Ruby default assignment ( ||= ) vs Rescuing error

Question

Since a ||= 1 is equivalent to a || a = 1, one can state that this is synctactic sugar for this:

if a.nil?
  a = 1
end

Likewise, given that session is a hash-like object, the following:

def increment_session_counter
  session[:counter] ||= 0
  session[:counter] += 1
end

is equivalent to:

def increment_session_counter
  if session[:counter].nil?
    session[:counter] = 0
  end
  session[:counter] += 1
end

Does that mean that the implicit if statement will be executed every time in the original definition of increment_session_counter? Since session[:counter] will mostly likely be nil only the first time (i.e. << 1% of the time), I have the feeling that the following code is better, since the implicit if won't be fired every time:

def increment_session_counter
  session[:counter] += 1
rescue NoMethodError
  session[:counter] = 1
end

Is this code better in such sense?

Having said that, I have no idea how Rescue is implemented in ruby and whether it's actually relevant regarding the tiny little optimization that can bring if so.

Answer 1

session[:counter] += 1 does three things:

1. fetch the value (Hash#[])
2. increment the value (Integer#+)
3. store the incremented value (Hash#[]=)

That's pretty convenient, but its brevity also makes it inflexible.

Providing a default is much easier if you separate the steps:

def increment_session_counter
  session[:counter] = session.fetch(:counter, 0) + 1
end