This post originated from an RSS feed registered with Ruby Buzz by Florian Frank. Original Post: Appending lazy lists Feed Title: The Rubylution Feed URL: http://rubylution.ping.de/xml/rss/feed.xml Feed Description: The Rubylution is a weblog (mainly) about Ruby Programming. Latest Ruby Buzz Posts Latest Ruby Buzz Posts by Florian Frank Latest Posts From The Rubylution

I've just added a nice new feature to my lazylist library in Version 0.2.2. It's now possible to append one list to another.

A good example of how this can be useful is reading log files:

First I put a bunch of file names into the files array. Then I wrap them into opened File objects and the gzipped files into GzipReader objects as well, after that all of them into LazyList instances.

require 'lazylist'
require 'zlib'
files = [ 'syslog' ] + Dir['syslog.[0-9].gz']
files.map! { |f|
  LazyList[f =~ /.gz$/ ? Zlib::GzipReader.new(File.new(f)) : File.new(f)]
}

Then I append all the lazy lists into one new lazy list.

syslog = files.first.append(*files[1..-1])

I grep all lines containing error from this lazy list.

errors = syslog.grep /error/i

Now I can use this

puts errors.take(2)

to output the first two errors. (This takes a while.)

puts errors.take(3)

This takes a while and the first three errors are printed, but the first two have alread been read from the logfiles before.

puts errors.take(3)

This outputs those three errors again, but they are already in memory by now, and are displayed instantly.

puts errors.size

This greps for all the error lines in the syslog files and displays how many there were. This could take a while, because all files are read into memory.

puts *errors

This displays all the error lines.

The nice thing about appending is that several different log files can be treated like they were only one file - in an uniform way.

BTW: I aliased + to append, so it's possible to add them like ordinary Ruby Arrays:

l = LazyList[1..3] + LazyList[21..23] # => [1,...]

Of course if one of the appended lists is infinite, the lists on the right hand side, will never be part of the resulting list.