"convenience" methods of creating hashes

Question

Here is how I've been creating hashes thus far (per Codecademy)

my_hash = {
  "pay" => "low",
  "vacation" => "yes",
  "parking" => "no"
}

And here is rubymonk's new convenience method of creating hashes.

chuck_norris = Hash[:punch, 99, :kick, 98, :stops_bullets_with_hands, true]

Are the colons in the second technique part of the syntax or parts of the string? And if they are just parts of the strings, why aren't the strings in quotes?

it is indeed a duplicate. Didn't know they were symbols objects. Going to go read that post now. Thx — dwilbank, Jul 28 '13 at 15:53

Arup Rakshit · Accepted Answer · 2013-07-28T16:18:34.803

2

Are the colons in the second technique part of the syntax or part of the string?

The : are not the part of the Hash creation syntax,You are seeing it there as your second example use symbols as keys. :punch,:kick,:stops_bullets_with_hands are Symbol objects.Hash::[] is the method used to create the Hash object.

Hash[:punch, 99, :kick, 98, :stops_bullets_with_hands, true]
     |_________| |________|
     (key,value) (key,value)

In the second example,the keys are considered as symbols. But you can use string,fixnum etc. too.

edited Jul 28 '13 at 16:18

answered Jul 28 '13 at 15:45

Arup Rakshit

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ah - a totally new object type introduced into the lesson with no warning! So they are NOT part of the syntax for creating a hash. I should just look at :punch, :kick, etc as if they are integers or strings. Right? – dwilbank Jul 28 '13 at 15:47
@Taymon How? `Hash::[]` method need only `(key,value)` pairs,it doesn't matters what objects the `key,value` combinations should be.. – Arup Rakshit Jul 28 '13 at 15:57
This should be actually in the Rubymonk forum. But there is no such forum. The problem was, they introduced symbols (with colons) into a very basic-level lesson about creating hashes using a new technique. The colons looked like part of the new syntax. Thanks for clearing it up sirs! – dwilbank Jul 28 '13 at 16:04

score 0 · Answer 2 · edited May 23 '17 at 12:05

Are the colons in the second technique part of the syntax or parts of the string? And if they are just parts of the strings, why aren't the strings in quotes?

Looking at the Rubydoc we can see that the Hash::[] method has three forms:

Hash[ key, value, ... ] → new_hash
Hash[ [ [key, value], ... ] ] → new_hash
Hash[ object ] → new_hash

Your specific example is an instance of the first case where a list of objects are interpreted as pairs of key-values. So calling:

Hash[a, b, c, d, e, f]

will pair:

a => b
c => d
e => f

I think you are confused by the : syntax, for which you can easily take a look at some topics here on SO. Just to summarize: the : in this case constructs a Symbol object.

In conclusion:

Hash[:punch, 99, :kick, 98, :stops_bullets_with_hands, true]

can be represented as the following hash:

{
    :punch => 99,
    :kick => 98,
    :stops_bullets_with_hands => true
}

where :punch, :kick and :stops_bullets_with_hands are symbols and 99, 98 and true the corresponding values.

To return the values you can simply use the Hash#[] method like this:

chuck_norris[:punch] # 99
chuck_norris[:kick] # 98
chuck_norris[:stops_bullets_with_hands] # true

Remember that you can convert a symbol to a string via the #to_s method:

:punch.to_s # "punch"
:kick.to_s # "kick"
:stops_bullets_with_hands.to_s # "stops_bullets_with_hands"

"convenience" methods of creating hashes

2 Answers2