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Blue Eyes

edited April 2008 in Everything Else
Via xkcd.
Can you figure it out? I'll admit, I couldn't, but I wish I would've worked harder before I looked at the solution. The answer is very elegant, and it isn't any bullshit like they're blind or something. Like he says, it's not a riddle or lateral thinking puzzle, it's just a straight-up logic problem. So, do your worst. Also, please don't spoil it for others.
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Comments

  • I'm lame. I didn't even try, and just like you I'm regretting not thinking more about it. It is an elegant puzzle, though.

    Advice to the next guy: Read the puzzle and take a week to casually ponder it.
  • SPOILER!All blue eyed people leave on the same ferry on the 100th night after the Guru said she saw someone with blue eyes./SPOILER!

    It's actually a really easy puzzle once you know it. As the note at the bottom says, this logic puzzle has roamed the world for ages. He just picked eye colours and an island. I've also heard the same logic puzzle with a line of prisoners all facing the same direction with either a green or a red hat on (or whatever two colours), and a version with a group of children where a certain number has dirty faces. It's quite simple once you know how to think. It took me some time to think up the proper method again and prove it to myself, but it didn't take me a week. MUAHAHAHAHA! That's what you get for studying Computer Science.
  • Apparently computer science doesn't teach you reading comprehension. She's only allowed to speak once, ever.
  • Apparently computer science doesn't teach you reading comprehension. She's only allowed to speak once, ever.
    I know she only speaks once. What makes you think she talks again? I did not say such thing at all. Heck, she doesn't even have to be on that island since it's just a useless hint.
  • No! It's not useless, she has to say it for anyone to leave.
  • No! It's not useless, she has to say it for anyone to leave.
    Nah. Only if the other people are brought to the island on seperate days. Which isn't stated specifically. She's just a start moment. If all people are brought at the same moment to the island and they are told the rules all at the same time is the same. Since they're perfect logicians.
  • It doesn't matter when any of them get on the island. She NEEDS to tell them the hint. That's part of the beauty of the puzzle.
  • It doesn't matter when any of them get on the island. She NEEDS to tell them the hint. That's part of the beauty of the puzzle.
    Not persay iirc. All that needs to be said is the existance of a state, a certain coloured hat, a certain eye colour, whatever, but that's basically part of the rules if you ask me, it's just an existing fact to start the logic.

    I do have to say she's a shitty guru. A real guru is wise and helpful, and would've said "I only see a person with blue eyes and a person with brown eyes." She's a bitch.
  • I worked it out within minutes.
  • I know she only speaks once. What makes you think she talks again? I did not say such thing at all. Heck, she doesn't even have to be on that island since it's just a useless hint.
    Hoisted by my own petard, then. :( I should've known better than to post so hastily, but I will console myself with the knowledge that there's one more person on the internet that doesn't know how to spell per se.
  • I worked it out within minutes.
    Same here.
  • edited April 2008
    How about this:-

    Two of the people (let's call them Rym and Scott), upon looking one another in the eyes, realise they could not bear to be seperated.
    How, in keeping with the rules, should Rym ensure (with absolute certainty) they won't be seperated?

    Note: This isn't difficult, and it is perhaps bending (albeit not breaking) the rules, but it is workable.
    Post edited by lackofcheese on
  • No! It's not useless, she has to say it for anyone to leave.
    I don’t understand how the guru’s hint gives the island people any more information then they already had. How does the hint start the hundred day count down?
  • How about this:-

    Two of the people (let's call them Rym and Scott), upon looking one another in the eyes, realise they could not bear to be seperated.
    How, in keeping with the rules, should Rym ensure (with absolute certainty) they won't be seperated?

    Note: This isn't difficult, and it is perhaps bending (albeit not breaking) the rules, but it is workable.
    1. Kill Scott
    2. Eat Scott
    3. ???
    4. Profit
  • edited April 2008
    Spoiler Alert! - Your post and mine
    No! It's not useless, she has to say it for anyone to leave.I don’t understand how the guru’s hint gives the island people any more information then they already had. How does the hint start the hundred day count down?
    What the guru does is bring the information into the realm of "common knowledge".
    Consider a case with only two people having blue eyes. Before the guru said anything, if blue-eyed person #1 noticed there was only one other person (#2) with blue eyes, they would think "I know that someone has blue eyes". However, since from #1's point of view it's possible #1 doesn't have blue eyes, which would mean #2 doesn't see any blue eyes at all, #1 doesn't know whether #2 knows that someone has blue eyes. However, if the guru said "Someone has blue eyes", #1 can now think, "I know that #2 knows that someone has blue eyes".
    Now, with that second bit of knowledge, the fact that the other blue-eyed person doesn't kill themselves on the first day means that #1 must deduce that they themselves have blue eyes.

    In the case of 100 blue-eyed people, any given blue-eyed person #1 could before only say
    "I know that #2 knows that #3 knows that #99 knows that someone has blue eyes"
    but now they can say
    "I know that #2 knows that #3 knows that #99 knows that #100 knows that someone has blue eyes"
    That makes all the difference.
    Post edited by lackofcheese on
  • edited April 2008
    How about this:-

    Two of the people (let's call them Rym and Scott), upon looking one another in the eyes, realise they could not bear to be seperated.
    How, in keeping with the rules, should Rym ensure (with absolute certainty) they won't be seperated?

    Note: This isn't difficult, and it is perhaps bending (albeit not breaking) the rules, but it is workable.
    1. Kill Scott
    2. Eat Scott
    3. ???
    4. Profit
    Well, perhaps I should have elaborated on the separation; let's just say they both have to be alive in the same place. The closer the better I'm sure.
    Post edited by lackofcheese on
  • ...ohh snap. Thanks cheese i get it now.
  • Wouldn't everyone just assume they have blue eyes and try to leave on the next ferry? Those who can't leave will know they have brown eyes and leave the next night.
  • Wouldn't everyone just assume they have blue eyes and try to leave on the next ferry? Those who can't leave will know they have brown eyes and leave the next night.
    They are perfect logicians, so they have no solid proof that they are blue eyed. They have to test it to see, and, once they've tested it, and proven to themselves that they are blue eyed, they will leave.
  • Wouldn't everyone just assume they have blue eyes and try to leave on the next ferry? Those who can't leave will know they have brown eyes and leave the next night.
    They are perfect logicians, so they have no solid proof that they are blue eyed. They have to test it to see, and, once they've tested it, and proven to themselves that they are blue eyed, they will leave.
    Don't they just test it by going to the boat believing they are blue-eyed? They only test they have is to go to the boat. If the boat lets them on they are right, if it does not they are wrong.
  • ......
    edited April 2008
    Don't they just test it by going to the boat believing they are blue-eyed? They only test they have is to go to the boat. If the boat lets them on they are right, if it does not they are wrong.
    God damnit. Read the entire damned page for god's sake.
    It doesn't depend on tricky wording or anyone lying or guessing,
    Also, to all of those who have also solved this. If we assume that every perfect logician on the island knows this logic puzzle and thinks practically, i.e. they all want to leave the island, but will follow the rules and only board the ferry if they know their own eye colour. Can we then say that everyone, except the guru, leaves on the 100th night?
    Post edited by ... on
  • I know the solution to the puzzle and it makes sense at low numbers. (Only one person with blue eyes and I see no one with blue eyes? Must be me!). As you add to it though it stops making sense. In part because the people can not communicate and the solution requires the people to communicate with each other.
  • edited April 2008
    Also, to all of those who have also solved this. If we assume that every perfect logician on the island knows this logic puzzle and thinks practically, i.e. they all want to leave the island, but will follow the rules and only board the ferry if they know their own eye colour. Can we then say that everyone, except the guru, leaves on the 100th night?
    No, the brown-eyed people would not leave, because they still can't be sure what color their eyes are
    Post edited by Linkigi(Link-ee-jee) on
  • edited April 2008
    I know the solution to the puzzle and it makes sense at low numbers. (Only one person with blue eyes and I see no one with blue eyes? Must be me!). As you add to it though it stops making sense. In part because the people can not communicate and the solution requires the people to communicate with each other.
    You are forgetting that they are perfect logicians, and can always remember how many people there are with each eye color. How does it require them to communicate?
    Post edited by Linkigi(Link-ee-jee) on
  • I know the solution to the puzzle and it makes sense at low numbers. (Only one person with blue eyes and I see no one with blue eyes? Must be me!). As you add to it though it stops making sense. In part because the people can not communicate and the solution requires the people to communicate with each other.
    You are forgetting that they are perfect logicians, and can always remember how many people there are with each eye color.
    True. The problem also states that the people can not communicate with each other. Making eye contact is a form of communication. Knowing what eye color the others have is not a form of communication.
  • The problem also states that the people can not communicate with each other. Making eye contact is a form of communication. Knowing what eye color the others have is not a form of communication.
    Ugh.
    Everyone can see everyone else at all times and keeps a count of the number of people they see with each eye color (excluding themselves), but they cannot otherwise communicate.
    The key word being "otherwise."
  • On the car ride home I ran this thing through my head and the low numbered answers are quick to figure out. Once you have more than a few blue eyed people it gets a lot more complicated.
  • Well, the spoilers are piling up in this thread, but Steve, it works. It's called induction. It works. It can be proven. Not in the scientific sense, in the mathematical sense.
  • alledinduction. It
    I know it works, I just have trouble following it after a certain point.
  • I know it works, I just have trouble following it after a certain point.
    The pattern should be obvious after the first few nights.
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