------------------------------------------------
Ran 3 tests in 0.000s

OK

Final Code

Final code is in challenge.py:

#! /usr/bin/env python
from sys import argv
from sys import stderr
import json


def firstFrost(daily_temps, drop):
    result = []
    for i, temp in enumerate(daily_temps):
        days_to_frost = 0
        for j in range(i + 1, len(daily_temps)):
            if daily_temps[j] <= temp - drop:
                days_to_frost = j - i
                break
        result.append(days_to_frost)
    return result


if __name__ == '__main__':
    if len(argv) == 3:
        daily_temps = json.loads(argv[1])
        drop = int(argv[2])
        print("# Given:", file=stderr)
        print("# * Daily temperatures:", daily_temps, file=stderr)
        print("# * Drop:", drop, file=stderr)
        print("# Days to wait for frost:", file=stderr)
        print(firstFrost(daily_temps, drop))
    else:
        print("Usage: challenge.py '<array of daily temperatures>' '<drop>'", file=stderr)

Credits and References

cassidoo’s interview question of the week (2026-09-28)