From: Robert Klemme <shortcutter@...>
Date: 2006-11-26T05:50:07+09:00
Subject: Re: coding practise

On 25.11.2006 11:59, Paul Lutus wrote:
> sempsteen wrote:
> 
>> Hi all,
>> First of all sorry for my english.
>> I'm a Ruby newbie, trying to learn the language from the book, "The
>> Pragmatic Programmer's Guide". I loved the language very much. Now i
>> have some question marks about some issues.
>> If you help me understand this concept i'll be very happy.
>>
>> 1-) What does "Hash#has_key?" actually do?
> 
> It returns true if provided with a key that is present in the hash.
> 
>> Why do we need such a method in spite of using the result of "Hash#[]"
>> method which will return nil for a non-present key.

As others have pointed out if Hash#[] returns nil you do not know 
whether the key was in the hash with nil associated or whether it was 
missing.

irb(main):001:0> h={}
=> {}
irb(main):002:0> h.has_key? :foo
=> false
irb(main):003:0> h[:foo] = nil
=> nil
irb(main):004:0> h[:foo]
=> nil
irb(main):005:0> h.has_key? :foo
=> true
irb(main):006:0> h.size
=> 1
irb(main):007:0> h
=> {:foo=>nil}

However, in practice this is often negligible since you rarely use nil 
as a value in a Hash.

> The method has_key? is faster than using the key to find and return a value,
> which is what Hash#[] must do.

This is wrong: a hash lookup has to be done for both.

Kind regards

	robert