Integer#digits returns the digits in reverse

TL;DR

Integer#digits returns the digits of the receiver as an array of integers, least significant digit first. 12345.digits returns [5, 4, 3, 2, 1]: the units digit 5 at index 0, the tens digit 4 at index 1, and so on.

Step by step

12345.digits # => [5, 4, 3, 2, 1]

A rough string-based equivalent:

12345.to_s.chars.map(&:to_i).reverse # => [5, 4, 3, 2, 1]

The reversed order is not an accident. It makes the index meaningful: digits[i] is the coefficient of 10**i, so the place value of each digit is encoded in its position. That also makes the number trivial to reconstruct:

d = 12345.digits

d.each_with_index.sum { |digit, i| digit * 10**i } # => 12345

And the units digit is simply first, without knowing the number's length:

12345.digits.first # => 5
12345 % 10         # => 5

Edge cases

digits accepts a base. The result is the positional representation in that base, still least significant first:

255.digits(16) # => [15, 15]
255.digits(2)  # => [1, 1, 1, 1, 1, 1, 1, 1]
0.digits       # => [0]

Negative numbers have no well-defined digit expansion, so digits raises:

begin
  -123.digits
rescue Math::DomainError => e
  e.message # => "out of domain"
end

digits is defined on Integer only; call to_i first if you are starting from a Float.