Iterate over just some elements of a Hash in Ruby - ruby-on-rails

Question

I've got a Hash that looks like this:
card = {
name: "Mrs.Jones",
number: "4242 4242 4242 4242",
exp_month: "12",
exp_year: "2014",
address: "90210 Beverly Hills",
added: "2014-11-09 09:14:23"
}
I'd like to iterate over just the number,exp_month and exp_year fields and update them. What's the most Ruby-like way of doing that?
This is what my code looks like at present:
card.each do |key,value|
card[key] = encrypt(value) # Only apply to number, exp_month and exp_year
end

Answer 1

I'd do it the following way:
ENCRYPTED_FIELDS = [:number, :exp_month, :exp_year]
card.each do |key,value|
card[key] = encrypt(value) if ENCRYPTED_FIELDS.include?(key)
end
But a better option would be to make a class for CreditCardDetails and define the setters to encrypt the data:
class CreditCardDetails
def initialize(hash)
hash.each do |k, v|
self.send("#{k}=", v)
end
end
#example for not encrypted field
def name=(value)
#name = value
end
#example for encrypted field
def number=(value)
#number = encrypted(value)
end
end

Answer 2

Since you already know which keys you'd like to encrypt, you can iterate over the wanted key names, instead of the hash:
ENCRYPTED_FIELDS = [:number, :exp_month, :exp_year]
ENCRYPTED_FIELDS.each do |key|
card[key] = encrypt(card[key])
end

Answer 3

Here are a couple of ways to do it. I've replaced the method encrypt with size, for purposes of illustration. They both use the form of Hash#merge that takes a block. The second approach does not use keys. Instead, it processes the value if the value is all digits (and spaces). I included that mainly to illustrate what you might do in other applications.
#1
card.merge(card) do |k,_,v|
case k
when :number, :exp_month, :exp_year
v.size
else
v
end
end
#=> {:name=>"Mrs.Jones", :number=>19, :exp_month=>2, :exp_year=>4,
# :address=>"90210 Beverly Hills", :added=>"2014-11-09 09:14:23"}
#2
card.merge(card) { |*_,v| v[/^[\s\d]+$/] ? v.size : v }
#=> {:name=>"Mrs.Jones", :number=>19, :exp_month=>2, :exp_year=>4,
# :address=>"90210 Beverly Hills", :added=>"2014-11-09 09:14:23"}
If you want to mutate card, use Hash#update (a.k.a. merge!) rather than merge:
#1a
card.update(card) do |k,_,v|
case k
when :number, :exp_month, :exp_year
v.size
else
v
end
end
#=> {:name=>"Mrs.Jones", :number=>19, :exp_month=>2, :exp_year=>4,
# :address=>"90210 Beverly Hills", :added=>"2014-11-09 09:14:23"}
card
#=> {:name=>"Mrs.Jones", :number=>19, :exp_month=>2, :exp_year=>4,
# :address=>"90210 Beverly Hills", :added=>"2014-11-09 09:14:23"}