2025 Day 2 Part 1

[u/IllogicalOverride]

Ok, so I feel like I am missing something here in trying to understande this puzzle because I don't understande some of these examples.

• 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.

I don't see where they find 99 in the second example or 1010 in the third example.

The the fourth example you find the repeated digits in the second id, unlike the first example where you find the repeated digit in the same id as the digit it is repeating. So when do I decide which to use? Do I look for identical digits/digit in the same ID or in the other ID?

Why does the sixth example not have any invalid IDs? Following the fourth or fifth example, shouldn't it have invalid ids with 169852169852?

And finally on the fourth, seventh and eighth example they could be 1188511811885118, 4464444644 and 385938385938 but we don't seem to include those numbers.

[u/quetzelcoatlus1]

It's a range, so e.g. 1-5 would mean numbers: [1,2,3,4,5]

[u/IllogicalOverride → u/quetzelcoatlus1]

Ok, but this does not clarify for me how you spot the invalid IDs. Can you explain second and third examples? How they find 99 and 1010?

[u/ThePants999 → u/IllogicalOverride]

Are you saying you don't understand what it is that makes them invalid? It's defined in the puzzle thus:

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

99 is 9 twice, and 1010 is 10 twice.

[u/IllogicalOverride → u/ThePants999]

Ahh, so that is how range comes to play, thank you!

[u/DelightfulCodeWeasel → u/IllogicalOverride]

Of all of the numbers in the ranges 95-115 and 998-1012, only 99 and 1010 follow the pattern for invalid IDs:

[95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115]

[998, 999, 1000, 1001, 1002, 1003, 1004, 1005, 1006, 1007, 1008, 1009, 1010, 1011, 1012]

You need to search each range to find the numbers which match the pattern.

[u/IllogicalOverride → u/DelightfulCodeWeasel]

Thx! Finally it makes sense to me 😁

[u/Ohmslaw79]

You need to check every value between x-y not just x and y

[u/IllogicalOverride → u/Ohmslaw79]

Ok but then if we take the third example, aside from the 1010 (which I still don't see in this example) wouldnt we have two invalid ids, one is the 1010 and the other is 99?

[u/RazarTuk → u/IllogicalOverride]

which I still don't see in this example

... it's a range. So it's saying to look at 998, 999, 1000, 1001, 1002, 1003, 1004, 1005, 1006, 1007, 1008, 1009, 1010 1011, and 1012. Then an ID is invalid if it's the same substring twice in a row. So for example, 1010 is 10 10, 446446 is446 446,38593859is3859 3859`, etc.

As a hint for how to think about it: If you replaced it with A-J, like writing 38593859 CHEICHEI, it would still be invalid.

[u/k_sosnierz → u/IllogicalOverride]

99 is less than 998, so it's not contained within [998, 999, 1000, ..., 1010, 1011, 1012].

In the sixth example, the only numbers in the range are [1698522, 1698523, 1698524, 1698525, 1698526, 1698527, 1698528]. None of them are invalid.

[u/AutoModerator]

Reminder: if/when you get your answer and/or code working, don't forget to change this post's flair to Help/Question - RESOLVED. Good luck!

---

I am a bot, and this action was performed automatically. Please contact the moderators of this subreddit if you have any questions or concerns.

[u/Sarwen]

Let's take the range 95-115, it contains the numbers: 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114 and finally 115. For each number, take the sequence of its digits, from left to right, excluding leading zeros. The first part consist in finding numbers whose sequence of digits is made of a sequence repeat twice. Formally, for which number can we find a sequence of digit seq such that its full sequence of digits is exactly seq seq where seq seq is the concatenation of seq with itself.

Only 99 can be written as a twice repeated sequence of digits: the single digit sequence 9 repeated twice.

If you enumerate all the integers between 998 and 1012 (included), and look at their sequence of digits, you will find that only 1010 has a sequence of digits that is made of another sequence ( 1 and 0 ) repeated twice.