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comp.lang.ruby

Hash as Checklist

basi

4/28/2005 2:18:00 AM

Hello,
I need to check if a word is in a list. I'm using hash because the
lists can be long (i'm under the impression hash is faster than array).
Basically I'm only interested that a key exists. I have to create
entries like:

h = {
"one" => true,
"two" => true,
"three" => true,
"four" => true,
"five" => true
}

So h["one"] gives true, and h["seven"] returns nil. But typing the
dummy value "true" is a waste of effort, and there's an ugliness in
there. Is there a better way to design a simple checklist?

Thank you for your help
basi
5 Answers

Assaph Mehr

4/28/2005 2:37:00 AM

0


basi wrote:
> Hello,
> I need to check if a word is in a list. I'm using hash because the
> lists can be long (i'm under the impression hash is faster than
array).
> Basically I'm only interested that a key exists. I have to create
> entries like:
>
> h = {
> "one" => true,
> "two" => true,
> "three" => true,
> "four" => true,
> "five" => true
> }
>
> So h["one"] gives true, and h["seven"] returns nil. But typing the
> dummy value "true" is a waste of effort, and there's an ugliness in
> there. Is there a better way to design a simple checklist?

irb(main):001:0> require 'set'
=> true
irb(main):002:0> s = Set.new %w{one two three four}
=> #<Set: {"three", "two", "one", "four"}>
irb(main):005:0> s.member? 'one'
=> true
irb(main):006:0> s.member? 'seven'
=> false

Jeremy Kemper

4/28/2005 2:45:00 AM

0

basi wrote:
> I need to check if a word is in a list. I'm using hash because the
> lists can be long (i'm under the impression hash is faster than array).
> Basically I'm only interested that a key exists. I have to create
> entries like:
>
> h = {
> "one" => true,
> "two" => true,
> "three" => true,
> "four" => true,
> "five" => true
> }
>
> So h["one"] gives true, and h["seven"] returns nil. But typing the
> dummy value "true" is a waste of effort, and there's an ugliness in
> there. Is there a better way to design a simple checklist?

Use a set:
require 'set'
words = %w(one two three four five).to_set
words.include? 'one' # => true
words.include? 'seven' # => false

http://www.ruby-doc.org/stdlib/libdoc...

jeremy

basi

4/28/2005 3:25:00 AM

0

Thanks to both Assaph and Jeremy for the quick reply. This is just what
I need!

Cheers!
basi

Balwinder S Dheeman

4/28/2005 6:12:00 AM

0

On 04/28/2005 07:48 AM, basi wrote:
> Hello,
> I need to check if a word is in a list. I'm using hash because the
> lists can be long (i'm under the impression hash is faster than array).

A hash have to store "key" => value pairs, how will be more efficient
than an array? and why?

> Basically I'm only interested that a key exists. I have to create
> entries like:
>
> h = {
> "one" => true,
> "two" => true,
> "three" => true,
> "four" => true,
> "five" => true
> }
>
> So h["one"] gives true, and h["seven"] returns nil. But typing the
> dummy value "true" is a waste of effort, and there's an ugliness in
> there. Is there a better way to design a simple checklist?
>
> Thank you for your help

irb(main):001:0> a = %w{one two three four five}
=> ["one", "two", "three", "four", "five"]
irb(main):002:0> a.member? 'one'
=> true
irb(main):003:0> a.member? 'seven'
=> false

Hope that helps.
--
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CLLO (Chief Linux Learning Officer) Machines: #168573, 170593, 259192
Anu's Linux@HOME Distros: Ubuntu, Fedora, Knoppix
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Brian Schröder

4/28/2005 1:33:00 PM

0

On 28/04/05, Dr Balwinder S Dheeman <bsd.SANSPAM@cto.homelinux.net> wrote:
> On 04/28/2005 07:48 AM, basi wrote:
> > Hello,
> > I need to check if a word is in a list. I'm using hash because the
> > lists can be long (i'm under the impression hash is faster than array).
>
> A hash have to store "key" => value pairs, how will be more efficient
> than an array? and why?
>

because Array lookups are O(n) and Hash lookups should be O(1). To
find something in an Array you have to traverse it completely and will
find the entry you are searching in expected n/2 steps. For hashes
there exist different implementations, but basically you compute the
possible position of an object from the object and just take a look if
it is there. (Oversimplified).

best regards,

Brian

> [snip]

--
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