Passing a method as a parameter in Ruby

Question

I am trying to mess around a little bit with Ruby  Therefor I try to implement the algorithms  given in Python  from the book  Programming Collective Intelligence  Ruby   In chapter 8 the author passes a method a as parameter  This seems to work in Python but not in Ruby   I have here the method  def gaussian dist  sigma 10 0    foo end   and want to call this with another method  def weightedknn data  vec1  k   5  weightf   gaussian    foo   weight   weightf dist    foo end   All I got is an error  ArgumentError  wrong number of arguments  0 for 1

User · Answer

You have to call the method  call  of the function object   weight   weightf call  dist     EDIT  as explained in the comments  this approach is wrong  It would work if you re using Procs instead of normal functions

User · Answer

I would recommend to use ampersand to have an access to named blocks within a function  Following the recommendations given in this article you can write something like this  this is a real scrap from my working program        Returns a valid hash for html form select element  combined of all entities     for the given  model   where only id and name attributes are taken as     values and keys correspondingly  Provide block returning boolean if you     need to select only specific entities             Args             -  model  - gt  ORM interface for specific entities        -   amp cond  - gt  block   x  boolean   filtering entities upon iterations        Returns          - hash of  entity id   gt  entity name        def make select list  model   amp cond       cond     proc   true     cond defaults to proc   true         Entities filtered by cond  followed by filtration by  id  name      model all map do  x        cond   x       x id   gt  x name            end reduce Hash new do  memo  e  memo merge  e   end   end   Afterwerds  you can call this function like this    contests   make select list Contest do  contest    logged admin  or contest organizer     current user end   If you don t need to filter your selection  you simply omit the block    categories   make select list  Category     selects all categories   So much for the power of Ruby blocks

User · Answer

The comments referring to blocks and Procs are correct in that they are more usual in Ruby  But you can pass a method if you want  You call method to get the method and  call to call it   def weightedknn  data  vec1  k   5  weightf   method  gaussian            weight   weightf call  dist         end

User · Answer

You want a proc object   gaussian   Proc new do  dist   args    sigma   args first    10 0       end  def weightedknn data  vec1  k   5  weightf   gaussian          weight   weightf call dist        end   Just note that you can t set a default argument in a block declaration like that   So you need to use a splat and setup the default in the proc code itself     Or  depending on your scope of all this  it may be easier to pass in a method name instead   def weightedknn data  vec1  k   5  weightf    gaussian          weight   self send weightf        end   In this case you are just calling a method that is defined on an object rather than passing in a complete chunk of code   Depending on how you structure this you may need replace self send with object that has the these math methods send    Last but not least  you can hang a block off the method   def weightedknn data  vec1  k   5          weight        if block given        yield dist      else       gaussian call dist      end   end       end  weightedknn foo  bar  do  dist      square the dist   dist   dist end   But it sounds like you would like more reusable chunks of code here

User · Answer

you also can use  eval   and pass the method as a string argument  and then simply eval it in the other method

User · Answer

The normal Ruby way to do this is to use a block   So it would be something like   def weightedknn  data  vec1  k   5     foo   weight   yield  dist     foo end   And used like   weightenknn  data  vec1      dist  gaussian  dist       This pattern is used extensively in Ruby

User · Answer

You can pass a method as parameter with method  function  way  Below is a very simple example    def double a    return a   2  end    nil  def method with function as param  callback  number     callback call number   end     nil  method with function as param  method  double    10       20

User · Answer

You can use the  amp  operator on the Method instance of your method to convert the method to a block   Example   def foo arg    p arg end  def bar  amp block    p  bar    block call  foo   end  bar  amp method  foo     More details at http   weblog raganwald com 2008 06 what-does-do-when-used-as-unary html

[ruby] Passing a method as a parameter in Ruby

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