Set vs Hash

ruby-talk
1

Are there any general guidelines on when a "Set" should be used and when a
"Hash" should be used. Are there any limitations to "Set"

Kiran Kumar

2

SunRaySon wrote:

Are there any general guidelines on when a "Set" should be used and when a
"Hash" should be used. Are there any limitations to "Set"

A Set is similar to an Array - the two major differences are that a Set is unordered (because it's implementation is based on a Hash) and that a Set contains every element only once (i.e. if you add an element which is already there, the Set won't change).

You should use a Hash if you have a dictionary-like structure (i.e. key => value pairs) and a Set/Array if you have single objects. You can think about a Set as a Hash where every value is nil - i.e. you can represent a Set with a Hash but it makes no sense if you are not using any values at all. In this case you should use a Set.

I hope this answers your question...

Cheers,
Peter

···

__
http://www.rubyrailways.com :: Ruby and Web2.0 blog
http://scrubyt.org :: Ruby web scraping framework
http://rubykitchensink.ca/ :: The indexed archive of all things Ruby

3

Thanks for your response. I think now the differences and similarities
between Set, Hash and Array are clear to me. But with respect to memory
consumption which one of among the following is better:

s = Set.new([1,2,3])
h = { 1=>nil, 2=>nil, 3=> nil}

···

On 2/17/07, Peter Szinek <peter@rubyrailways.com> wrote:

SunRaySon wrote:
> Are there any general guidelines on when a "Set" should be used and when
a
> "Hash" should be used. Are there any limitations to "Set"

A Set is similar to an Array - the two major differences are that a Set
is unordered (because it's implementation is based on a Hash) and that a
Set contains every element only once (i.e. if you add an element which
is already there, the Set won't change).

You should use a Hash if you have a dictionary-like structure (i.e. key
=> value pairs) and a Set/Array if you have single objects. You can
think about a Set as a Hash where every value is nil - i.e. you can
represent a Set with a Hash but it makes no sense if you are not using
any values at all. In this case you should use a Set.

I hope this answers your question...

Cheers,
Peter

__
http://www.rubyrailways.com :: Ruby and Web2.0 blog
http://scrubyt.org :: Ruby web scraping framework
http://rubykitchensink.ca/ :: The indexed archive of all things Ruby

4

It should be the same as a Set is implemented as a Hash with every value being true. Storing true and storing nil uses the same amount of memory.

Honestly, it doesn't matter. Use which ever is semantically correct; if you need to store values with the keys then use a Hash, if you don't then use a Set.

Dan

SunRaySon wrote:

···

Thanks for your response. I think now the differences and similarities
between Set, Hash and Array are clear to me. But with respect to memory
consumption which one of among the following is better:

s = Set.new([1,2,3])
h = { 1=>nil, 2=>nil, 3=> nil}

On 2/17/07, Peter Szinek <peter@rubyrailways.com> wrote:

SunRaySon wrote:
> Are there any general guidelines on when a "Set" should be used and when
a
> "Hash" should be used. Are there any limitations to "Set"

A Set is similar to an Array - the two major differences are that a Set
is unordered (because it's implementation is based on a Hash) and that a
Set contains every element only once (i.e. if you add an element which
is already there, the Set won't change).

You should use a Hash if you have a dictionary-like structure (i.e. key
=> value pairs) and a Set/Array if you have single objects. You can
think about a Set as a Hash where every value is nil - i.e. you can
represent a Set with a Hash but it makes no sense if you are not using
any values at all. In this case you should use a Set.

I hope this answers your question...

Cheers,
Peter

__
http://www.rubyrailways.com :: Ruby and Web2.0 blog
http://scrubyt.org :: Ruby web scraping framework
http://rubykitchensink.ca/ :: The indexed archive of all things Ruby

5

nearly the same - set, in ruby, happens to be built on top of hash.

-a

···

On Sun, 18 Feb 2007, SunRaySon wrote:

Thanks for your response. I think now the differences and similarities
between Set, Hash and Array are clear to me. But with respect to memory
consumption which one of among the following is better:

s = Set.new([1,2,3])
h = { 1=>nil, 2=>nil, 3=> nil}

--
we can deny everything, except that we have the possibility of being better.
simply reflect on that.
- the dalai lama