Method: Numeric#choose

Defined in:

lib/epitools/core_ext/numbers.rb

#choose(r) ⇒ Object Also known as: combinations

Combinations: compute “n choose r” (self.choose®)

This represents number of ways to pick “r” items from a collection of “self” items (where the order of the items doesn’t matter, and items can’t be repeated.)

eg: 49.choose(6) is how many ways can we pick 6 lottery numbers from a set of 49.

Formula: n! / (r! * (n-r)!) == n * n-1 * … * n-r / r * r-1 * … * 2

# File 'lib/epitools/core_ext/numbers.rb', line 136

def choose(r)
  (self-r+1..self).reduce(:*) / (2..r).reduce(:*)
end