Set vs Hash


#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 K.


#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

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#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}


#4

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}

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

-a


#5

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