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[–] 67 points 6 months ago

This is a lot like when Boston PD was found to screen out all the smart applicants. Sometime the company wants an obedient idiot.

https://www.cbsnews.com/news/too-smart-to-be-a-cop/

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  • [+] 94 points 6 months ago (12 children)
  • [–] 17 points 6 months ago

    If the ant can only move a single direction in time, it cannot reach all the time corners. Every corner in 3 dimensional space has a twin corner, at the beginning and end of time. Since the ant can only walk forward in time, it will only reach 2 4D corners, where it started, and where it ended.

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  • [–] 7 points 6 months ago (4 children)
  • [–] 2 points 6 months ago (2 children)

    I wonder what would have happened if someone had attempted to explain sinusoids to that man. Like, they'd probably be called a dumb evil bastard and some racial and homo/transphobic slurs followed by the sort of logic that only schizophrenics can follow. But still, a chunk of this really is just a man mapping squares on circles

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  • [–] 2 points 6 months ago (1 child)

    Unfortunately I don’t think this is true. Every 3D face is the intersection of a 2D plane with the upper and lower bounds of the 3rd dimension. So I think a hypercube “face” would be every 3D “plane” at both the very start time AND the very end time. Meaning the ant would need to immediately accelerate to light speed - so no time would pass - and then (otherwise) normally traverse the faces, wait until the end time, and then repeat the process in reverse (still at light speed).

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  • [–] 33 points 6 months ago* (last edited 6 months ago)

    Too many people are obsessing about 4d topology in this thread. The real difficulty in the question is the non -deterministic pathfinding of the ant, in the absence of pheromones.

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  • [–] 23 points 6 months ago (6 children)

    making sure you cannot solve it, so you are perfect for the job

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  • [–] 31 points 6 months ago (4 children)

    Possible candidate responses:

    1. Solves it (too smart for job)
    2. "That's bullshit, who needs this for a $14.50/hr job?" (too intolerant of bullshit for job)
    3. Tries to solve it but fails (lacks self-awareness for job)
    4. Knows they can't solve it so doesn't even try (too lazy for job)
    5. Doesn't understand the question/comprehend what a hypercube is (too dumb for job)
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  • [–] 7 points 6 months ago*

    I believe this is sometimes the case. I was called for an interview with a group of 15 other people ones. We were like a class, being interviewed as a group, and were supposed to solve some problems together. Nobody in that group could solve even the simple, obvious problems - we're talking basic math and reading comprehension here. Got an email the next day informing me that they had I had not been selected for recruitment.

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  • [–] 19 points 6 months ago (10 children)

    Sure. Draw the cube for me and I will plot it's path.

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  • [–] 14 points 6 months ago* (9 children)
  • [–] 33 points 6 months ago (6 children)
  • [–] 0 points 6 months ago* (5 children)

    No shit? Next thing you say that there are no 3d games, because there are no 3d monitors. And those that say they are 3d as well as VR are just faking it, by using two 2d projections instead of one.

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  • [–] 1 point 6 months ago*

    There are 2D monitors though.

    You can project a 3d object into 2d space and you can do the same with 4d into 3d, but collapsing more than that generally loses too much information. Edit: If you include movement you can reduce this effect somewhat depending what you're doing.

    Your portrait is now just a colored line the height of your subject, and this "4D cube" doesn't mean anything because it looks like a 3d cube with a smaller cube cut out of the middle of it. Unless you're really into geometry I guess it you dropped a /s.

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  • [–] 31 points 6 months ago* (last edited 6 months ago) (3 children)

    Four dimensional? That is a tesseract. This is impossible to describe how an ant would even interact with let alone touch all eight cells only once.

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  • [–] 15 points 6 months ago (3 children)

    One does not simply walk into the 4th dimension

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  • [–] 15 points 6 months ago

    At $14.50 per hour, he's going to take the shortest route.

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  • [–] 14 points 6 months ago (2 children)

    Entry level positions to Gregg's (fast food sausage roll chain) require 1000 word personal statements as part of online applications

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  • [–] 8 points 6 months ago

    Yeah but you also get equity in the company so I think that's fair enough.

    You have to be proven worthy before you are handed the recipe for the vegan sausage roll. I want to know what addictive substance they put in there.

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  • [–] 34 points 6 months ago (2 children)

    This is actually quite fun and simple! Even if the problem and my following explanation look complicated :P

    Let's look at the three dimensional case. One can parametrize a 3 dimensional cube as the Cartesian product of intervals [0, 1] x [0, 1] x [0, 1]. This means a cube is a set of points (a, b, c) where a, b and c are real numbers between 0 and 1. The 2 dimensional sides of the cube are then given by fixing one coordinate. That is, the 6 sides are

    {0}    x [0, 1] x [0, 1], 
    {1}    x [0, 1] x [0, 1], 
    [0, 1] x {0}    x [0, 1], 
    [0, 1] x {1}    x [0, 1], 
    [0, 1] x [0, 1] x {0} and 
    [0, 1] x [0, 1] x {1}. 
    

    Now we just start in the middle of a side at (0, 0.5, 0.5). To get to the next side we walk towards an edge (0, 0, 0.5) and then to the middle of the next side (0.5, 0, 0.5). We iterate this process until we run out of sides with a fixed 0, then walk towards a side with a fixed 1 and continue there. That is:

       (0  , 0.5, 0.5)
    -> (0  , 0  , 0.5) 
    -> (0.5, 0  , 0.5) 
    -> (0.5, 0  , 0  ) 
    -> (0.5, 0.5, 0  ) 
    -> (1  , 0.5, 0  ) 
    -> (1  , 0.5, 0.5) 
    -> (1  , 1  , 0.5) 
    -> (0.5, 1  , 0.5) 
    -> (0.5, 1  , 1  ) 
    -> (0.5, 0.5, 1  ) 
    

    This path basically spirals around the cube, going through every side only once. Here's a visualization (sorry, I'm no artist :P) visualization of this path on a 3 dimensional cube

    The same procedure works on a 4 dimensional cube or any other higher dimension. For the 4 dimensional cube it goes like this:

       (0  , 0.5, 0.5, 0.5)
    -> (0  , 0  , 0.5, 0.5) 
    -> (0.5, 0  , 0.5, 0.5) 
    -> (0.5, 0  , 0  , 0.5) 
    -> ...
    -> (0.5, 0.5, 0.5, 0  )
    -> (1  , 0.5, 0.5, 0  )
    -> (1  , 0.5, 0.5, 0.5)
    -> (1  , 1  , 0.5, 0.5)
    -> ...
    -> (0.5, 0.5, 0.5, 1  )
    

    This works for arbitrary dimension except for the 1 dimensional cube (which is just a line) because the "sides" there are the two end points of the line and not connected at all. Additionally note, that it is never specified how edges count in this problem, whether they somehow count towards a face or whether you're allowed to go back and fourth on edges. You could technically only walk along edges and step into the sides every now and then.

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  • [–] 7 points 6 months ago

    I skipped all the blabla and looked at the drawing and was pleased to see the path I started visualising in my head was exactly like that. I do think I would've needed a cube in my hands to confirm it, or a bit longer thinking about it instead to complete it.

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  • [–] 12 points 6 months ago (1 child)

    Choose a starting face and remember it. Walk each face of a cell containing that face touching each face once much like you would a 3-cube.

    Pick any adjoining cell and move into one of its faces from there, walk each of its faces saving the one opposite the face you started on for last.

    From there you're on a shared face with the cell opposite your starting cell. Traverse this one in a similar manner to the last, but this time also visit the adjoining faces of each cell adjacent to the second cell you filled, before once again ending opposite the face you started on for this cell.

    Now you're on a shared face with the final cell, opposite the face you started on. Walk around the remaining four faces and you're done.

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  • [–] 12 points 6 months ago* (last edited 6 months ago) (3 children)

    Wait, isn’t this trivial?

    If we’re talking about “faces” as in the cubic faces of a tesseract then each of the 8 faces are connected to all other faces except the opposite face. So just spiral around from your starting face (keeping the faces you’ve visited on the inside of the spiral) and you’re fine.

    If you mean 2D faces connecting the 3D ones, then things get more difficult but not that much because you can do the exact same thing. Choose a 1D edge as your origin, pick a face touching that edge to start with, traverse that edge twice to get the next two faces. Then traverse three faces which share edges with those faces you already traversed (there are 6 faces with this property, 3 for each vertex of our origin edge, the set you pick determines the “direction” of your overall progress through/around the tesseract). Repeat that step again but for the faces that share edges with two of the three you just did. Repeat again and again and again until the last three faces share a vertex with the origin edge you started with. You’re done.

    Am I missing something? Did the prompt mean to say you can only traverse each edge once?


    Edit: the 2D face path I described would miss 6 faces. Those six faces should be traversed in the middle, so do the first three faces, the second three, then all six which touch both those three you just traversed and the three you would have done next on the original path. Then do the rest just like I originally mentioned.

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  • [–] 12 points 6 months ago (2 children)

    I understood some of those words.

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  • [–] 5 points 6 months ago* (last edited 6 months ago)

    Have you ever seen one of those images of a tesseract where it’s like a cube in a cube? (You can just look up “tesseract” to find an image)

    Now, pick one of the corners of the outer cube and find the line that connects it to a corner of the inner cube. That’s our origin “edge” and we’re basically just going to move in through the cube along that direction.

    There are three “faces” which share that “edge” (line). We do those ones first.

    Then we move deeper in and do the three faces of the inner cube which share the corner our origin line connects to.

    Then we have to zig zag around the six “faces” that exist between inner and outer cubes which are roughly perpendicular to our origin edge. (Imagine you broke the tesseract in half by cutting halfway between your starting corner and the corner opposite it. The “faces” we need to traverse would intersect that plane)

    After that, we do the three faces on the far side of the inner cube. (The ones opposite our starting corner)

    Then we do the three around the line which connects that far corner of the inner cube to the outer cube.

    Then we do the three faces on the outside of the large cube at that corner.

    Finally we do the three faces on the outside of the cube around our starting corner.

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  • [–] 10 points 6 months ago

    Well I think the ant would probably wander around until it found food

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  • [–] 19 points 6 months ago (2 children)

    tricky with only four dimentions, but I'd use a Grathenbour's loop with a transverse Z axis movement if gimbal locks are ignored, naturally.

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  • [–] 45 points 6 months ago* (6 children)

    This is a direct appliacation of the hairy ball theorem.

    I ain't even kidding

    https://en.wikipedia.org/wiki/Hairy_ball_theorem

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  • [–] 42 points 6 months ago (5 children)

    Hairy ball theorem applies to even-dimensional spheres (the ordinary sphere is the 2D surface of the 3D solid), but a cube in four-dimensional space is a three-dimensional surface, so it doesn't apply.

    This is a question about graph theory, not topology; it's asking for a Hamiltonian path on the surface of 4D cube (where faces are vertices, which is different than the normal polytope graph).

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  • [–] 5 points 6 months ago (2 children)

    You are right.

    However most proofs of the hairy ball theorem also prove the converse, so that there is a continous non vanishing tangent vector field on uneven dimensional sphere surfaces.

    This can be extended to all 3 dimensional surfaces in 4 dimensions homomorphic to the sphere. The ant walking can follow the vector field and solve this problem topologically.

    My point being that the HR goon following the expected leet code solution might not understand this because they might expect the "approved" graph theory solution rather than an alternative approach.

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  • [–] 1 point 6 months ago (1 child)

    Why does following a tangent vector field visit all faces of the hypercube? Surely it's not going to visit something like a dense subset of the hypersphere's surface? (Or is it? My intuition comes from thinking about the torus)

    I'm more interested in the maths ;)

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  • [–] 2 points 6 months ago

    My topology and maths are very rusty, am a software developer these days.

    I think that there are both tangent vector fields that don't and some that do. In the two dimnsional case (circle) certainly all do.

    In n I intuitively would say that you should be able to have a vector field that does but I am now less confident to think about a proof on my bus rides while I answer here. I tried twice already.

    I will try to think about this more, will ping here if I get more

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  • [–] 17 points 6 months ago (6 children)

    Okay, if you can explain to me in detail how four dimensional topology is going to be important to me while I'm stocking the shelves of your grocery store, I'll give you an answer.

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  • [–] 6 points 6 months ago (1 child)

    Listen, once you get the job, you'll discover the truth about those shelves. And all I'm saying is, it becomes relevant that you can find your way through four dimensional space. Okay?

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  • [–] -1 points 6 months ago (3 children)

    stocking the shelves of your grocery store

    See that's what's so ragebaity about the post. There's no mention of what the job was, which means people can just make up whatever bit of background allows them to feel the most superior.

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  • [–] -1 points 6 months ago (2 children)

    It has a pay rate that is less than a living wage in many places. In fact, any job that could justify such a question could would be salaried. So it couldn't even be described with an hourly rate.

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  • [–] 16 points 6 months ago

    Be glad you got the shitty interview instead of getting ghosted

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  • [–] 7 points 6 months ago (7 children)

    Forward, left, right, forward, left.

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  • [–] 10 points 6 months ago (5 children)

    That’s a three dimensional cube.

    Which I thought by definition was a cube.

    What is a four dimensional cube?

    What is a two dimensional cube?

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  • [–] 3 points 6 months ago

    What is a four dimensional cube?

    2 three dimensional cubes, A and B, and each corner in A is connected by a new edge to it's equivalent corner in B. This is also called a tesseract.

    What is a two dimensional cube?

    A square.

    In general, if you have an n-dimensional cube, you can get an n+1 dimensional cube by doubling it, and connecting each corner with it's equivalent corner.

    A 0-dimensional cube is just a single dot.

    A 1-dimensional cube is a two dots, connected by a line.

    A 2-dimensional cube is 2 lines, connected. Also called a square.

    A 3-dimensional cube is 2 squares, connected. Also called, well, a cube.

    A 4-dimensional cube is 2 cubes, connected. Also called a tesseract.

    A 5-dimensional cube is 2 tesseracts, connected.

    In general, this is called the n-dimensional hypercube.

    And continuing this, each edge on an n-dimensional cube will, together with its copy and the two edges connecting it to its copy, form another face on the n+1 dimensional cube. This new face will border exactly one face on the original cube, and one face on the copy. It's also bordered by two other connecting faces.

    So, basically, if I start out on a face on the original cube, I can walk onto a connecting face, and then walk around all four connecting faces, and walk to the copy face. From there, I walk to another copy face, over the four connecting faces, and then back to the original cube. This way I should be able to continue going back and fourth between the cube and the copy, always walking across all the connecting faces.

    I got a rubics cube here, with sides white, red, blue, orange, green, and yellow. For each of these faces, there exist 5 additional ones on the tesseract. For white: white, white_copy, and white_c1 to white_c4.

    The solution is white, white_c1, white_c2, white_c3, white_c4, white_copy, red_copy, red_c1, red_c2, red_c3, red_c4, red, green, green_c1, green_c2, green_c3, green_c4, green_copy, orange_copy, orange_c1, orange_c2, orange_c3, orange_c4, orange, blue, blue_c1, blue_c2, blue_c3, blue_c4, blue_copy, yellow_copy, yellow_c1, yellow_c2, yellow_c3, yellow_c5, yellow.

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  • [–] 7 points 6 months ago*
    [–] 7 points 6 months ago

    Am I fucking stupid? Just walk in a shallow spiral?

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  • [–] 5 points 6 months ago

    "it would start by crawling up your ass."

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  • [–] 12 points 6 months ago (2 children)

    Isn't a cube by definition a 3 dimensional object? If it were 4 dimensional, it would no longer be a cube.

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  • [–] 23 points 6 months ago (1 child)

    Its a generalisation. A 4d cube is a shape that has the same length in all 4 dimensions. You can also talk of 5d cubes, 6d cubes, etc. These are commonly called n-cubes: a 4-cube is a 4d cube.

    There are also 4D spheres, even though spheres are definitionally 3D. They are called n-spheres.

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  • [–] 4 points 6 months ago

    "They would head towards the exit" and before they process youre already standing up and walking away.

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  • [–] 4 points 6 months ago

    "Being that the fourth dimension is time, it would need a flux capacitor and have to hit 88mph but where he's going he doesn't need 'sides.'"

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  • [–] 4 points 6 months ago

    We gotta start laughing in their faces

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  • [+] 4 points 6 months ago (2 children)
  • [–] 7 points 6 months ago (1 child)
  • [–] 3 points 6 months ago* (1 child)

    They specifically didn't say it was a hypercube, so I'm going to assume its a normal cube traveling through time and say that it could get to a max of 5 faces by moving forward 3 times, turning 90 degrees and moving forward again. The 6th face can be reached, if the cube's gravity-well is high enough, by getting a really good running start and jumping over the next to land on the 6th, Otherwise the ant is fucked and his father is very disappointed in him.

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  • [–] 1 point 6 months ago

    isn't 4d a line of cubes? would need 6d for cubes of cubes?

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