Ruby optional parameters

Question

If I define a Ruby functions like this   def ldap get   base dn  filter  scope LDAP  LDAP SCOPE SUBTREE  attrs nil     How can I call it supplying only the first 2 and the last args  Why isn t something like  ldap get  base dn  filter    X    possible or if it is possible  how can it be done

User · Accepted Answer

This isn t possible with ruby currently  You can t pass  empty  attributes to methods   The closest you can get is to pass nil   ldap get base dn  filter  nil  X    However  this will set the scope to nil  not LDAP  LDAP SCOPE SUBTREE   What you can do is set the default value within your method   def ldap get base dn  filter  scope   nil  attrs   nil    scope     LDAP  LDAP SCOPE SUBTREE       do something     end   Now if you call the method as above  the behaviour will be as you expect

User · Answer

You are almost always better off using an options hash   def ldap get base dn  filter  options         options  scope      LDAP  LDAP SCOPE SUBTREE       end  ldap get base dn  filter   attrs   gt  X

User · Answer

Recently I found a way around this  I wanted to create a method in the array class with an optional parameter  to keep or discard elements in the array   The way I simulated this was by passing an array as the parameter  and then checking if the value at that index was nil or not   class Array   def ascii to text params      param len   params length     if param len  gt  3 or param len  lt  2 then raise  Invalid number of arguments   param len  for 2    3   end     bottom    params 0      top       params 1      keep      params 2      if keep nil     false       if keep    1         self map  x  if x  gt   bottom and x  lt   top then x   x chr else x   x to s end        else         raise  Invalid option   keep  at argument position 3 in   p params   must be 1 or nil        end     else       self map  x  if x  gt   bottom and x  lt   top then x   x chr end  compact     end   end end   Trying out our class method with different parameters   array    1  2  97  98  99  p array ascii to text  32  126  1     Convert all ASCII values of 32-126 to their chr value otherwise keep it the same  That s what the optional 1 is for    output    1    2    a    b    c    Okay  cool that works as planned  Now let s check and see what happens if we don t pass in the the third parameter option  1  in the array   array    1  2  97  98  99  p array ascii to text  32  126     Convert all ASCII values of 32-126 to their chr value else remove it  1 isn t a parameter option    output    a    b    c    As you can see  the third option in the array has been removed  thus initiating a different section in the method and removing all ASCII values that are not in our range  32-126   Alternatively  we could had issued the value as nil in the parameters  Which would look similar to the following code block   def ascii to text top  bottom  keep   nil    if keep nil      self map  x  if x  gt   bottom and x  lt   top then x   x chr end  compact   else     self map  x  if x  gt   bottom and x  lt   top then x   x chr else x   x to s end  end

User · Answer

It isn t possible to do it the way you ve defined ldap get  However  if you define ldap get like this   def ldap get   base dn  filter  attrs nil  scope LDAP  LDAP SCOPE SUBTREE     Now you can   ldap get  base dn  filter  X     But now you have problem that you can t call it with the first two args and the last arg  the same problem as before but now the last arg is different     The rationale for this is simple  Every argument in Ruby isn t required to have a default value  so you can t call it the way you ve specified  In your case  for example  the first two arguments don t have default values

User · Answer

It is possible    Just change definition  def ldap get   base dn  filter  scope LDAP  LDAP SCOPE SUBTREE  attrs nil     to   def ldap get   base dn  filter   param array  attrs nil   scope   param array first    LDAP  LDAP SCOPE SUBTREE   scope will be now in array on its first place  When you provide 3 arguments  then you will have assigned base dn  filter and attrs and param array will be    When 4 and more arguments then param array will be  argument1  or more  and more   Downside is    it is unclear solution  really ugly  This is to answer that it is possible to ommit argument in the middle of function call in ruby     Another thing you have to do is to rewrite default value of scope

User · Answer

1  You cannot overload the method  Why doesn  39 t ruby support method overloading   so why not write a new method altogether   2  I solved a similar problem using the splat operator   for an array of zero or more length  Then  if I want to pass a parameter s  I can  it is interpreted as an array  but if I want to call the method without any parameter then I don t have to pass anything  See Ruby Programming Language pages 186 187

User · Answer

You could do this with partial application  although using named variables definitely leads to more readable code  John Resig wrote a blog article in 2008 about how to do it in JavaScript  http   ejohn org blog partial-functions-in-javascript   Function prototype partial   function      var fn   this  args   Array prototype slice call arguments     return function        var arg   0      for   var i   0  i  lt  args length  amp  amp  arg  lt  arguments length  i           if   args i      undefined           args i    arguments arg         return fn apply this  args             It would probably be possible to apply the same principle in Ruby  except for the prototypal inheritance

User · Answer

Time has moved on and since version 2 Ruby supports named parameters   def ldap get   base dn  filter  scope   some scope   attrs  nil     p attrs end  ldap get  first arg    second arg   attrs   attr1  attr2       gt   attr1  attr2

[ruby] Ruby optional parameters

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