无法写入红宝石文件

Question

class PostCallbacks < XML::SAX::Document
  test = "";
  out = File.open("output.txt","w");
  def start_element(element = "", attributes = [])
      @out << element
  end
end

gives error: undefined method <<' for nil:NilClass`

why can I not write to file this way?

Answer 1

Try this:

class PostCallbacks < XML::SAX::Document
  test = "";
  def initialize
    @out = File.open("output.txt","w");
  end
  def start_element(element = "", attributes = [])
    @out << element
  end
end

The error you're getting tells you that @out isn't initialized properly. The safest way to do what you're attempting to do is to initialize the instance variable @out in the constructor, which is to say, the initialize method.

Even better, of course, would be to pass the name of the output file to the constructor, rather than hard-coding it.