Question

I’ve a method like:

def method (a=[],b=[])
 ...
end

I am calling this method with variables like this:

method(h[0].values_at("value"), h[1].values_at("value")) 

where h[0] and h[1] are hashes. It works fine.

I dont know if h[1] is going to be there in the next run, so it’s giving me error if hash h[1] is not there.

How can I modify it so it makes the call dynamically depending on whether h[0], h[1] are there or not, and call the method with the correct number of parameters.

Answer 1

Hope I understood your problem right:

method(h[0].values_at("value"), 
        h[1] ? h[1].values_at("value") : []
      ) 

Your problem: if h[1]does not exist, h[1].values_at will raise an exception. So you have to test first, if the value is available. In the code snipllet above I used a ternary operator.

An extended version would be:

par2 = []
par2 = h[1].values_at("value") if h[1]

method(h[0].values_at("value"), 
        par2
      ) 

With my solution you don’t need the default values in the method definition.

---

In your comment you extended your question.

With four parameters you could use it like this:

def method(p1,p2,p3,p4)
  #...
end

method(
    h[0] ? h[0].values_at("value") : [],
    h[1] ? h[1].values_at("value") : [],
    h[2] ? h[2].values_at("value") : [],
    h[3] ? h[3].values_at("value") : [],
  ) 

But I would recommend a more generic version:

def method(*pars)
  #pars is an Array with all values (including empty arrays.
  #The next check make only sense, if you need exactly 4 values.
  raise "Expected 4 values, get #{pars.size}"  unless pars.size == 4
end

method( *(h.map{|i|i.values_at("x")})

---

And another – probably not so good – idea:

Extend nil (the result of h[1] if h has not this element) to return [] for values_at:

class NilClass  
  def values_at(x)
    []
  end
end