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Extended External Iterators (forward and backward)


Module Stream defines an interface for external iterators. A stream can be seen as an iterator on a sequence of objects x1,…,xn. The state of the stream is uniquely determined by the following methods:

  • at_beginning?

  • at_end?

  • current

  • peek

State changes are done with the following operations:

  • set_to_begin

  • set_to_end

  • forward

  • backward

With the help of the method current_edge the state of a stream s can be exactly defined

s.current_edge == [s.current, s.peek]

If s a stream on [x1,…,xn]. Consider the edges [xi,xi+1] i=1,…,n and


and [xn,xn+1] (x0 and xn+1 are helper elements to define the boundary

conditions). Then if s is non empty, the following conditions must be true:

s.at_beginning? <=> s.current_edge == [x0,x1]
s.at_end? <=> s.current_edge == [xn,xn+1]
s.isEmpty? <=> s.at_beginning? && s.at_end? <=> s.current_edge == [x0,x1] <=> n = 0
s.set_to_end => s.at_end?
s.set_to_begin => s.at_beginning?

If 0 <= i < n and s.current_edge == [xi, xi+1] , then:

[s.forward, s.current_edge] == [xi+1, [xi+1, xi+2]]

If 1 <= i < n and s.current_edge == [xi, xi+1] , then:

[s.backward, s.current_edge] == [xi, [xi-1, xi]]

The result of peek is the same as of forward without changing state. The result of current is the same as of backward without changing state.

Module Stream includes Enumerable implementing #each in the obvious way.

Not every stream needs to implement #backward and #at_beginning? thus being not reversable. If they are reversable peek can easily be implemented using forward and backward, as is done in module Stream. If a stream is not reversable all derived streams provided by the stream module (filter, mapping, concatenation) can be used anyway. Explicit or implicit (via peek or current) uses of backward would throw a NotImplementedError.

Classes implementing the stream interface must implement the following methods:

  • basic_forward

  • basic_backward

  • at_end?

  • at_beginning?

The methods set_to_end and set_to_begin are by default implemented as:

set_to_end   :  until at_end?; do basic_forward end
set_to_begin :  until at_beginning?; do basic_backward end

The methods forward and backward are by default implemented as:

forward:	   raise EndOfStreamException if at_end?; basic_forward.
backward:   raise EndOfStreamException if at_beginning?; basic_backward

Thus subclasses must only implement four methods. Efficiency sometimes demands better implementations.

There are several concrete classes implementing the stream interface:

  • Stream::EmptyStream (boring)

  • Stream::CollectionStream created by the method Array#create_stream

  • Stream::FilteredStream created by the method Stream#filtered

  • Stream::ReversedStream created by the method Stream#reverse

  • Stream::ConcatenatedStream created by the method Stream#concatenate

  • Stream::ImplicitStream using closures for the basic methods to implement


The latest version of stream.rb can be found at


Normal Installation

You can install stream with the following command.

% ruby install.rb

from its distribution directory.

GEM Installation

Download the GEM file and install it with ..

gem -i stream-VERSION.gem

Use the correct version number for VERSION (e.g. 0.5). You may need root privileges to install.

See also


g = ('a'..'f').create_stream
h = (1..10).create_stream
i = (10..20).create_stream

until g.at_end? || h.at_end? || i.at_end?
  p [g.forward, h.forward, i.forward]

def filestream fname { |s|
    f = open(fname)
    s.at_end_proc = proc {f.eof?}
    s.forward_proc = proc {f.readline}
    # Need not implement backward moving to use the framework

(filestream("/etc/passwd") + ('a'..'f').create_stream + filestream("/etc/group")).each do |l|
  puts l

puts "\nTwo filtered collection streams concatenated and reversed:\n\n"

def newstream; (1..6).create_stream; end
s = newstream.filtered { |x| x % 2 == 0 } + newstream.filtered { |x| x % 2 != 0 }
s = s.reverse
puts "Contents      : #{s.to_a.join ' '}"
puts "At end?       : #{s.at_end?}"
puts "At beginning? : #{s.at_beginning?}"
puts "2xBackwards   : #{s.backward} #{s.backward}"
puts "Forward       : #{s.forward}"
puts "Peek          : #{s.peek}"
puts "Current       : #{s.current}"
puts "set_to_begin    : Peek=#{s.set_to_begin;s.peek}"

# an infinite stream (do not use set_to_end!)
def randomStream { |s|
    s.set_to_begin_proc = proc {srand 1234}
    s.at_end_proc = proc {false}
    s.forward_proc = proc {rand}
s = randomStream.filtered { |x| x >= 0.5 }.collect { |x| sprintf("%5.2f ",x*100) }
puts "5 random numbers: #{(1..5).collect {|x| s.forward}}\n" # =>

  5 random numbers: 74.05 94.80 87.87 86.07 83.70

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Horst Duchene


Copyright © 2001, 2013, 2016, 2020 Horst Duchene

Released under the same license as Ruby