# Question about sub-class of array

**URL:** <https://www.ruby-forum.com/t/question-about-sub-class-of-array/192905>\
**Category:** Ruby\
**Created:** [August 8, 2010, 12:40am UTC](https://www.ruby-forum.com/t/question-about-sub-class-of-array/192905 "2010-08-08T00:40:01Z")\
**Posts on this page:** 7\
**Page:** 1

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**Author:** ![warren\_ferguson](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/7c8e57/32.png) [@warren\_ferguson](https://www.ruby-forum.com/u/warren_ferguson)\
**Post date:** [August 8, 2010, 12:40am UTC](https://www.ruby-forum.com/t/question-about-sub-class-of-array/192905/1 "2010-08-08T00:40:01Z")

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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

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<div class="post-metadata">

**Author:** ![warren\_ferguson](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/7c8e57/32.png) [@warren\_ferguson](https://www.ruby-forum.com/u/warren_ferguson)\
**Post date:** [August 8, 2010, 1:12am UTC](https://www.ruby-forum.com/t/question-about-sub-class-of-array/192905/2 "2010-08-08T01:12:48Z")

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Hi –

On Sun, 8 Aug 2010, warren ferguson wrote:

> # to string of 0s and 1s, ordered msb to lsb
> 
> # ----------------------------
> 
> ## 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](http://www.compleatrubyist.com)

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<div class="post-metadata">

**Author:** ![warren\_ferguson](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/7c8e57/32.png) [@warren\_ferguson](https://www.ruby-forum.com/u/warren_ferguson)\
**Post date:** [August 8, 2010, 1:26am UTC](https://www.ruby-forum.com/t/question-about-sub-class-of-array/192905/3 "2010-08-08T01:26:09Z")

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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!

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<div class="post-metadata">

**Author:** ![warren\_ferguson](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/7c8e57/32.png) [@warren\_ferguson](https://www.ruby-forum.com/u/warren_ferguson)\
**Post date:** [August 8, 2010, 1:39am UTC](https://www.ruby-forum.com/t/question-about-sub-class-of-array/192905/4 "2010-08-08T01:39:44Z")

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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.

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<div class="post-metadata">

**Author:** ![warren\_ferguson](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/7c8e57/32.png) [@warren\_ferguson](https://www.ruby-forum.com/u/warren_ferguson)\
**Post date:** [August 8, 2010, 1:51am UTC](https://www.ruby-forum.com/t/question-about-sub-class-of-array/192905/5 "2010-08-08T01:51:41Z")

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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?

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<div class="post-metadata">

**Author:** ![warren\_ferguson](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/7c8e57/32.png) [@warren\_ferguson](https://www.ruby-forum.com/u/warren_ferguson)\
**Post date:** [August 8, 2010, 2:05am UTC](https://www.ruby-forum.com/t/question-about-sub-class-of-array/192905/6 "2010-08-08T02:05:29Z")

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Hi –

On Sun, 8 Aug 2010, Warren Ferguson wrote:

> 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?

No; it’s because you weren’t doing what you think you were doing 🙂

Here are the two versions:

```
 Bitvector.new(self.reverse.split(//)).map{|k| k.to_i(2)}

 v = self.reverse.split(//).map{|k| k.to_i(2)}
 Bitvector.new(v)

```

In the first one, you’re calling map on a Bitvector object. In essence  
it’s this:

```
 bv = Bitvector.new(self.reverse.split(//))
 return bv.map{|k| k.to_i(2)}

```

In the second one, you’re doing the map operation first, and then  
sending the result of that operation to the BV initializer.

Move the parentheses around in the first one and you’ll see what I mean:

```
 Bitvector.new(self.reverse.split(//).map{|k| k.to_i(2)})

```

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](http://www.compleatrubyist.com)

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<div class="post-metadata">

**Author:** ![warren\_ferguson](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/w/7c8e57/32.png) [@warren\_ferguson](https://www.ruby-forum.com/u/warren_ferguson)\
**Post date:** [August 8, 2010, 2:11am UTC](https://www.ruby-forum.com/t/question-about-sub-class-of-array/192905/7 "2010-08-08T02:11:06Z")

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Give me a dope slap.  
I never saw the incorrect closing paren 🙂  
I must have added it, in the incorrect position,  
when the parser complained.

Thanks again.