How to convert an integer from base 36 to base 10?

Question

I'd like to encode an integer as base 36, and then decode it to base 10. The encoding step is easy:

12345.to_s(36)
=> "9ix"

Now I'd like to decode that back into a base 10 integer. But this doesn't work:

"9ix".to_i(10)
=> 9

So how do I write

def base36to10(36)
  # ?
end

so that

r = rand(100000)
base36to10(r.to_s(36)) == r

?

Answer 1

You're not converting from base 10, you're converting from base 36, e.g., in Ruby console or irb:

> "9ix".to_i(36)
#=> 12345

Paraphrased from the docs:

Returns result of interpreting str as integer base base (between 2 and 36).

That said, isn't converting numbers between bases trivial? Iterate over the chars, multiply, add.