Rumor Simulation

A couple months ago someone posted an interesting programming homework problem on reddit, asking for help. Help had already been provided before I got there, but I thought the problem was an interesting one.

Write a program that simulates the spreading of a rumor among a group of people. At any given time, each person in the group is in one of three categories:

  • IGNORANT - the person has not yet heard the rumor
  • SPREADER - the person has heard the rumor and is eager to spread it
  • STIFLER - the person has heard the rumor but considers it old news and will not spread it

At the very beginning, there is one spreader; everyone else is ignorant. Then people begin to encounter each other.

So the encounters go like this:

  • If a SPREADER and an IGNORANT meet, IGNORANT becomes a SPREADER.
  • If a SPREADER and a STIFLER meet, the SPREADER becomes a STIFLER.
  • If a SPREADER and a SPREADER meet, they both become STIFLERS.
  • In all other encounters nothing changes.

Your program should simulate this by repeatedly selecting two people randomly and having them “meet.”

There are three questions we want to answer:

  • Will everyone eventually hear the rumor, or will it die out before everyone hears it?
  • If it does die out, what percentage of the population hears it?
  • How long does it take? i.e. How many encounters occur before the rumor dies out?

I wrote a very thorough version to produce videos of the simulation in action.

It accepts some command line arguments, so you don’t need to edit any code just to try out some simple things.

And here are a couple of videos. Each individual is a cell in a 2D grid. IGNORANT is black, SPREADER is red, and STIFLER is white. Note that this is not a cellular automata, because cell neighborship does not come into play.

Here’s are the statistics for ten different rumors.

Rumor(n=10000, meetups=132380, knowing=0.789)
Rumor(n=10000, meetups=123944, knowing=0.7911)
Rumor(n=10000, meetups=117459, knowing=0.7985)
Rumor(n=10000, meetups=127063, knowing=0.79)
Rumor(n=10000, meetups=124116, knowing=0.8025)
Rumor(n=10000, meetups=115903, knowing=0.7952)
Rumor(n=10000, meetups=137222, knowing=0.7927)
Rumor(n=10000, meetups=134354, knowing=0.797)
Rumor(n=10000, meetups=113887, knowing=0.8025)
Rumor(n=10000, meetups=139534, knowing=0.7938)

Except for very small populations, the simulation always terminates very close to 80% rumor coverage. I don’t understand (yet) why this is, but I find it very interesting.

Have a comment on this article? Start a discussion in my public inbox by sending an email to ~skeeto/ [mailing list etiquette] , or see existing discussions.

This post has archived comments.

null program

Chris Wellons (PGP)
~skeeto/ (view)