Variable getting initialized with nil

Question

p b #undefined local variable or method b for main:Object
a = nil
if a and (b=3)
  do_something_with b
end
p b # nil

Why is b getting the value nil after the execution of if block, while expected result would be undefined local variable or method b for main:Object, Does Ruby initialize all the variables to nil in the memory beforehand ?

The same case with the following code

if nil
  bb = 10
end
p bb # nil

someone please throw some light on how ruby initializes the variables and what is going on in this case, Thanks

Answer 1

"[A local variable] is initialized if it appears on the left‐hand side (before the equals sign (U＋003D)) of an assignment expression, even if the expression does not actually execute. Variables of the latter sort have the value nil."

EDIT: This answer used to point to a fairly good Ruby reference, which has apparently been replaced by a malware site. I've removed the link but retained the quotation of the answer.