December 21, 2012

Meta-programming in Ruby : Part 16 : instance_eval

There are altogether 16 posts on this topic of meta-programming in Ruby language. Our approach encourages hands-on experimentation. The exercises here are specially designed to facilitate self-study and other independent modes of learning.

Provided we can find time amidst our busy schedule(well, who is not busy these days?), we will also discuss meta-programming in other languages such as Lisp and python.

Code 1

instance_eval and self.


# instance_eval evaluates a block of code
# with self set to the object that's receiving the instance_eval call.

add_two = Proc.new { self + 2 }

puts 1.instance_eval(&add_two) #=> 3
puts 2.instance_eval(&add_two) #=> 4

"stellar coding staff".instance_eval{p self}
[1970, 1980, 1990, 2000].instance_eval{p self}
{'a'=> 1970, 'water'=> 1990, 'gold'=>2010}.instance_eval{p self}
puts "\n\n"


# Here's a second example using an implicit method receiver.
call_reverse = Proc.new { self.reverse }

p "alan akhoe kid".instance_eval(&call_reverse)
p ["x", "y", "z"].instance_eval(&call_reverse) #=> ["z", "y", "x"]
p [1, 34, 117, 997].instance_eval(&call_reverse)
# OK BAD # p 12121.instance_eval(&call_reverse)


Result 1



[root@localhost ruby_tutor]# ruby ie_self.rb
3
4
"stellar coding staff"
[1970, 1980, 1990, 2000]
{"a"=>1970, "water"=>1990, "gold"=>2010}


"dik eohka nala"
["z", "y", "x"]
[997, 117, 34, 1]


Code 2

We can display private attributes using instance_eval.


class C
def initialize
@a = 0
end
def next
@a += 1
end
end
w = C.new
p w.next
p w.next; puts;puts

p w.instance_eval "@a" #=> 2
p w.instance_eval { @a } #=> 2
p C.new.instance_eval { @a } #=> 0


Result 2



[root@localhost ruby_tutor]# ruby ie_and_private_vars.rb
1
2


2
2
0


2 comments:

  1. Here is a site you should study.

    BackYard Chickens is powered by Huddler Families

    http://www.huddler.com/

    BackYard Chickens ; Most likey in PHP.

    ReplyDelete
    Replies
    1. Thank you very much.I've seen it. I learn alot from their business.

      Delete

A Tip for Job Search: Gold Rush Skills

  If you need to make some money very quickly, what would you do? Your answer points to the kind of problems you can solve. They give you so...