Tuples/Records/Quicky Objects + Read Only Arrays?


#1

Hi,

Does Ruby have a tuple or quicky object mechanism? I’m new to Ruby and
have become stuck on how to solve the following.

I’m testing adding items to a container, where each item is a pair of
strings. One is a name and another an alias. As they’re added a check
should be performed to see if they’re in the container, etc.

I’ve been using OCaml for a while, and for this I’d just use a list of
tuples of strings. What’s the ruby way of doing this? Iterating over
a container/array is a doddle, that’s not the problem. The problem is
really can you have quick pairs without making classes and so on?

While I’m at it, is there a read only array like container? Don’t want
anyone messing with the internal array, they should not know that what’s
returned is actually the internal representation or fiddle with it.
Don’t care what they do with the elements in the array, only the array
itself.

class X
def initialize
@elements = Array.new
end

def elements
@elements.clone
end
end

Returning a clone of an array is fine, just wondered if you could
“write-protect” stuff in general.

Thanks,
Chris


#2

Christopher C. wrote:

Returning a clone of an array is fine, just wondered if you could
“write-protect” stuff in general.

Just use Object#freeze. Note that you will not usually be able to change
objects once they have become frozen.


#3

On 12/31/05, Christopher C. removed_email_address@domain.invalid wrote:

tuples of strings. What’s the ruby way of doing this? Iterating over
def initialize
Returning a clone of an array is fine, just wondered if you could
“write-protect” stuff in general.

The Ruby equivalent of a tuple is pretty much an Array, since the
syntax is so lightweight.
a = [1, 2]
a.first == 1
a.last == 2

Possibly due to champagne, I can’t think of a built-in object that
behaves the way you want… but luckily, a quick implementation of it
shouldn’t be hard:
class FunBox
def initialize
@names = {}
@aliases = {}
end

def <<(pair)
@names[pair.first] = pair.last
@aliases[pair.last] = pair.first
end

def alias_for_name(n)
@names[n]
end

def name_for_alias(a)
@aliases[a]
end
end

irb(main):060:0> f = FunBox.new
=> #<FunBox:0x2e15f00 @aliases={}, @names={}>
irb(main):061:0> f<<[‘joe’, ‘jim’]
=> “joe”
irb(main):062:0> f.alias_for_name(‘joe’)
=> “jim”
irb(main):063:0> f.name_for_alias(‘jim’)
=> “joe”
irb(main):064:0>

Of course, given that you know OCaml, I’m probably telling you exactly
nothing you don’t know.
I’m not aware of a way to freeze instance variables, such that you
couldn’t still change them with Object#instance_variable_set


#4

Christopher C. removed_email_address@domain.invalid wrote:

Hi,

Does Ruby have a tuple or quicky object mechanism? I’m new to Ruby
and have become stuck on how to solve the following.

Easiest is to use arrays. But you can as well use a Struct:

Tuple=Struct.new :name, :alias
=> Tuple

t1=Tuple.new(“name”, “alias”).freeze
=> #<struct Tuple name=“name”, alias=“alias”>

t2=Tuple.new(“name”, “alias”).freeze
=> #<struct Tuple name=“name”, alias=“alias”>

t1.name = “changed?”
TypeError: can’t modify frozen Struct
from (irb):14:in `name=’
from (irb):14
from ?:0

t1 == t2
=> true

I’m testing adding items to a container, where each item is a pair of
strings. One is a name and another an alias. As they’re added a
check should be performed to see if they’re in the container, etc.

I guess you rather want a Set so no multiple insertions can occur.
Alternatively you can use a Hash with your tuple as key if there is a
meaningful value.

end

Returning a clone of an array is fine, just wondered if you could
“write-protect” stuff in general.

Use #freeze as Florian pointed out.

require ‘set’
=> true

container = Set.new
=> #<Set: {}>

container << [“name”, “alias”].freeze
=> #<Set: {[“name”, “alias”]}>

container << [“name”, “alias”].freeze
=> #<Set: {[“name”, “alias”]}>

container << [“no name”, “alias”].freeze
=> #<Set: {[“name”, “alias”], [“no name”, “alias”]}>

container.each {|nm,al| print “name=”, nm, " alias=", al,"\n"}
name=name alias=alias
name=no name alias=alias
=> #<Set: {[“name”, “alias”], [“no name”, “alias”]}>

container.each {|a| a << “does not work”}
TypeError: can’t modify frozen array
from (irb):7:in <<' from (irb):7 from /usr/lib/ruby/1.8/set.rb:189:ineach’
from /usr/lib/ruby/1.8/set.rb:189:in `each’
from (irb):7
from :0

HTH

Kind regards

robert

#5

Thanks guys.

I see now we can use arrays and hashes to get

[{‘name’ => ‘hi’, ‘c_alias’ => ‘c_hi’}]

or

[{:name => ‘hi’, :c_alias => ‘c_hi’}]

which works dandy. I think my brain evaporated with the festivities
last night, heh.


#6

Christopher C. removed_email_address@domain.invalid wrote:

which works dandy. I think my brain evaporated with the festivities
last night, heh.

Are you sure you recovered? I mean, what’s the point in stuffing a hash
into a one element array?

robert

#7

Robert K. wrote:

which works dandy. I think my brain evaporated with the festivities
last night, heh.

Are you sure you recovered? I mean, what’s the point in stuffing a hash
into a one element array?

Heh… it was just an example, I will be stuffing lots of hashes into an
array.

def test_add_many_aliased
sample = [{:name => “abc”, :c_alias => “c_abc”},
{:name => “hoop”, :c_alias => “c_hoop”},
{:name => “blah”, :c_alias => “c_blah”}]
elist = EnumList.new(“many_aliased”)

 sample.each_index {|e| elist.add_alias(sample[e][:name],
                                        sample[e][:c_alias])
                        assert_equal(sample[e][:name], 

elist[e].name)
assert_equal(sample[e][:c_alias],
elist[e].c_alias)}
end