What is the opposite of string.next?

Question

Is there a method in Ruby that does the opposite String#succ?

If you run succ or next you get this:

a = "4.4.10"
a.succ
=> "4.4.11"

I want the opposite:

a = "4.4.10"
a.previous
=> "4.4.09"

Is this possible? I was having a hard time finding this in the Ruby docs. Maybe it doesn't exist?

Answer 1

prev or something similar is not in the standard API because succ and a hypoethetical prev are surjective. Despite this, "Implement Ruby String Class Prev / Pred / Prev! / Pred! - Opposite Of Next / Succ Methods" is a possible version you could use.

The root problem is that succ is not inversible. This means, once you applied succ, then prev would end up with some ambiguity.