From: James Edward Gray II <james@...>
Date: 2006-08-28T01:13:57+09:00
Subject: Re: map and join or inject?

On Aug 26, 2006, at 10:05 PM, ako... wrote:

> it looks like map+join use fewer objects than inject. even though map
> creates a whole new array. i would expect inject to use fewer objects.
> strange...

It has to do with the block you gave inject() and the definition of  
String.+().  Watch:

 >> ("a".."j").inject(String.new) do |res, let|
?>   new_str = res + let
 >>   puts "#{new_str.object_id}: #{new_str.inspect}"
 >>   new_str
 >> end
1669064: "a"
1668834: "ab"
1668624: "abc"
1668424: "abcd"
1668304: "abcde"
1668144: "abcdef"
1668064: "abcdefg"
1667854: "abcdefgh"
1667764: "abcdefghi"
1667534: "abcdefghij"
=> "abcdefghij"

Each of those intermediate Strings is a new object, which is right.   
That's what String.+() does:

$ ri -T String#+
--------------------------------------------------------------- String#+
      str + other_str   => new_str
------------------------------------------------------------------------
      Concatenation---Returns a new String containing other_str
      concatenated to str.

         "Hello from " + self.to_s   #=> "Hello from main"

The keywords there being "new String."  String does have an append  
method though:

$ ri -T 'String#<<'
-------------------------------------------------------------- String#<<
      str << fixnum        => str
      str.concat(fixnum)   => str
      str << obj           => str
      str.concat(obj)      => str
------------------------------------------------------------------------
      Append---Concatenates the given object to str. If the object is a
      Fixnum between 0 and 255, it is converted to a character before
      concatenation.

         a = "hello "
         a << "world"   #=> "hello world"
         a.concat(33)   #=> "hello world!"

And if we use that, we can get down to less objects than map(),  
because we don't need the Array:

 >> ("a".."j").inject(String.new) do |res, let|
?>   same_str = res << let
 >>   puts "#{same_str.object_id}: #{same_str.inspect}"
 >>   same_str
 >> end
938158: "a"
938158: "ab"
938158: "abc"
938158: "abcd"
938158: "abcde"
938158: "abcdef"
938158: "abcdefg"
938158: "abcdefgh"
938158: "abcdefghi"
938158: "abcdefghij"
=> "abcdefghij"

Hope that helps.

James Edward Gray II