chunk

Creates an enumerator for each chunked elements. The consecutive elements which have same block value are chunked.

The result enumerator yields the block value and an array of chunked elements. So “each” method can be called as follows.

enum.chunk {|elt| key }.each {|key, ary| ... }
enum.chunk(initial_state) {|elt, state| key }.each {|key, ary| ... }

For example, consecutive even numbers and odd numbers can be splitted as follows.

[3,1,4,1,5,9,2,6,5,3,5].chunk {|n|
  n.even?
}.each {|even, ary|
  p [even, ary]
}
#=> [false, [3, 1]]
#   [true, [4]]
#   [false, [1, 5, 9]]
#   [true, [2, 6]]
#   [false, [5, 3, 5]]

This method is especially useful for sorted series of elements. The following example counts words for each initial letter.

open("/usr/share/dict/words", "r:iso-8859-1") {|f|
  f.chunk {|line| line.ord }.each {|ch, lines| p [ch.chr, lines.length] }
}
#=> ["\n", 1]
#   ["A", 1327]
#   ["B", 1372]
#   ["C", 1507]
#   ["D", 791]
#   ...

The following key values has special meaning:

• nil and :_separator specifies that the elements are dropped.

• :_alone specifies that the element should be chunked as a singleton.

Other symbols which begins an underscore are reserved.

nil and :_separator can be used to ignore some elements. For example, the sequence of hyphens in svn log can be eliminated as follows.

sep = "-"*72 + "\n"
IO.popen("svn log README") {|f|
  f.chunk {|line|
    line != sep || nil
  }.each {|_, lines|
    pp lines
  }
}
#=> ["r20018 | knu | 2008-10-29 13:20:42 +0900 (Wed, 29 Oct 2008) | 2 lines\n",
#    "\n",
#    "* README, README.ja: Update the portability section.\n",
#    "\n"]
#   ["r16725 | knu | 2008-05-31 23:34:23 +0900 (Sat, 31 May 2008) | 2 lines\n",
#    "\n",
#    "* README, README.ja: Add a note about default C flags.\n",
#    "\n"]
#   ...

paragraphs separated by empty lines can be parsed as follows.

File.foreach("README").chunk {|line|
  /\A\s*\z/ !~ line || nil
}.each {|_, lines|
  pp lines
}

:_alone can be used to pass through bunch of elements. For example, sort consecutive lines formed as Foo#bar and pass other lines, chunk can be used as follows.

pat = /\A[A-Z][A-Za-z0-9_]+\#/
open(filename) {|f|
  f.chunk {|line| pat =~ line ? $& : :_alone }.each {|key, lines|
    if key != :_alone
      print lines.sort.join('')
    else
      print lines.join('')
    end
  }
}

If the block needs to maintain state over multiple elements, initial_state argument can be used. If non-nil value is given, it is duplicated for each “each” method invocation of the enumerator. The duplicated object is passed to 2nd argument of the block for “chunk” method.