From: Paul Brannan <pbrannan@...>
Date: 2004-05-25T04:51:28+09:00
Subject: Re: Why have a Symbol class?

On Tue, May 25, 2004 at 01:01:25AM +0900, rolo wrote:
> Hi
> 
> As far as I understand Symbols are interned Strings. Why have a different
> Symbol class? Any advantages?

It is for efficiency.  A Symbol is just an entry in the symbol table;
it's hash value is the same as its object id:

irb(main):001:0> :foo.id
=> 3979534
irb(main):002:0> :foo.hash
=> 3979534

Compare this to a String, where the hash value is computed based on the
String's value:

irb(main):003:0> "foo".id
=> 537781650
irb(main):004:0> "foo".hash
=> 876516207

The result is that hash operations with Symbols are much faster:

irb(main):005:0> require 'benchmark'
=> true
irb(main):006:0> puts Benchmark.measure {
irb(main):007:1* h = {}; 1000000.times { h[:foo] = 1 }
irb(main):008:1> }
  0.960000   0.010000   0.970000 (  0.975577)
=> nil
irb(main):009:0> puts Benchmark.measure {
irb(main):010:1* h = {}; 1000000.times { h["foo"] = 1 }
irb(main):011:1> }
  2.410000   0.000000   2.410000 (  2.427366)
=> nil

Paul