Difference between update(other_hash) and merge(other_hash)

Question

Update Code:

irb(main):001:0> h1 = { "a" => 100, "b" => 200 }
=> {"a"=>100, "b"=>200}
irb(main):002:0> h2 = { "b" => 254, "c" => 300 }
=> {"b"=>254, "c"=>300}
irb(main):003:0> h1.update(h2)
=> {"a"=>100, "b"=>254, "c"=>300}

Merge Code:

irb(main):001:0> h1 = { "a" => 100, "b" => 200 }
=> {"a"=>100, "b"=>200}
irb(main):002:0> h2 = { "b" => 254, "c" => 300 }
=> {"b"=>254, "c"=>300}
irb(main):003:0> h1.merge(h2)
=> {"a"=>100, "b"=>254, "c"=>300}
irb(main):004:0>

I ran the above merge and update method on the same hash. But got the same output. So here my question is: are update and merge works with same logic? if not same then how the output came same for those?

Answer 1

Are update and merge works with same logic?

No, they are not the same. update is an alias for merge!, which is the in-place variant of merge.

if not same then how the output came same for those?

Because in both cases you are using the return value of the call, however, the value of h1 is different in each case:

h1 = { "a" => 100, "b" => 200 }
h2 = { "b" => 254, "c" => 300 }
h1.update(h2)
h1  #=> { "a" => 100, "b" => 254, "c" => 300 }

h1 = { "a" => 100, "b" => 200 }
h2 = { "b" => 254, "c" => 300 }
h1.merge(h2)
h1  #=> { "a" => 100, "b" => 200 }

Answer 2

Hash#update is an alias for Hash#merge! http://ruby-doc.org/core-1.9.3/Hash.html#method-i-update