Building a hash in a conditional way

I am using Ruby on Rails 3.0.10 and I would like to build an hash key\value pairs in a conditional way. That is, I would like to add a key and its related 开发者_Go百科value if a condition is matched:

hash = {
  :key1 => value1,
  :key2 => value2, # This key2\value2 pair should be added only 'if condition' is 'true'
  :key3 => value3,
  ...
}

How can I do that and keep a "good" readability for the code? Am I "forced" to use the merge method?

I prefer tap, as I think it provides a cleaner solution than the ones described here by not requiring any hacky deleting of elements and by clearly defining the scope in which the hash is being built.

It also means you don't need to declare an unnecessary local variable, which I always hate.

In case you haven't come across it before, tap is very simple - it's a method on Object that accepts a block and always returns the object it was called on. So to build up a hash conditionally you could do this:

Hash.new.tap do |my_hash|
  my_hash[:x] = 1 if condition_1
  my_hash[:y] = 2 if condition_2
  ...
end

There are many interesting uses for tap, this is just one.

A functional approach with Hash.compact:

hash = {
  :key1 => 1,
  :key2 => (2 if condition),
  :key3 => 3,
}.compact 

Probably best to keep it simple if you're concerned about readability:

hash = {}
hash[:key1] = value1
hash[:key2] = value2 if condition?
hash[:key3] = value3
...

Keep it simple:

hash = {
  key1: value1,
  key3: value3,
}

hash[:key2] = value2 if condition

This way you also visually separate your special case, which might get unnoticed if it is buried within hash literal assignment.

I use merge and the ternary operator for that situation,

hash = {
  :key1 => value1,
  :key3 => value3,
  ...
}.merge(condition ? {:key2 => value2} : {})

Simple as this:

hash = {
  :key1 => value1,
  **(condition ? {key2: value2} : {})
}

Hope it helps!

IF you build hash from some kind of Enumerable data, you can use inject, for example:

raw_data.inject({}){ |a,e| a[e.name] = e.value if expr; a }

In case you want to add few keys under single condition, you can use merge:

hash = {
  :key1 => value1,
  :key2 => value2,
  :key3 => value3
}

if condition
  hash.merge!(
    :key5 => value4,
    :key5 => value5,
    :key6 => value6
  )
end

hash

First build your hash thusly:

hash = {
  :key1 => value1,
  :key2 => condition ? value2 : :delete_me,
  :key3 => value3
}

Then do this after building your hash:

hash.delete_if {|_, v| v == :delete_me}

Unless your hash is frozen or otherwise immutable, this would effectively only keep values that are present.

Using fetch can be useful if you're populating a hash from optional attributes somewhere else. Look at this example:

def create_watchable_data(attrs = {})
  return WatchableData.new({
    id:             attrs.fetch(:id, '/catalog/titles/breaking_bad_2_737'),
    titles:         attrs.fetch(:titles, ['737']),
    url:            attrs.fetch(:url, 'http://www.netflix.com/shows/breaking_bad/3423432'),
    year:           attrs.fetch(:year, '1993'),
    watchable_type: attrs.fetch(:watchable_type, 'Show'),
    season_title:   attrs.fetch(:season_title, 'Season 2'),
    show_title:     attrs.fetch(:id, 'Breaking Bad')
  })
end

Same idea as Chris Jester-Young, with a slight readability trick

def cond(x)
  condition ? x : :delete_me
end

hash = {
  :key1 => value1,
  :key2 => cond(value2),
  :key3 => value3
}

and then postprocess to remove the :delete_me entries