I'm just a newbie to ruby. I've seen a string method (String).hash .
For example, in irb, I've tried
>> "mgpyone".hash returns
=> 144611910 how does this method works ?
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2 Answers
The hash method is defined for all objects. See documentation:
Generates a
Fixnumhash value for this object. This function must have the property thata.eql?(b)impliesa.hash == b.hash. The hash value is used by classHash. Any hash value that exceeds the capacity of aFixnumwill be truncated before being used.
So the String.hash method is defined in C-Code. Basically (over-simplified) it just sums up the characters in that string.
1 Comment
One thing to realize is that if you run a program more than once, the hashes change. That is, it's not just computing a hash based on the characters contained in the string. E.g., try doing ruby -e "print 'a'.hash" twice in a row. If you want a reproducible hash, you can use something like Digest::MD5.
If you need to get a consistent hashing output I would recommend NOT to use 'string.hash but instead consider using Digest::MD5 which will be safe in multi-instance cloud applications for example you can test this as mentioned in comment in previous by @BenCrowell
Run this 2x from your terminal, you will get different output each time:
ruby -e "puts 'a'.hash" But if you run this the output will be consistent:
ruby -e "require 'digest'; puts Digest::MD5.hexdigest 'a'" 1 Comment
Jake
The use cases for each are different. String#hash gives an int used for picking a hash bucket when storing a value in a Hash. MD5 was for giving objects a deterministic unique fingerprint. Since it returns a chunky string, not an int, it wouldn't be very useful for a hashmap data structure.