From: Michael Edgar <adgar@...>
Date: 2011-03-30T05:36:36+09:00
Subject: Re: Why can a floating point number be used as an array index?

Keep in mind that arrays use the implicit integer conversion protocol, #to_int, to make this conversion. You
can't use any old object with #to_i:

>> [1, 2, 3]["1"]
TypeError: can't convert String into Integer

Very few classes implement this protocol:

>> ObjectSpace.each_object { |a| p(a) if Module === a && a.instance_methods(false).include?(:to_int) }
Float
Integer
Numeric

I suspect the primary reason for allowing Arrays to convert floats to appropriate indices is to simplify calculated array
indices in *any* case. Sam gives a good example, but there are even more common, simple cases where
one might calculate array indices and not want to have to call #to_i every time. Here's one for determining
what item in a list was clicked:

chosen_item = items[ pixel_clicked.y / item_size ]

Michael Edgar
adgar@carboni.ca
http://carboni.ca/

On Mar 29, 2011, at 4:17 PM, Sam Duncan wrote:

> How about statistical bucketing. Imagine your indexes 1, 2, and 3 are Gb of RSS or something.
> 
> > irb
> ruby-1.9.2-p0 > rss = [0.123, 1.223, 5.33, 2.294, 7.66, 5.6, 3.222, 3.88, 3.44, 1.78] # incoming rss values
> => [0.123, 1.223, 5.33, 2.294, 7.66, 5.6, 3.222, 3.88, 3.44, 1.78]
> 
> ruby-1.9.2-p0 > buckets = (0..rss.max).collect {|i| i} # bucket indexes
> => [0, 1, 2, 3, 4, 5, 6, 7]
> 
> ruby-1.9.2-p0 > rss.reduce(Hash.new(0)) {|h, v| h.tap{|h| h[buckets[v]]+=1}} # statistics!
> => {0=>1, 1=>2, 5=>2, 2=>1, 7=>1, 3=>3}
> 
> I'm sure there are lots of (probably simpler) ways to do the same thing, but it was a fun exercise nonetheless =]
> 
> Sam
> 
> 
> On 30/03/11 07:35, Jeff Dik wrote:
>> Why can a floating point number be used as an array index?  Anybody
>> know of a good use case for this?
>> 
>> irb(main):020:0>  [1, 2, 3][0.3]
>> 1
>> irb(main):021:0>  [1, 2, 3][0.9]
>> 1
>> 
>> Just curious,
>> Jeff
>> 
>>   
>