check hash value within an array

Question

I have an array (array1) of hashes that looks like this :

array1 = [
           {:ID=>"1", :value=>"abc"},
           {:ID=>"2", :value=>"def"}
         ]

I can loop over each hash and check each hash value manually:

array1.each do |h|
  if h.has_value?("def")
    #do something
  end
end

Is there a way to check if the hash value "abc" exists within a hash inside the array without having to iterate over the array?

Yes, don't use an array. It takes away the `O(1)` point of lookup for hash tables. Just use a JS Object literal for this. — Spencer Wieczorek, Apr 21 '17 at 20:34
Where does `array1` come from, how is it generated? Is it possible to change the structure (for example to an hash) at the origin of `array1`? — spickermann, Apr 21 '17 at 21:27
Do you consider `"1"` to be a value inside a hash of your array? — Eric Duminil, Apr 21 '17 at 22:35
yes, by definition "1" and "abc" are values of the hash. Where "ID" and "value" are keys. — danynl, Apr 22 '17 at 06:47

score 2 · Answer 1 · answered Apr 21 '17 at 20:42

find method is the most concise way with an array.

array1.find { |item| item[:value] == 'abc' }

Anyway, if you can start directly with an hash is better but if you have to switch to an hash from an array to have the O(1) lookup, it will probably be slower anyway.

score 2 · Answer 2 · answered Apr 21 '17 at 20:43

2

use any? instead of each, it breaks in early stage if found, best case O(1) and worst case O(n)

word = "def"
is_present = array1.any? { |h| h.has_value?(word) }

is_present is true if word is found in hash otherwise false

answered Apr 21 '17 at 20:43

aqfaridi

719
7
11

But this iterates over the array, which the OP doesn't want to do and is still `O(n)`. – Spencer Wieczorek Apr 21 '17 at 20:52
yes, you have to iterate ultimately, but ruby has any? which ease our work, how will u find element in array by iterating right. – aqfaridi Apr 21 '17 at 20:54
@SpencerWieczorek There is no avoiding iteration, however `any?` will stop processing at the first `true` value, avoiding a full array scan. – Mark Thomas Apr 21 '17 at 20:55
@MarkThomas You could certainly avoid iteration, OP is asking for a solution to an XY Problem. – Spencer Wieczorek Apr 21 '17 at 20:56
@SpencerWieczorek Agreed. An array is not the best starting point. – Mark Thomas Apr 21 '17 at 20:57
@MarkThomas, I agree that an array is ideal to start with. What would you recommend? storing everything in a hash as opposed to array? – danynl Apr 21 '17 at 21:01
@danynl if the typical use case is to check for the existence of a value, then the values you are using should actually be the keys of a hash. – Mark Thomas Apr 23 '17 at 00:01

score 1 · Answer 3 · answered Apr 21 '17 at 22:10

1

One way or another, you can't avoid iterating over the array. Here's one way.

array1.flat_map(&:values).include?("def")
  #=> true

Note

array1.flat_map(&:values)
  #=> ["1", "abc", "2", "def"]

answered Apr 21 '17 at 22:10

Cary Swoveland

106,649
6
63
100

From the question, I don't know if `"1"` and `"2"` should be here. – Eric Duminil Apr 21 '17 at 22:40

Eric Duminil · Answer 4 · 2017-04-21T22:36:08.873

You obviously have to iterate at least once over the elements of your array.

If you do this often, you should use another data format:

array1 = [
  { ID: '1', value: 'abc' },
  { ID: '2', value: 'def' },
  { ID: '3', value: 'abc' }
]

lookup_table = array1.each_with_object(Hash.new { |h, k| h[k] = [] }) do |hash, table|
  table[hash[:value]] << hash[:ID]
end

p lookup_table
# {"abc"=>["1", "3"], "def"=>["2"]}
p lookup_table['abc']
# ["1", "3"]

As a bonus, it gives you all the IDs for which this value is found, and it gives it fast.

If you just want to know if there's a value somewhere in your hashes, you could use a Set:

require 'set'
all_values = Set.new(array1.map{|h| h[:value]})
p all_values.include? 'abc'
# true
p all_values.include? 'xyz'
# false

Again, the lookup will be much faster than with an array of hashes.

check hash value within an array

4 Answers4