From: Simon Harrison <simon@...>
Date: 2011-01-15T23:10:00+09:00
Subject: Re: Retrieving and copying element from array

That works great, thanks Josh. A couple of questions if you don't mind.

1. What is the purpose of the [] in line below? Does it mean collect 
whatever matches into an array?

categories << [ line.sub(/^category: /,'').chomp ]

2. I've tried to achieve the same result using an existing array, rather 
than reading from the file and I'm stuck. I'm using JRuby 1.6RC1 and 
getting this error about NilClass. Any ideas?


irb(main):038:0> arr2 = []
irb(main):039:0> arr
=> [["cat1", "1", "2", "3"], ["cat2", "1", "2"], ["cat3", "1", "2"]]

irb(main):040:0> arr.map do |item|
irb(main):041:1*  if item =~ /^cat/
irb(main):042:2>   arr2 << [ item ]
irb(main):043:2>  else
irb(main):044:2*   arr2.last << item
irb(main):045:2>  end
irb(main):046:1> end

NoMethodError: undefined method `<<' for nil:NilClass
        from (irb):44:in `evaluate'
        from org/jruby/RubyArray.java:2460:in `collect'
        from (irb):40:in `evaluate'
        from org/jruby/RubyKernel.java:1091:in `eval'
        from /opt/jruby/lib/ruby/1.8/irb.rb:158:in `eval_input'
        from /opt/jruby/lib/ruby/1.8/irb.rb:271:in `signal_status'
        from /opt/jruby/lib/ruby/1.8/irb.rb:270:in `signal_status'
        from /opt/jruby/lib/ruby/1.8/irb.rb:155:in `eval_input'
        from org/jruby/RubyKernel.java:1421:in `loop'
        from org/jruby/RubyKernel.java:1194:in `rbCatch'
        from /opt/jruby/lib/ruby/1.8/irb.rb:154:in `eval_input'
        from /opt/jruby/lib/ruby/1.8/irb.rb:71:in `start'
        from org/jruby/RubyKernel.java:1194:in `rbCatch'
        from /opt/jruby/lib/ruby/1.8/irb.rb:70:in `start'

irb(main):047:0> arr
=> [["cat1", "1", "2", "3"], ["cat2", "1", "2"], ["cat3", "1", "2"]]
irb(main):048:0> arr2
=> []
irb(main):049:0> arr2.empty?
=> true

irb(main):052:0> arr.each do |item|
irb(main):053:1*  if item =~ /^cat/
irb(main):054:2>   arr2 << item
irb(main):055:2>  else
irb(main):056:2*   arr2.last << item
irb(main):057:2>  end
irb(main):058:1> end

NoMethodError: undefined method `<<' for nil:NilClass
        from (irb):56:in `evaluate'
        from org/jruby/RubyArray.java:1671:in `each'
        from (irb):52:in `evaluate'
        from org/jruby/RubyKernel.java:1091:in `eval'
        from /opt/jruby/lib/ruby/1.8/irb.rb:158:in `eval_input'
        from /opt/jruby/lib/ruby/1.8/irb.rb:271:in `signal_status'
        from /opt/jruby/lib/ruby/1.8/irb.rb:270:in `signal_status'
        from /opt/jruby/lib/ruby/1.8/irb.rb:155:in `eval_input'
        from org/jruby/RubyKernel.java:1421:in `loop'
        from org/jruby/RubyKernel.java:1194:in `rbCatch'
        from /opt/jruby/lib/ruby/1.8/irb.rb:154:in `eval_input'
        from /opt/jruby/lib/ruby/1.8/irb.rb:71:in `start'
        from org/jruby/RubyKernel.java:1194:in `rbCatch'
        from /opt/jruby/lib/ruby/1.8/irb.rb:70:in `start'
irb(main):059:0>

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