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Ruby Ruby Foundations Procs & Lambdas Working With Procs & Lambdas

Posted by Unsubscribed User
Unsubscribed User
3,309 Points

Procs & Lambdas Extra Credit: How to write your class without using a class variable?

Assuming I understood what was being asked for in the Extra Credit...

When I needed to call my proc from another method, I made an instance variable and kept getting no output. Since it works with class variables, this is a scope problem - is that correct? My code with an instance variable:

class ExtraCredit
  def pass_a_proc(a_proc)
    @my_proc = a_proc
  end

  def call_a_proc(n)
    @my_proc.call n
  end
end

sqr = ExtraCredit.new
sqr.pass_a_proc(proc {|x| x ** 2})
sqr.call_a_proc 4 # =>

Outputs nothing, but I checked and @my_proc.nil? == false and @my_proc.respond_to? "call" == true, inside both methods.

My code using class variables:

class ExtraCredit
  def pass_a_proc(a_proc)
    @@my_proc = a_proc
  end

  def call_a_proc(n)
    @@my_proc.call n
  end
end

sqr = ExtraCredit.new
sqr.pass_a_proc(proc {|x| x ** 2})
p sqr.call_a_proc 4 # => 25
p sqr.call_a_proc 7 # => 49

It feels clunky to have to use a class variable. Anyone know what the question is asking for? Can it be done with instance variables?

2 Answers

Unsubscribed User
Unsubscribed User
3,309 Points

To answer my own question:

Class instance variables are available only to class methods and not instance methods. Class variables are avilable to both class methods and instance methods.

That being said, http://stackoverflow.com/questions/3787154/why-should-class-variables-be-avoided-in-ruby/3787327#3787327 This in mind, I re-wrote my code to use class methods with a class instance variable:

class ExtraCredit
  def self.pass_a_proc(a_proc)
    @my_proc = a_proc
  end

  def self.call_a_proc(n)
    @my_proc.call n
  end
end

ExtraCredit.pass_a_proc(proc {|x| x ** 2})
p ExtraCredit.call_a_proc 7 # => 49
p ExtraCredit.call_a_proc 6 # => 36

I welcome any comments.

markd2
markd2
5,253 Points
markd2
markd2
5,253 Points

This is my code using the ampersand argument prefix to indicate that the method is expecting a proc. It may solve the scope issue above??

class ExtraCredit <br>
&nbsp;&nbsp;def saves_proc(&aproc) <br>
&nbsp;&nbsp;&nbsp;&nbsp; x = aproc <br>
&nbsp;&nbsp;  end <br>

&nbsp;&nbsp;  def calls_proc(&a) <br>
&nbsp;&nbsp;&nbsp;&nbsp;    saves_proc(&a).call <br>
&nbsp;&nbsp;  end <br>
end <br>

x = ExtraCredit.new <br>
a = Proc.new { puts "Hello world!" } <br>
x.calls_proc(&a) <br>
</p>