From: Robert Klemme <shortcutter@...>
Date: 2010-01-07T18:50:11+09:00
Subject: Re: group in the array

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

Not providing an argument can be useful as well:

irb(main):008:0> (1..5).inject {|a,b| a.to_s << "," << b.to_s}
=> "1,2,3,4,5"

(Of course, that's better done with #join.)

And Ruby's pattern matching for block arguments makes it easy to process 
Hash and other collections that have more than one object per iteration:

irb(main):001:0> h={1=>2,3=>4}
=> {1=>2, 3=>4}
irb(main):002:0> h.inject(0) {|sum, (key, val)| sum + key + val}
=> 10

Kind regards

	robert

-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/