From: Josh Cheek <josh.cheek@...>
Date: 2009-10-20T19:25:56+09:00
Subject: Re: recursive function problem

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On Tue, Oct 20, 2009 at 4:32 AM, Maurizio De santis <
desantis.maurizio@gmail.com> wrote:

> Hi all!
>
> I have a problem with this function, which should return a formatted
> string of an hash given as an argument:
>
>  def show_params(params, tabs_num)
>    params_str = String.new if tabs_num == -1
>    tabs_num++
>    params.each do |key, value|
>      tabs_num.times { params_str += "\t" }
>      if params.class != value.class
>        params_str += "#{key} => " + value.to_s + "\n"
>      else
>        params_str += "#{key} => \n"
>        show_params(value, tabs_num)
>      end
>    end
>    return params_str
>  end
>
> I call the function with this:
>  show_params(params, -1)
>
> obtaining thish error (in Rails):
>
>  undefined method `+@' for {"action"=>"edit", "id"=>"51",
> "controller"=>"items"}:HashWithIndifferentAccess
>
> I cannot guess the reason! Any idea?
>
> Thanks!
> --
> Posted via http://www.ruby-forum.com/.
>
>
In Ruby ++ isn't defined
tabs_num++
should instead use += like this
tabs_num+=1

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Also, you can specify default values for parameters, so
def show_params(params, tabs_num)
could be rewritten:
def show_params(params, tabs_num = -1)

Which would allow you to take the current call of
show_params(params, -1)
and instead call it like this
show_params(params)

This allows you to keep internal knowledge inside, people calling the
function shouldn't have to know that they need to initialize some internal
variable to -1

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You still have an issue in that you only initialize the variable params_str
on the first call to the function, so in the future calls, this variable is
not defined, and you cannot then concatenate additional string to it. To
resolve this, remove the if on it, so that it is initialized for each
recursive call. Then you actually get some output. Now, what to do with this
variable? Well, we want to take the results of the recursive calls, and add
them into the current string. Notice right now you are saying

params_str += "#{key} => \n"
show_params(value, tabs_num)

But this second line does nothing, it calculates the substring, then returns
it but nothing receives it or does anything with it. So instead, try

params_str += "#{key} => \n"
params_str += show_params(value, tabs_num)

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Anyway, here is what I have, and it does what I think you are wanting it to
do:

def show_params( params , tabs_num = -1 )
  tabs_num += 1
  params_str = String.new
  params.each do |key, value|
    params_str << "\t" * tabs_num
    if value.is_a? Hash
      params_str << "#{key.inspect} =>\n#{show_params value , tabs_num}"
    else
      params_str << "#{key.inspect} => #{value.inspect}\n"
    end
  end
  params_str
end

params = { :abc => 'def' , 3 => {1=>true,2=>{'xyz'=>/xyz/}} , 5 => 5 }
puts show_params(params)

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I don't really know how to read the docs, but this seems like the kind of
thing that I would expect the pp library to do for us. I tried about 15
different ways to get it to do this, and only ever got the same as the
default p function. If pp does address this issue, can someone explain how
to get it to work?

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