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From: warren ferguson on 7 Aug 2010 18:39

I don't understand why thsi isn't working. I wanted to introduce a subclass of the Array class called Bitvector. A bit vector is an array of 0s and 1s, ordered from lsb at index 0 to msb at the highest index, so
"10011" as a string would be [1,1,0,0,1] as an Bitvector. Here's what I tried:

class Bitvector < Array
def initialize(*)
@a = super
end
def to_s
# convert Bitvector, ordered lsb to msb,
# to string of 0s and 1s, ordered msb to lsb
@a.map{|i| i.to_s(2)}.reverse.join
end
end
class String
def to_bv
# convert bit string of 0s and 1s, ordered msb to lsb,
# to Bitvector, ordered lsb to msb
Bitvector.new(self.reverse.split(//)).map{|k| k.to_i(2)}
end
end
# ----------------------------
v = "10011".to_bv
puts "v.class = #{v .class}"
puts "v = #{v}"
puts "--"

v = Bitvector.new([0,1])
puts "v.class = #{v .class}"
puts "v = #{v}"

The output from this program is

v.class = Array
v = [1, 1, 0, 0, 1]
--
v.class = Bitvector
v = 10

What I don't understand is why the first part reports v is of class Array, instead of class Bitvector.

Thanks, Warren
From: David A. Black on 7 Aug 2010 19:12
Hi --

On Sun, 8 Aug 2010, warren ferguson wrote:

>
> I don't understand why thsi isn't working. I wanted to introduce a subclass of the Array class called Bitvector. A bit vector is an array of 0s and 1s, ordered from lsb at index 0 to msb at the highest index, so
> "10011" as a string would be [1,1,0,0,1] as an Bitvector. Here's what I tried:
>
> class Bitvector < Array
> def initialize(*)
> @a = super
> end
> def to_s
> # convert Bitvector, ordered lsb to msb,
> # to string of 0s and 1s, ordered msb to lsb
> @a.map{|i| i.to_s(2)}.reverse.join
> end
> end
> class String
> def to_bv
> # convert bit string of 0s and 1s, ordered msb to lsb,
> # to Bitvector, ordered lsb to msb
> Bitvector.new(self.reverse.split(//)).map{|k| k.to_i(2)}
> end
> end
> # ----------------------------
> v = "10011".to_bv
> puts "v.class = #{v .class}"
> puts "v = #{v}"
> puts "--"
>
> v = Bitvector.new([0,1])
> puts "v.class = #{v .class}"
> puts "v = #{v}"
>
> The output from this program is
>
> v.class = Array
> v = [1, 1, 0, 0, 1]
> --
> v.class = Bitvector
> v = 10
>
> What I don't understand is why the first part reports v is of class Array, instead of class Bitvector.

It's because of the map operation, which returns an Array.


David

--
David A. Black, Senior Developer, Cyrus Innovation Inc.

The Ruby training with Black/Brown/McAnally
Compleat Philadelphia, PA, October 1-2, 2010
Rubyist http://www.compleatrubyist.com

From: Warren Ferguson on 7 Aug 2010 19:26
You're right, and I'm enjoying your book. Just to be clear, this solved
the problem, replace the earlier to_bv method with this

class String
def to_bv
# convert bit string of 0s and 1s, ordered msb to lsb,
# to Bitvector, ordered lsb to msb
v = self.reverse.split(//).map{|k| k.to_i(2)}
Bitvector.new(v)
end
end

So, I hope I understood your answer correctly. Thanks!
--
Posted via http://www.ruby-forum.com/.

From: Aaron D. Gifford on 7 Aug 2010 19:38
Unrelated to your question, your code example made me curious:

class Bitvector < Array
def initialize(*)
@a = super
end
def to_s
# convert Bitvector, ordered lsb to msb,
# to string of 0s and 1s, ordered msb to lsb
@a.map{|i| i.to_s(2)}.reverse.join
end
end

Q: Is there any benefit to storing a copy of self in @a? Is there any
drawback to eliminating the initialize definition in the child class
and replacing the "@a" in the to_s method with "self"? What am I
missing?

Wondering,
Aaron out.

From: Warren Ferguson on 7 Aug 2010 19:51
Aaron D. Gifford wrote:

> Q: Is there any benefit to storing a copy of self in @a? Is there any
> drawback to eliminating the initialize definition in the child class
> and replacing the "@a" in the to_s method with "self"? What am I
> missing?
>
> Wondering,
> Aaron out.

No, there isn't, my bad. I added the initialize method because I thought
it might be the source of the original problem. Taking it out and using
self instead works fine.

I'm stil mystified why substituting the expression for v into
Bitvector.new(v) fails. Is it because Ruby optimizes that complex
expression by realizing the expression is of class Array and Bitvector
is a subclass of array?
--
Posted via http://www.ruby-forum.com/.

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