From: Rick DeNatale <rick.denatale@...>
Date: 2010-01-08T00:31:34+09:00
Subject: Re: group in the array

On Thu, Jan 7, 2010 at 4:50 AM, Robert Klemme
<shortcutter@googlemail.com> wrote:
> On 01/07/2010 05:00 AM, Ruby Newbee wrote:
>>
>> On Thu, Jan 7, 2010 at 11:19 AM, Ruby Newbee <rubynewbee@gmail.com> wrote:
>>>
>>> On Wed, Jan 6, 2010 at 11:20 PM, Robert Klemme
>>> <shortcutter@googlemail.com> wrote:
>>>
>>>> inject
>>>> http://ruby-doc.org/core-1.8.7/classes/Enumerable.html#M001147
>
>>> I have checked the document for inject, but still can't understand for.
>>> Can you help explain it with general words by a little samples?
>
>> Now I checked some other help documents wigh google, and find Ruby's
>> inject is similiar to Python's reduce, which is known by me.
>
> I don't know Python's reduce but a few things are probably noteworthy about
> Ruby's #inject:
>
> It does matter whether you provide an argument or not:
>
> irb(main):001:0> (1..5).inject {|*a| p a; nil}
> [1, 2]
> [nil, 3]
> [nil, 4]
> [nil, 5]
> => nil
> irb(main):002:0> (1..5).inject(0) {|*a| p a; nil}
> [0, 1]
> [nil, 2]
> [nil, 3]
> [nil, 4]
> [nil, 5]
> => nil
>
> This means that when summing up you usually want to provide 0 as argument.
>  For other use cases, e.g. concatenating you probably do not want to:
>
> irb(main):003:0> (1..5).inject(0) {|sum,x| sum + x}
> => 15
> irb(main):004:0> [].inject(0) {|sum,x| sum + x}
> => 0
>
> Whithout you do not get a sum for the empty collection:
>
> irb(main):005:0> [].inject {|sum,x| sum + x}
> => nil

Which you may or may not want.

I'm not sure I see the logic of why you example alone leads to the
recommendation to provide the 0 argument:

ruby-1.8.7-p174 > (1..5).inject(0) {|sum,x| p [sum, x]; sum + x}
[0, 1]
[1, 2]
[3, 3]
[6, 4]
[10, 5]
 => 15
ruby-1.8.7-p174 > (1..5).inject {|sum,x| p [sum, x]; sum + x}
[1, 2]
[3, 3]
[6, 4]
[10, 5]
 => 15

I've come to use inject or reduce depending on whether or not I'm
providing the argument, assuming I'm using a version of Ruby which
aliases them.  I think that inject without an argument is a bit
mysterious, because that argument is what you are injecting.

I think that most languages which have a form of reduce don't allow
that argument, as far as I know, it came from Smaltalk whose form of
reduce came from the inject:initialValue into: aBlock method on
collection classes. For an old Smaltalker Ruby's inject was no
mystery, but using reduce when not supplying the argument makes more
sense to me.

-- 
Rick DeNatale

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