kuroshivo
Joined: Jun 8, 2019
• Posts: 11
Thanks for this post from:
May 13th, 2021 at 6:40:47 AM permalink
I just read today's newsletter from the Wizard. He says he wants to know if a string is an anagram of another.
I can't post a link to the method, but it's from the website "Fermat's Library."

Say we want to check if "12345" is an anagram of "54321";
1. map each possible character to a prime number: 1->2, 2->3, 3->5; 4->7; 5->11
2. take the product of all the characters in each string: "12345" -> 2*3*5*7*11, "54321" -> 11*7*5*3*2
3. If the products are equal, the two strings are anagrams.
Last edited by: kuroshivo on May 13, 2021
OnceDear

Joined: Jun 1, 2014
• Posts: 6341
May 13th, 2021 at 4:56:38 PM permalink
Quote: kuroshivo

I just read today's newsletter from the Wizard. He says he wants to know if a string is an anagram of another.
I can't post a link to the method, but it's from the website "Fermat's Library."

Say we want to check if "12345" is an anagram of "54321";
1. map each possible character to a primer number: 1->2, 2->3, 3->5; 4->7; 5->11
2. take the product of all the characters in each string: "12345" -> 2*3*5*7*11, "54321" -> 11*7*5*3*2
3. If the products are equal, the two strings are anagrams.

Very very neat
Beware. The earth is NOT flat. Hit and run is not a winning strategy: Pressing into trends IS not a winning strategy: Progressives are not a winning strategy: Don't Buy It! .Don't even take it for free.
Wizard
Joined: Oct 14, 2009
• Posts: 24221
May 13th, 2021 at 6:17:23 PM permalink
Quote: kuroshivo

I just read today's newsletter from the Wizard. He says he wants to know if a string is an anagram of another.
I can't post a link to the method, but it's from the website "Fermat's Library."

Say we want to check if "12345" is an anagram of "54321";
1. map each possible character to a prime number: 1->2, 2->3, 3->5; 4->7; 5->11
2. take the product of all the characters in each string: "12345" -> 2*3*5*7*11, "54321" -> 11*7*5*3*2
3. If the products are equal, the two strings are anagrams.

That is super cool in my book! Thank you.

Here is a related question.

Suppose you have two decks of cards, each with an ace to 10. You shuffle both decks. What is the probability that there are no positional matches? For example:

0-1-2-3-4-5-6-7-8-9
2-4-6-8-0-1-3-5-9-7

Please don't say (9/10)^10. There are positional correlations.
It's not whether you win or lose; it's whether or not you had a good bet.
EdCollins
Joined: Oct 21, 2011
• Posts: 1553
May 13th, 2021 at 10:04:10 PM permalink
I think the probability is .367879.

Assume the order of the first deck is 0-1-2-3-4-5-6-7-8-9.

There are 3,628,800 possible arrangements in Deck #2 and exactly 1,334,961 of them contain no positional matches with that particular order. 1,334,961 / 3,628,880 = .367879.

What I found interesting is that seems to be the general answer for most decks of n cards each.

Two decks of 4 cards; 24 possible arrangements and 9 of them contain no positional matches. 9 / 24 = .375
Two decks of 5 cards; 120 possible arrangements and 44 of them contain no positional matches. 44 / 120 = .3666
Two decks of 6 cards; 720 possible arrangements and 265 of them contain no positional matches. 265 / 720 = .3680
Two decks of 7 cards; 5,040 possible arrangements and 1,854 of them contain no positional matches. 1,854 / 5,040 = .36785
Wizard
Joined: Oct 14, 2009
• Posts: 24221
May 13th, 2021 at 10:18:33 PM permalink
Quote: EdCollins

I think the probability is .367879.

Assume the order of the first deck is 0-1-2-3-4-5-6-7-8-9.

There are 3,628,800 possible arrangements in Deck #2 and exactly 1,334,961 of them contain no positional matches with that particular order. 1,334,961 / 3,628,880 = .367879.

Thank you! Can you show or explain a solution other than brute force looping?
It's not whether you win or lose; it's whether or not you had a good bet.
EdCollins
Joined: Oct 21, 2011
• Posts: 1553
May 13th, 2021 at 11:34:30 PM permalink
I cannot. But I bet it's something rather simple.
charliepatrick
Joined: Jun 17, 2011
• Posts: 2577
May 14th, 2021 at 2:04:20 AM permalink
It approaches 1/e as n gets larger and is also known as the Hat Check Problem. link
Quote: https://en.wikipedia.org/wiki/Derangement

Derangements
Main article: Derangement

Another application of e, also discovered in part by Jacob Bernoulli along with Pierre Remond de Montmort, is in the problem of derangements, also known as the hat check problem:[18] n guests are invited to a party, and at the door, the guests all check their hats with the butler, who in turn places the hats into n boxes, each labelled with the name of one guest. But the butler has not asked the identities of the guests, and so he puts the hats into boxes selected at random. The problem of de Montmort is to find the probability that none of the hats gets put into the right box. This probability, denoted by p n {\displaystyle p_{n}\!} {\displaystyle p_{n}\!}, is:

p n = 1 − 1 1 ! + 1 2 ! − 1 3 ! + ⋯ + ( − 1 ) n n ! = ∑ k = 0 n ( − 1 ) k k ! . {\displaystyle p_{n}=1-{\frac {1}{1!}}+{\frac {1}{2!}}-{\frac {1}{3!}}+\cdots +{\frac {(-1)^{n}}{n!}}=\sum _{k=0}^{n}{\frac {(-1)^{k}}{k!}}.} {\displaystyle p_{n}=1-{\frac {1}{1!}}+{\frac {1}{2!}}-{\frac {1}{3!}}+\cdots +{\frac {(-1)^{n}}{n!}}=\sum _{k=0}^{n}{\frac {(-1)^{k}}{k!}}.}

As the number n of guests tends to infinity, pn approaches 1/e. Furthermore, the number of ways the hats can be placed into the boxes so that none of the hats are in the right box is n!/e (rounded to the nearest integer for every positive n).[19]

kuroshivo
Joined: Jun 8, 2019
• Posts: 11
May 14th, 2021 at 2:38:50 AM permalink
Quote: charliepatrick

It approaches 1/e as n gets larger and is also known as the Hat Check Problem. link

Cool! Here's another one related e:

Prove this: Select numbers between 0 and 1 randomly until sum is > 1. The expected number of selections needed is equal to e.
Wizard
Joined: Oct 14, 2009
• Posts: 24221
May 14th, 2021 at 5:49:27 AM permalink
Quote: kuroshivo

Prove this: Select numbers between 0 and 1 randomly until sum is > 1. The expected number of selections needed is equal to e.

I know this one, but I don't have time at the moment to go through it. I'll leave this hint, which is an infinite series needed for my proof.

I have a feeling there may also be a calculus proof, but I don't know it. Don or Ace?

The sum for n=1 to infinity of n/n! = e

In other words 1/1 + 2/2 + 3/6 + 4/24 + 5/120 + 6/720 + ... = e

It's not whether you win or lose; it's whether or not you had a good bet.
ThatDonGuy
Joined: Jun 22, 2011