Bash Shell Script: Building a Better March Madness Bracket

Last year, I wrote an article for Linux Journal titled "Building Your March Madness Bracket" My article was timely, arriving just in time for the "March Madness" college basketball series. You see, I don't follow college basketball (or really, any sports at all), but I do like to participate in office pools. And every year, it seems my office likes to fill out the March Madness brackets to see who can best predict the outcomes.

Since I don't follow college basketball, I am not a good judge of which teams might perform better than others. But fortunately, the NCAA ranks the teams for you, so I wrote a Bash script that filled out my March Madness bracket for me. Since teams were ranked 1–16, I used a "D16" method borrowed from tabletop gaming. I thought this was an elegant method to predict the outcomes.

But, there's a bug in my script. Specifically, there's an error in a key assumption for the D16 algorithm, so I'd like to correct that with an improved March Madness script here.

Let's Review What Went Wrong

My Bash script predicted the outcome of a match by comparing the ranking of each team. So, you can throw a D16 "die" to determine if team A wins and another D16 "die" to determine if team B loses, or vice versa. If the two throws agree, you know the outcome of the game: team A wins and team B loses, or team A loses and team B wins.

I asserted that a #1 team should be a strong team, so I assumed the #1 team had 15 out of 16 "chances" to win, and one out of 16 "chances" to lose. Without any other inputs, the #1 ranked team would win if its D16 throw is two or greater, and the #1 team could lose only if the D16 value was one. With that assumption, I wrote this function:


function guesswinner {
  rankA=$1
  rankB=$2

  d16A=$(( ( $RANDOM % 16 ) + 1 ))
  d16B=$(( ( $RANDOM % 16 ) + 1 ))

  if [ $d16A -gt $rankA -a $d16B -le $rankB ] ; then
    # team A wins and team B loses
    return $rankA
  elif [ $d16A -le $rankA -a $d16B -gt $rankB ] ; then
    # team A loses and team B wins
    return $rankB
  else
    # no winner
    return 0
  fi
}

In the guesswinner function, each D16 roll generates a random number 1–16. If the rank of team A is "rankA" and the rank of team B is "rankB," and the D16 roll for team A is "A" and the roll for team B is "B," the function tests two D16 rolls like this:

  • If A greater than rankA (team A wins) and B less than or equal to rankB (team B loses), then team A wins.

  • If A less than or equal to rankA (team A loses) and B greater rankB (team B wins), then team B wins.

But look at what happens if team A is ranked #1 and team B is ranked #16. Team A will always win:

  • A roll 1–16 will have a 15 out of 16 chance to be greater than 1 (team A wins), and a 1–16 roll will always be less than or equal to 16 (team B loses).

  • A roll 1–16 will have a 1 out of 16 chance to be less than or equal to 1 (team A loses) but a 1–16 roll will never be greater than 16 (team B wins).

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Jim Hall is an advocate for free and open-source software, best known for his work on the FreeDOS Project. Jim earned his MS in Scientific and Technical Communication from the University of Minnesota, focusing on the usability of open-source software.