Why does this method return three values?

It's because of the return statement, Ruby tries to interpret the right-hand side as an array and your code:

return a,b = b,a

gets evaluated as: (parentheses added for clarity)

return [a, (b=b), a]

i.e. a 3-element array [a, b, a] (assigning b to itself does nothing)

Remove the keyword return. It is actually doing: return a, (b), a. Where (b = b)

def swap(a,b)
  a, b = b, a
end

puts swap(5,3)

Output

=> 3, 5

In your program: in swap function the return you have given is return a,b = b,a remove a,band diretcly write return b,a. you will get your expected output. Reason: in return a,b = b,a is return first the value of a then executing b=b,a. Hope this might help you out

def swap (a,b)
   return b,a
end

puts swap(5,3)

Output

3
5