From: Brian Candler <b.candler@...>
Date: 2009-02-24T23:09:25+09:00
Subject: Re: Error in While Loop

Naresh Ramaswamy wrote:
> Following is my Ruby Code Wherein I am reading an XML file.
> I am afraid why my loop is not getting incremented for value of i.

STDERR.puts "About to increment $i:"
>     $i +=1;
STDERR.puts "Value is now #{$i}"

I think you're going about debugging in a reasonable way: just keep 
putting more debugging lines in until you find the problem. Don't be 
afraid to put your debugging code straight inline. I tend to put it 
lined up with the left-hand column, so it's easy to spot and rip out 
later.

Incidentally, you have a strange mixture of global variables ($i) and 
instance variables (@a). Normally you would just use local variables, 
and pass in any parameters as required, like this:

def check(txn)
  i = 0
  a = tnx.size
  while i <= a #|| txn[i] != 0
    .. etc
  end
end

doc = Document.new File.new("script_data.xml")
txn = XPath.match(doc, "//TXN[@value]")
check(txn)

OR: you would make a class, and keep only those values you wish to 
persist in instance variables. e.g.

class TXN
  def initialize(txn)
    @txn = txn
  end

  def check
    i = 0
    a = @txn.size
    while i < a
      ... etc
    end
  end
end

doc = Document.new File.new("script_data.xml")
txn = TXN.new XPath.match(doc, "//TXN[@value]")
txn.check

This latter case is useful when you want to add more methods into your 
TXN class, which operate on the same data, or you wish to bundle 
together the txn with additional objects or data, and pass them around 
as a single entity.

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