Hash#select returns an array but Hash#reject returns a hash

#1

Hello,

Is there any design reason for select returning an array and reject
returning a hash? Why this disparity?

Thank you,

Jayanth

#2

Srijayanth S. wrote:

Is there any design reason for select returning an array and reject
returning a hash? Why this disparity?

In module Enumerable, both select and reject return an array (according
to the Pickaxe).

#3

irb(main):001:0> a=Hash.new
=> {}
irb(main):002:0> a[1]=2
=> 2
irb(main):003:0> a[2]=2
=> 2
irb(main):004:0> a[3]=4
=> 4
irb(main):005:0> a.select { |key,value| value > 2 }
=> [[3, 4]]
irb(main):006:0> a.reject { |key,value| value <= 2 }
=> {3=>4}

#4

=> 2
irb(main):019:0> a[3]=4
=> 4
irb(main):020:0> a.select { |key,value| value > 2 }
=> {3=>4}
irb(main):021:0> a.reject { |key,value| value <= 2 }
=> {3=>4}

kind regards -botp

Thanks.

So this is just some sort of artefact/legacy code that never got
changed?

Jayanth

#5

From: Srijayanth S. [mailto:removed_email_address@domain.invalid]

irb(main):001:0> a=Hash.new

=> {}

irb(main):002:0> a[1]=2

=> 2

irb(main):003:0> a[2]=2

=> 2

irb(main):004:0> a[3]=4

=> 4

irb(main):005:0> a.select { |key,value| value > 2 }

=> [[3, 4]]

irb(main):006:0> a.reject { |key,value| value <= 2 }

=> {3=>4}

that will change in newer version of ruby, eg ruby1.9

irb(main):015:0> RUBY_VERSION
=> “1.9.0”
irb(main):016:0> a=Hash.new
=> {}
irb(main):017:0> a[1]=2
=> 2
irb(main):018:0> a[2]=2
=> 2
irb(main):019:0> a[3]=4
=> 4
irb(main):020:0> a.select { |key,value| value > 2 }
=> {3=>4}
irb(main):021:0> a.reject { |key,value| value <= 2 }
=> {3=>4}

kind regards -botp

#6

Hi –

On Tue, 1 Jul 2008, Srijayanth S. wrote:

=> 2

Thanks.

So this is just some sort of artefact/legacy code that never got changed?

You can actually make a case that select and reject are not exactly
symmetrical operations. Imagine a line of people:

 Joe John Joan David Jim Jenny Jeff Matz

If I tell everyone in the line whose name does not begin with J to
step backwards (reject), the original line is smaller but it’s still
the same line.

If I tell everyone whose name does begin with J to step forwards
(select), I’ve got a new line of J people.

“Same line” and “new line” don’t necessarily map to “same object” and
“new object” in Ruby (since the post-reject hash is a different hash).
But it suggests that there’s a difference, arguably, between select
and reject, in terms of the formal disturbance of the object, which in
turn makes it easier to understand why select would return objects in
a different container, while reject would leave the container in the
same form but just contain fewer things.

However, that doesn’t mean it’s bad for select to return a hash (which
it does, as was mentioned, as of 1.9). Just that the behavior of one
doesn’t necessarily imply the behavior of the other.

David

#7

Well put. Thanks again.

Jayanth

#8

On Tue, Jul 1, 2008 at 8:48 AM, David A. Black removed_email_address@domain.invalid
wrote:

irb(main):016:0> a=Hash.new
=> {3=>4}
You can actually make a case that select and reject are not exactly
(select), I’ve got a new line of J people.
I’m having a hard time making the connection between this analogy and
the
methods.

In the first case one could say that we end up with two lines, the
‘original’ one and the line of rejects.

But first of all, x.reject leaves x alone so the original ‘line’ is
unchanged.

And why can’t we see your select example exactly the same way, except
with
the resultant line of ‘selects’ just closer to you.

“Same line” and “new line” don’t necessarily map to “same object” and
“new object” in Ruby (since the post-reject hash is a different hash).
But it suggests that there’s a difference, arguably, between select
and reject, in terms of the formal disturbance of the object, which in
turn makes it easier to understand why select would return objects in
a different container, while reject would leave the container in the
same form but just contain fewer things.

Except that both select and reject leave the original container the
same.
The analogy might be more apt for reject! and select! (if the latter
method
existed).

However, that doesn’t mean it’s bad for select to return a hash (which
it does, as was mentioned, as of 1.9). Just that the behavior of one
doesn’t necessarily imply the behavior of the other.

The real change, it seems to me is that pre 1.9 the Ruby enumerator
methods
had/have a preference for returning arrays rather than an instance of
the
same class as the receiver, whereas 1.9 seems to be shifting to a
preference
for returning an instance of the same class as the receiver where that
makes
sense.


Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/

#9

On Tue, Jul 1, 2008 at 7:26 PM, Robert D. removed_email_address@domain.invalid
wrote:

On Tue, Jul 1, 2008 at 3:42 PM, Rick DeNatale removed_email_address@domain.invalid wrote:
I rather agree with Rick although David has a point too.

Yeah I agree with Rick as well or I wouldn’t have asked the question
in the first place.

I therefore implemented hselect in Labrador (also kselect for keys and
vselect for values) all returning hashes. Parts of Labrador are
however highly experimental - and I intend to put some order into this
soon - therefore maybe you can find what you want in Facets which is a
highly tested and widely used framework or just borrow my code from
Labrador (BSD license).

I am just hacking up the classes as of now. Don’t really intend to use
it anywhere.

Thank you all,

Jayanth

#10

On Tue, Jul 1, 2008 at 3:42 PM, Rick DeNatale removed_email_address@domain.invalid
wrote:
I rather agree with Rick although David has a point too.

I therefore implemented hselect in Labrador (also kselect for keys and
vselect for values) all returning hashes. Parts of Labrador are
however highly experimental - and I intend to put some order into this
soon - therefore maybe you can find what you want in Facets which is a
highly tested and widely used framework or just borrow my code from
Labrador (BSD license).

HTH
Robert


http://ruby-smalltalk.blogspot.com/


AALST (n.) One who changes his name to be further to the front
D.Adams; The Meaning of LIFF

#11

Hi –

On Tue, 1 Jul 2008, Rick DeNatale wrote:

irb(main):015:0> RUBY_VERSION
=> {3=>4}

But first of all, x.reject leaves x alone so the original ‘line’ is

turn makes it easier to understand why select would return objects in
a different container, while reject would leave the container in the
same form but just contain fewer things.

Except that both select and reject leave the original container the same.
The analogy might be more apt for reject! and select! (if the latter method
existed).

It’s specifically not about the exact objects, though. As I said,
“same line” in this model doesn’t map to “same object”. Rather, the
analogy has to do with types of structure or container.

Here’s another, perhaps better analogy. You’ve got a shelf of books,
and you’ve got an empty box. You select some books from the shelf, and
put them in the box. Now you’ve got two structures: a shelf, and a
box.

If you were describing this operation, you’d say: “I selected some
books and put them in a box.” The box of books is the result of the
operation, and is structurally different from the shelf. The point is
not that this is the only way it can be done, but that it can be
done this way. It’s reasonable to select from a collection into
something that is not only a different collection, but a different
container type.

Now, if you do it reject-wise, you take the books you don’t want and
throw them out the window. The box never enters into it. You’re not
selecting, so you don’t care about observing or measuring the books
that you’re removing from the shelf.

And the shelf is still a shelf. There’s no new type of container
involved, just the same type of container with different contents. The
reject operation does not imply the transfer of objects to a different
type of container. Also, the fact that Ruby can do this in terms of
duplicate objects, rather than in place, isn’t relevant. The change or
conservation of the container type is what’s important.

That’s all I’m saying :slight_smile:

And thereafter, to make sense of the new 1.9 behavior, all you have to
do is decide that instead of a box, you’re putting the selected books
on another shelf.

Again, this has nothing to do with the identity of objects vs. copies.
It’s about the proliferation (or not) of structures.

However, that doesn’t mean it’s bad for select to return a hash (which
it does, as was mentioned, as of 1.9). Just that the behavior of one
doesn’t necessarily imply the behavior of the other.

The real change, it seems to me is that pre 1.9 the Ruby enumerator methods
had/have a preference for returning arrays rather than an instance of the
same class as the receiver, whereas 1.9 seems to be shifting to a preference
for returning an instance of the same class as the receiver where that makes
sense.

Yes, that’s clearly the change. I wasn’t talking about the change,
though; I was suggesting a model for understanding why the pre-change
way might not have been as irrational as has sometimes been suggested.
It’s all moot as of 1.9.

David

#12

Hi –

On Tue, 1 Jul 2008, Srijayanth S. wrote:

On Tue, Jul 1, 2008 at 7:26 PM, Robert D. removed_email_address@domain.invalid wrote:

On Tue, Jul 1, 2008 at 3:42 PM, Rick DeNatale removed_email_address@domain.invalid wrote:
I rather agree with Rick although David has a point too.

Yeah I agree with Rick as well or I wouldn’t have asked the question
in the first place.

Rick made some points about in-place changes vs. copies of Ruby
objects that I agree with, but that wasn’t what my post was about :slight_smile:
See my last one too.

David

#13

On Jul 1, 9:18 am, “David A. Black” removed_email_address@domain.invalid wrote:

Now, if you do it reject-wise, you take the books you don’t want and
throw them out the window. The box never enters into it. You’re not
selecting, so you don’t care about observing or measuring the books
that you’re removing from the shelf.

Throw them out the window? You could hurt someone.

Let’s not be so hasty, David. You could put those books in the box and
take them to a used-book store instead.

#14

Hi –

On Tue, 1 Jul 2008, Yossef M. wrote:

On Jul 1, 9:18 am, “David A. Black” removed_email_address@domain.invalid wrote:

Now, if you do it reject-wise, you take the books you don’t want and
throw them out the window. The box never enters into it. You’re not
selecting, so you don’t care about observing or measuring the books
that you’re removing from the shelf.

Throw them out the window? You could hurt someone.

Let’s not be so hasty, David. You could put those books in the box and
take them to a used-book store instead.

Yeah, I thought of that, but I figured the window would make the point
better :slight_smile: Besides, used book stores have gotten so picky lately…

David

#15

On Tue, Jul 1, 2008 at 10:18 AM, David A. Black removed_email_address@domain.invalid
wrote:

‘original’ one and the line of rejects.
analogy has to do with types of structure or container.
done this way. It’s reasonable to select from a collection into
something that is not only a different collection, but a different
container type.

Now, if you do it reject-wise, you take the books you don’t want and
throw them out the window. The box never enters into it. You’re not
selecting, so you don’t care about observing or measuring the books
that you’re removing from the shelf.

I still have the same problem with this analogy.

reject and select are not about the books. They don’t move books from
one
collection to another. They BOTH return a new collection which
references
some of the objects referenced by the receiver.

select returns a collection of all of the objects in the receiver which
meet
the criterion specified by the block, reject returns a collection of all
the
objects in the receiver which DON’T meet the criterion specified by the
block.

The fact that Ruby <1.9 tends to return arrays rather than something of
the
same species as the receiver on such methods was something I had to get
used
to after my Smalltalk experience.

And, compatibility aside, I think that 1.9 is going in the right
direction
here.


Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/

#16

Hi –

On Wed, 2 Jul 2008, Rick DeNatale wrote:

It’s specifically not about the exact objects, though. As I said,
operation, and is structurally different from the shelf. The point is

I still have the same problem with this analogy.

reject and select are not about the books. They don’t move books from one
collection to another. They BOTH return a new collection which references
some of the objects referenced by the receiver.

It’s an analogy, not a roman à clef about some Ruby objects :slight_smile:

here.
I must not be making it clear that what I’m discussing has nothing to
do with whether Object a and Object b are the same object in Ruby’s
memory, and I’m certainly not arguing that 1.9 should not change what
it’s changing. I think we’re all so used to discussions that are about
what should and should not be in the language that it becomes hard to
make points that aren’t about that kind of advocacy :slight_smile:

If you have a shelf of books, and you take some off, and then you look
back up at your shelf and it has turned into a canvas sack – that
would be kind of weird. Similarly, if you remove items from a hash and
the non-removed items get transferred to an array, that would be weird.

If, however, you select some books from the shelf, and they end up in
a canvas sack (1.8), that isn’t necessarily weird. If they end up on
another shelf (1.9), that also isn’t weird.

David

#17

On Tue, Jul 1, 2008 at 4:24 PM, David A. Black removed_email_address@domain.invalid
wrote:

Yeah I agree with Rick as well or I wouldn’t have asked the question
in the first place.

Rick made some points about in-place changes vs. copies of Ruby
objects that I agree with, but that wasn’t what my post was about :slight_smile:
See my last one too.

Actually I think the most important thing about this is, yes I know
you guessed and you disagree, consistency;)
But give me a chance, it can be consistent that whatever.reject.class
== whatever.class and
whatever.select.class == Array. That was exactly why I named my Hash
returning methods hselect, vselect and kselect.

However personally I would prefer a different kind of consistency on
this one, as some others would, I do not even daresay that we are a
majority ;).

Cheers
Robert

#18

Hi –

On Wed, 2 Jul 2008, Robert D. wrote:

I rather agree with Rick although David has a point too.
you guessed and you disagree, consistency;)
But give me a chance, it can be consistent that whatever.reject.class
== whatever.class and
whatever.select.class == Array. That was exactly why I named my Hash
returning methods hselect, vselect and kselect.

It depends on the class, though. Neither File#select nor File#reject
is going to be return a File object.

David

#19

On Wed, Jul 2, 2008 at 4:07 AM, David A. Black removed_email_address@domain.invalid
wrote:

It depends on the class, though. Neither File#select nor File#reject
is going to be return a File object.
OMG, but are these names really baring the same semantic meaning?
Robert

#20

OK, then there’s a box of books. You get another box. You move books
that you like from the first box to the second and ditch the others.
It’s all the same and the minutia aren’t really that important when
describing the general behavior.

I think the analogy works perfectly fine to get the point across about
the behavior of the method.

–Jeremy

On Tue, Jul 1, 2008 at 10:28 AM, Rick DeNatale removed_email_address@domain.invalid
wrote:

It’s specifically not about the exact objects, though. As I said,
operation, and is structurally different from the shelf. The point is

My blog on Ruby
http://talklikeaduck.denhaven2.com/


http://jeremymcanally.com/
http://entp.com

Read my books:
Ruby in Practice (http://manning.com/mcanally/)
My free Ruby e-book (http://humblelittlerubybook.com/)

Or, my blogs:
http://mrneighborly.com
http://rubyinpractice.com