Explanation of # and @#

ruby-talk
1

I am relatively new to Ruby. In the below code which redefines the
attr_accessor method i cannot understand the #{attr} and @#{attr}. I
know # as comment and @ as required for a instance variable in Ruby.

class Class
  def attr_access(*attrs)
    attrs.each do |attr|
      class_eval %Q{
      def #{attr}
        @#{attr}
      end
      def #{attr}=(value)
        @#{attr} = value
      end
      }
    end
  end
end

class Foo
attr_access :a,:b
end

Foo.instance_methods(false) #=> ["b=", "a=", "b", "a"]

···

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2

I am relatively new to Ruby. In the below code which redefines the
attr_accessor method i cannot understand the #{attr} and @#{attr}. I

Those are string interpolation.

know # as comment and @ as required for a instance variable in Ruby.

1 class Class
2 def attr_access(*attrs)
3 attrs.each do |attr|
4 class_eval %Q{
5 def #{attr}
6 @#{attr}
7 end
8 def #{attr}=(value)
9 @#{attr} = value
10 end
11 }
12 end
  end
end

I numbered a few of the lines to make this easier to talk about.
line 4 starts of an eval note the %Q which roughly means double quote this
string.
lines 5- 10 are basically in a string. from there the string interpolation
/ variable injection happens anywhere you see #{}
after line 11 the previous 5 through 10 are evaluated.

try doing this in irb
variable = "world"
p "hello #{variable}"

you should see
hello world

Hope this helps.

Andrew McElroy

···

On Thu, Oct 11, 2012 at 3:33 PM, Muhammad Salman <lists@ruby-forum.com>wrote:

class Foo
attr_access :a,:b
end

Foo.instance_methods(false) #=> ["b=", "a=", "b", "a"]

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3

%Q{
     def #{attr}
       @#{attr}
     end
     def #{attr}=(value)
       @#{attr} = value
     end
     }

The # symbol when used with {} is for string interpolation, e.g..

f = "world"
puts "hello #{f}"

%Q is another way of defining a string with interpolation, e.g.

f = %Q{world}
puts "hello #{f}"

Henry

···

On 12/10/2012, at 9:33 AM, Muhammad Salman <lists@ruby-forum.com> wrote:

I am relatively new to Ruby. In the below code which redefines the
attr_accessor method i cannot understand the #{attr} and @#{attr}. I
know # as comment and @ as required for a instance variable in Ruby.

4

greeting = 'hello'
my_string = "#{greeting} world"
puts my_string

--output:--
hello world

attr = 'color'
my_string =<<ENDOFSTRING
  def #{attr}
    @#{attr}
  end
ENDOFSTRING

puts my_string

--output:--
  def color
    @color
  end

···

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5

Oh, yeah...%Q is just another syntax for double quotes:

verb = 'yelled'
puts %Q{He #{verb}, "It's all Ruby!"}

--output:--
He yelled, "It's all Ruby!"

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