Fastest way to find a String into an array of string

Question

The script has to verify if one predefined IP is present in a big array of IPs. Currently I code that function like this (saying that "ips" is my array of IP and "ip" is the predefined ip)

ips.each do |existsip|
  if ip == existsip
    puts "ip exists"
    return 1
  end
end
puts "ip doesn't exist"
return nil

Is there a faster way to do the same thing?

Edit : I might have wrongly expressed myself. I can do array.include? but what I'd like to know is : Is array.include? the method that will give me the fastest result?

Read http://ruby-doc.org/core-1.9.3/Enumerable.html before any Ruby programming. — tokland, Feb 16 '12 at 15:30
You can use the `include?` method defined in class `Array` to make this operation look neater, I am not sure if it will increase the speed of the lookup much — Hunter McMillen, Feb 16 '12 at 15:30
possible duplicate of http://stackoverflow.com/questions/6140554/ruby-on-rails-2-search-string-in-hash — Joseph Le Brech, Feb 16 '12 at 15:31
@JosephLeBrech The question here is about searching an array. — Hunter McMillen, Feb 16 '12 at 15:38
Given the updated question, the answer is to not use an array of strings at all. The answer below that uses the `Set` class is a much better method. — Don Cruickshank, Feb 16 '12 at 15:44
If absolute speed is critical, using symbols instead of strings for the IPs may provide a small benefit (after you first switch to use either a Hash or Set). — Phrogz, Feb 16 '12 at 15:52

score 36 · Accepted Answer · answered Feb 16 '12 at 15:29

36

You can use Set. It is implemented on top of Hash and will be faster for big datasets - O(1).

require 'set'
s = Set.new ['1.1.1.1', '1.2.3.4']
# => #<Set: {"1.1.1.1", "1.2.3.4"}> 
s.include? '1.1.1.1'
# => true

answered Feb 16 '12 at 15:29

Aliaksei Kliuchnikau

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1

Or in your case: `s = Set.new(ips)` – Phrogz Feb 16 '12 at 15:32
Hello again Alex :) .include method source code seems to do almost the same thing as mine. Or is it actually faster? – Cocotton Feb 16 '12 at 15:36
2

@Cocotton: [Much faster](http://stackoverflow.com/questions/5551168/performance-of-arrays-and-hashes-in-ruby/5552062#5552062). You could also use a Hash with ip's as keys and 'true' as values. – steenslag Feb 16 '12 at 15:49
@steenslag,Phrogz : For now I'll leave it like this (.include), but when I'll be done will my script, I'll switch it for a hash – Cocotton Feb 16 '12 at 15:53
1

The obvious caveat here is that the Set is faster, but building the Set can be an expensive operation, so you wouldn't want to build a set to query it a small number of times. – Marc Talbot Feb 16 '12 at 16:11
3

For an array of 12.4 million short strings: `a=('a'..'zzzzz').to_a; time{ a.include?('0') } #=> 0.71s; time{ Set.new(a) } #=> 11.2s`; so yes, the overhead of creating the set needs to be worth the performance gains of instantaneous queries. – Phrogz May 15 '12 at 19:49
Backing up @Phrogz statement, check the source code of both #include? methods and this gist as proof: https://gist.github.com/vadviktor/66e524c591b2604ce2d7 – Ikon Jul 14 '15 at 14:14

ericraio · Answer 2 · 2012-02-21T21:12:59.713

6

You could use the Array#include method to return you a true/false.

http://ruby-doc.org/core-1.9.3/Array.html#method-i-include-3F

if ips.include?(ip) #=> true
  puts 'ip exists'
else
  puts 'ip  doesn\'t exist'
end

edited Feb 21 '12 at 21:12

answered Feb 16 '12 at 15:34

ericraio

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dku.rajkumar · Answer 3 · 2012-02-16T15:36:49.130

3

ips = ['10.10.10.10','10.10.10.11','10.10.10.12']

ip = '10.10.10.10'
ips.include?(ip) => true

ip = '10.10.10.13'
ips.include?(ip) => false

check Documentaion here

edited Feb 16 '12 at 15:36

answered Feb 16 '12 at 15:29

dku.rajkumar

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But is this actually faster than my method? Because the source code of this seems to do practically the same thing as mine. – Cocotton Feb 16 '12 at 15:36
ofcourse it is faster.. i have using in my project.. moreover when there is a method in ruby, why should we write extra code. – dku.rajkumar Feb 16 '12 at 15:39
@dku.rajkumar wanted to say, it should be faster as `.include?` is implemented on C level on the Array class. – Ikon Jul 14 '15 at 13:40

score 3 · Answer 4 · answered Feb 16 '12 at 15:29

3

A faster way would be:

if ips.include?(ip)
  puts "ip exists"
  return 1
else
  puts "ip doesn't exist"
  return nil
end

answered Feb 16 '12 at 15:29

Don Cruickshank

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Slightly faster, since the `each` occurs in C instead of Ruby, but it's still O(n) versus O(1) for a Hash or Set. – Phrogz Feb 16 '12 at 15:33

score 2 · Answer 5 · answered Feb 16 '12 at 15:29

2

have you tried the Array#include? function?

http://ruby-doc.org/core-1.9.3/Array.html#method-i-include-3F

You can see from the source it does almost exactly the same thing, except natively.

answered Feb 16 '12 at 15:29

Peter Ehrlich

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This is still an O(n) time operation, as it must search through every item in the array (even if it's in C). – Phrogz Feb 16 '12 at 15:33
I know that an enum can be sorted, but I don't know how to search such a sorted array. One could make an indexed database column to do the job. – Peter Ehrlich Feb 16 '12 at 17:54
1

Even a binary search is O(log n). Hashing an item and looking it up in a hash table is a constant-time operation that is unrelated to the number of items stored. – Phrogz Feb 16 '12 at 22:04

Fastest way to find a String into an array of string

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