Forcing default values for method

Question

Given the method:

def foo(a,b=5,c=1)
    return a+(b*c)
end

Running foo(1) should return 6. However, how would you go about doing something like this: foo(1,DEFAULT,2). I need to change the third value, but use the default second value.

How would you do this? (Note: I can't just change the order of the variables because they are arguments for a method from a gem)

Answer 1

You can't do it, in the terms posed. This sort of situation is exactly why named (keyword) parameters were introduced in Ruby 2. But your parameters with default values, according to the terms of the question, are not named.

Therefore, they are positional — that is why the optional parameters must come last — and the rule is, accordingly, that this method must be called with at least one argument (because a is not optional), and any further arguments will be used in the order supplied to fill in the corresponding parameters.

Thus, you can supply a, or a and b, or a and b and c. You cannot supply a and c alone, as you could easily do if these parameters were named.

Two obvious solutions spring to mind.

• Call the method, supplying the default value of the second parameter as the second argument. Presumably you know what it is, so it's not much of a hardship:

  foo(1,5,2)
  

• Write a trampoline method that does the same thing, but where the parameters are named:

  def foo(a,b=5,c=1)
    return a+(b*c)
  end
  def bar(a,b:5,c:1)
    return foo(a,b,c)
  end
  bar(1,c:2) # => 11