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Lotto Picker - The Ruby Way

Using a lotto picker example to demonstrate random numbers and set operations the ruby way


Picking numbers for a lottery is a simple demonstration of two language features:

  • random number generation
  • producing a unique set

This is a little demonstration of how to do this the ruby way.

See also:

Random Number Generation

The Random standard library provides a pseudo-random number generator (PRNG), currently implemented as a modified Mersenne Twister with a period of 2**19937-1.

This is exposed as Kernel#rand and Random#rand:

 => 5
rand(10) # same as Kernel.rand
 => 6
 => 4

SecureRandom also provides a random number genreator interface:

 => 7

How do these rate?

  • Random#rand is better than Kernel#rand because forces a re-seeding of the PRNG
  • Random#rand is a very good random number generator:
    • Mersenne Twister is a solid PRNG used by many, many, many languages and libraries.
    • It’s much, much, much better than most system PRNGs.
  • SecureRandom#rand isn’t a better PRNG, but it is much more secure (i.e. hard to crack the seed or state of the random number generator). That matters for cryptographic purposes, but not really here as we are not concerned with security, just randomness.

So for the example, I’ll use Random#rand.

Unique Sets

We need to pick a unique set of numbers i.e. no repeats. There are many ways of doing this. Since we are picking from a continuous series, we can simplify things by just ensuring that the results set does is unique i.e. does not contain an repeating elements.

That can be done with array#uniq!:

my_numbers = []
 => []
my_numbers << 5
 => [5]
my_numbers <> 5
 => [5, 5]
 => [5]

There is an easier way using the Set library. I think this was added to ruby in one of the 2.x releases. The Set class provides collection of unordered values with no duplicates.

my_numbers = Set[]
 => #<Set: {}>
 => #<Set: {5}>
 => #<Set: {5}>

So for the example, I’ll use Set for ensuring a unique results set.

The Example

See lpickr.rb. The core routine uses Random#rand for random number generation and Set for unique result set

def pick(number_count, upper_limit)
  raise 'Upper limit must be >= the number to pick' if number_count > upper_limit

  prng =

  choices = Set[]
  while choices.length < number_count do
    choices.add 1 + prng.rand(upper_limit)

Using the Example

The lpickr.rb script supports a simple command line to pick “n” numbers from 1 to a max number “m”

Call for instructions:

$ ./lpickr.rb ?
Usage: ruby ./lpickr.rb <number_to_pick> <max>

  ruby ./lpickr.rb 6 40
  5, 6, 14, 28, 30, 38

Some sample runs:

$ ./lpickr.rb 3 10
1, 9, 10
$ ./lpickr.rb 3 10
2, 3, 4
$ ./lpickr.rb 3 10
4, 5, 9
$ ./lpickr.rb 3 10
4, 6, 7
$ ./lpickr.rb 7 49
1, 2, 18, 22, 29, 37, 49

Running the Tests

Some basic tests are included in test_lpickr.rb

$ ./test_lpickr.rb
Run options: --seed 26915

# Running:


Finished in 0.006796s, 588.5815 runs/s, 30164.8031 assertions/s.

4 runs, 205 assertions, 0 failures, 0 errors, 0 skips

Credits and References

About LCK#250 ruby
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This page is a web-friendly rendering of my project notes shared in the LittleCodingKata GitHub repository.

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