173: Movie Seating
Title text: It's like the traveling salesman problem, but the endpoints are different and you can't ask your friends for help because they're sitting three seats down.
In most movie theaters in the US, seats are not reserved. That is, tickets are sold by screening without seat assignment, and therefore an entering patron can take any vacant seat. Therefore, a group of incoming patrons may walk in a column and take a section of consecutive seats in a row, each person taking one next to the person in front of them. The order of these people is thus determined by the order in which they walk into the theater, which is in most cases random.
Cueball is upset at the way he and his friends have sat down at the movie theater. Part of the problem is that two people who are apparently in a relationship do not sit together. He therefore tries to use a social graph to calculate the best way for him and his seven friends to sit in a row, while taking into account all of the social connections among them. In mathematics, this type of problem is called combinatorial optimisation. The most popular example, the travelling salesman problem, is referenced in the title text, as well as in comics 287 and 399.
The title text shows that another part of the problem is that Cueball's friends, who could have helped him calculate a solution, are each sitting three seats away from him, and so he cannot ask them for help.
 Starting in 2017, certain Marcus theaters around the U.S. now sell movie tickets with reserved seats, both online and when purchased at the theater. When you buy your ticket, you also specify which seat you would like to sit in. Therefore, if you buy multiple tickets, you can select seats that are adjacent to each other, provided that you purchase them early enough before the point when you cannot find two or more empty seats together any longer.
Two hypothetical solutions for the specific problem proposed in the comic are displayed:
- At the movies, I get frustrated when we file into our row haphazardly, ignoring the computationally difficulty problem of seating people together for maximum enjoyment.
- [Map of relationships between 8 people.]
- Single line: friends.
- Double line: in a relationship.
- Arrow: one-way crush.
- Dashed line: acquaintances
- [The eight friends sit in a row in a dark cinema. Cueball and one other are between two lovers.]
- Cueball: Guys! This is not socially optimal!
- Randall states in xkcd: volume 0 that this was his "passive-aggressive way of asking my friends to let me plan seating."
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