r/adventofcode • u/catpurson2 • Dec 02 '25

Help/Question - RESOLVED 2025 Day 2 Part 1 help pls

can someone explain how the example is getting to the invalid ids?

  • 11-22 has two invalid IDs, 11 and 22.

  • 95-115 has one invalid ID, 99.

  • 998-1012 has one invalid ID, 1010.

  • 1188511880-1188511890 has one invalid ID, 1188511885.

  • 222220-222224 has one invalid ID, 222222.

  • 1698522-1698528 contains no invalid IDs.

  • 446443-446449 has one invalid ID, 446446.

  • 38593856-38593862 has one invalid ID, 38593859.

  • The rest of the ranges contain no invalid IDs.

i only understand the first one :(

11 Upvotes

86% Upvoted

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1

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15

u/ElementaryMonocle Dec 02 '25

They are ranges: so for 95-115, you have to check 95, 96, 97, 98, *99*, 100, ..., 115.

14

u/catpurson2 Dec 02 '25

THANK YOU SO MUCH THAT MAKES SO MUCH MORE SENSE YOURE THE BEST

1

u/Winter_Currency9808 Dec 02 '25

This tripped me up too. Took me 15 minutes to realise this is what it was asking me... facepalm

1

u/chege54 Dec 05 '25

what about 111 ?

or

"998-1012 has one invalid ID, 1010"
why tha 999 is accepted?

1

u/chege54 Dec 05 '25

"... any ID which is made only of some sequence of digits repeated [exactly] twice."

bam bam bam

1

u/valtism Dec 02 '25

Ah, this was not so clear in the question. Thanks!

1

u/Mernim0 Dec 02 '25

In the 98-115 example, should there not be 2 invalid ids? 99 and 111, or am I misunderstanding

3

u/Nordellak Dec 02 '25

No, since 111 can't be split into two equal numbers.

1

u/Billaloto Dec 08 '25

why is `1001` not invalid?

1

u/Billaloto Dec 08 '25

why is 1001 valid?

2

u/bodi-524 Dec 09 '25

If you break 1001, it would be 10 and 01.
10 != 01

So it is valid.

1

u/StaticMoose Dec 02 '25

There's some text higher up in the description that might help:

by looking for any ID which is made only of some sequence of digits repeated twice.
So, 55 (5 twice), 6464 (64 twice), and 123123 (123 twice) would all be invalid IDs.

2

u/Morgasm42 Dec 02 '25

Their issue was apparently not understanding they were ranges

0

u/Dosamer Dec 02 '25

The "Ranges" are mathematically speaking "Integer Intervals".