Create an object/Native demonstration
From Rosetta Code
Create an object/Native demonstration is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.
Create a Hash/Associative Array/Dictionary-like object, initialized with some default key/value pairs using the languages native method of object creation. The object should behave like a native Hash/Associative Array/Dictionary of the language, if any, but with the following differences:
- No new item can be added;
- Item cannot be deleted, (but native delete method may used to reset the item's value to default) ;
If the language supports Magic Methods, then show how these work.
Contents |
[edit] D
struct DefaultAA(TK, TV) {
TV[TK] standard, current;
this(TV[TK] default_) /*pure nothrow*/ {
this.standard = default_;
this.current = default_.dup;
}
alias current this;
void remove(in TK key) pure nothrow {
current[key] = standard[key];
}
void clear() /*pure nothrow*/ {
current = standard.dup;
}
}
void main() {
import std.stdio;
auto d = DefaultAA!(string, int)(["a": 1, "b": 2]);
writeln(d); // ["a":1, "b":2]
d["a"] = 55; d["b"] = 66;
writeln(d); // ["a":55, "b":66]
d.clear();
writeln(d); // ["a":1, "b":2]
d["a"] = 55; d["b"] = 66;
writeln(d["a"]); // 55
d.remove("a");
writeln(d); // ["a":1, "b":66]
}
- Output:
["a":1, "b":2] ["a":55, "b":66] ["a":1, "b":2] 55 ["a":1, "b":66]
[edit] J
Given a list of keys and an associated list of values, the idiomatic way of expressing this concept in J would be:
lookup=: values {~ keys&i.
For example:
lookup=: 10 20 30 40 50 {~ (;:'this is a test')&i.
lookup ;:'a test'
30 40
Notes:
1) While the result can not be modified or deleted, the name used to refer to it can be made to refer to something else, and once all references are lost it will be garbage collected.
2) In the above example, we have 5 values and 4 keys. The extra value is used when no key is found. If no extra value was provided, the "key not found" case would be an error case.
3) In J, objects are always referenced, but all data is passed by value. This means that objects can never be passed to a function -- only a reference to an object (its name) can be passed. This means that objects exist only in the way things are named, in J. So for the most part, we do not call things "objects" in J, and this task has nothing to do with what are called "objects" in J. However, this does demonstrate how things are created in J -- you write their definition, and can use them and/or assign to names or inspect them or whatever else.
[edit] JavaScript
This is a first demonstration of the task, but only implemented the functionality, not any native behavior, eg indexing. JavaScript experts may want to replace this one.
var keyError = new Error("Invalid Key Error (FixedKeyDict)") ;
function FixedKeyDict(obj)
{
var myDefault = new Object() ;
var myData = new Object() ;
for(k in obj) {
myDefault[k] = obj[k] ;
myData[k] = obj[k] ;
}
var gotKey = function(k) {
for(kk in myDefault) {
if(kk == k) return true ;
}
return false ;
} ;
this.hasKey = gotKey ;
var checkKey = function(k) {
if(!gotKey(k))
throw keyError ;
} ;
this.getItem = function(k) {
checkKey(k) ;
return myData[k];
} ;
this.setItem = function(k, v) {
checkKey(k) ;
myData[k] = v ;
} ;
this.resetItem = function(k) {
checkKey(k) ;
myData[k] = myDefault[k] ;
} ;
this.delItem = this.resetItem ;
this.clear = function() {
for(k in myDefault)
myData[k] = myDefault[k] ;
} ;
this.iterator = function() {
for(k in myDefault)
yield (k);
} ;
this.clone = function() {
return new FixedKeyDict(myDefault) ;
}
this.toStr = function() {
var s = "" ;
for(key in myData)
s = s + key + " => " + myData[key] + ", " ;
return "FixedKeyDict{" + s + "}" ;
} ;
}
Test run:
const BR = "<BR>\n"
var pl = function(s) {
document.write(s + BR) ;
} ;
pl("<pre>") ;
var o = { foo:101, bar:102 } ;
var h = new FixedKeyDict(o) ;
pl("Fixed Key Dict Created") ;
pl("toString : " + h.toStr()) ;
pl("get an item: " + h.getItem("foo")) ;
pl("check a key: " + h.hasKey("boo")) ;
pl("ditto : " + h.hasKey("bar")) ;
h.setItem("bar", 999) ;
pl("set an item: " + h.toStr()) ;
pl("Test iterator (or whatever)") ;
for(k in h.iterator())
pl(" " + k + " => " + h.getItem(k)) ;
var g = h.clone() ;
pl("Clone a dict") ;
pl(" clone : " + g.toStr()) ;
pl(" original : " + h.toStr()) ;
h.clear() ;
pl("clear or reset the dict") ;
pl(" : " + h.toStr()) ;
try {
h.setItem("NoNewKey", 666 ) ;
} catch(e) {
pl("error test : " + e.message) ;
}
output :
Fixed Key Dict Created
toString : FixedKeyDict{foo => 101, bar => 102, }
get an item: 101
check a key: false
ditto : true
set an item: FixedKeyDict{foo => 101, bar => 999, }
Test iterator (or whatever)
foo => 101
bar => 999
Clone a dict
clone : FixedKeyDict{foo => 101, bar => 102, }
original : FixedKeyDict{foo => 101, bar => 999, }
clear or reset the dict
: FixedKeyDict{foo => 101, bar => 102, }
error test : Invalid Key Error (FixedKeyDict)
[edit] Mathematica
a[1] = "Do not modify after creation";
a[2] = "Native demonstration";
Protect[a];
Example usage:
a[3] = 2 ->Set::write: Tag a in a[1] is Protected. >>
[edit] Perl
package LockedHash;output:
use parent Tie::Hash;
use Carp;
use strict;
sub TIEHASH {
my $cls = shift;
my %h = @_;
bless \%h, ref $cls || $cls;
}
sub STORE {
my ($self, $k, $v) = @_;
croak "Can't add key $k" unless exists $self->{$k};
$self->{$k} = $v;
}
sub FETCH {
my ($self, $k) = @_;
croak "No key $k" unless exists $self->{$k};
$self->{$k};
}
sub DELETE {
my ($self, $k) = @_;
croak "No key $k" unless exists $self->{$k};
$self->{$k} = 0;
}
sub CLEAR { } # ignored
sub EXISTS { exists shift->{+shift} }
sub FIRSTKEY {
my $self = shift;
keys %$self;
each %$self;
}
sub NEXTKEY {
my $self = shift;
each %$self;
}
sub lock_hash(\%) {
my $ref = shift;
tie(%$ref, __PACKAGE__, %$ref);
}
1;
my %h = (a => 3, b => 4, c => 5);
# lock down %h
LockedHash::lock_hash(%h);
# show hash content and iteration
for (sort keys %h) { print "$_ => $h{$_}\n"; }
# try delete b
delete $h{b};
print "\nafter deleting b: b => $h{b}\n";
# change value of a
$h{a} = 100;
print "\na => $h{a}\n";
# add a new key x: will die
eval { $h{x} = 1 };
if ($@) { print "Operation error: $@" }
a => 3
b => 4
c => 5
after deleting b: b => 0
a => 100
operation error: Can't add key x at test.pl line 14
LockedHash::STORE('LockedHash=HASH(0x8cebe14)', 'x', 1) called at test.pl line 66
eval {...} called at test.pl line 66
[edit] Perl 6
Here we use delegation to handle all the normal hash methods that we don't need to override to define our new class.
class FixedHash {
has $.hash handles *;
method new(*@args) { self.bless: *, hash => Hash.new: @args }
method at_key(FixedHash:D: $key is copy) is rw {
$!hash.exists($key) ?? $!hash.at_key($key) !! Nil;
}
method delete($key) { $!hash.{$key} = Nil }
}
# Testing
my $fh = FixedHash.new: "a" => 1, "b" => 2;
say $fh<a b>; # 1 2
$fh<b>:delete;
say $fh<a b>; # 1 Nil
$fh<b> = 42;
say $fh<a b>; # 1 42
say $fh<c>; # Nil
$fh<c> = 43; # error
- Output:
1 2 1 Nil 1 42 Nil Cannot assign to a non-container in block at freezehash:21
[edit] Python
from collections import UserDict
import copy
class Dict(UserDict):
'''
>>> d = Dict(a=1, b=2)
>>> d
Dict({'a': 1, 'b': 2})
>>> d['a'] = 55; d['b'] = 66
>>> d
Dict({'a': 55, 'b': 66})
>>> d.clear()
>>> d
Dict({'a': 1, 'b': 2})
>>> d['a'] = 55; d['b'] = 66
>>> d['a']
55
>>> del d['a']
>>> d
Dict({'a': 1, 'b': 66})
'''
def __init__(self, dict=None, **kwargs):
self.__init = True
super().__init__(dict, **kwargs)
self.default = copy.deepcopy(self.data)
self.__init = False
def __delitem__(self, key):
if key in self.default:
self.data[key] = self.default[key]
else:
raise NotImplementedError
def __setitem__(self, key, item):
if self.__init:
super().__setitem__(key, item)
elif key in self.data:
self.data[key] = item
else:
raise KeyError
def __repr__(self):
return "%s(%s)" % (type(self).__name__, super().__repr__())
def fromkeys(cls, iterable, value=None):
if self.__init:
super().fromkeys(cls, iterable, value)
else:
for key in iterable:
if key in self.data:
self.data[key] = value
else:
raise KeyError
def clear(self):
self.data.update(copy.deepcopy(self.default))
def pop(self, key, default=None):
raise NotImplementedError
def popitem(self):
raise NotImplementedError
def update(self, E, **F):
if self.__init:
super().update(E, **F)
else:
haskeys = False
try:
keys = E.keys()
haskeys = Ture
except AttributeError:
pass
if haskeys:
for key in keys:
self[key] = E[key]
else:
for key, val in E:
self[key] = val
for key in F:
self[key] = F[key]
def setdefault(self, key, default=None):
if key not in self.data:
raise KeyError
else:
return super().setdefault(key, default)
[edit] Ruby
TODO: Write comments for FencedHash::new, FencedHash#delete and related methods. Add more methods (merge, merge!, reject, reject!, select, select!, update). Explain why FencedHash#replace and FencedHash#shift will not exist.
# fencedhash.rb
require 'forwardable'
# A FencedHash acts like a Hash, but with a fence around its keys.
# After the creation of a FencedHash, one cannot add nor remove keys.
# Any attempt to insert a new key will raise KeyError. Any attempt to
# delete a key-value pair will keep the key but will reset the value to
# the default value.
class FencedHash < Object
extend Forwardable
include Enumerable
#--
# @hash: our Hash inside the fence
# @default_proc: passes self, not @hash
#++
def_delegators(:@hash, :[], :assoc,
:compare_by_identity, :compare_by_identity?,
:default, :empty?, :fetch, :flatten,
:has_key?, :has_value?, :hash, :include?,
:key, :key?, :keys, :length, :member?,
:rassoc, :size, :to_a,
:values, :values_at, :value?)
attr_reader :default_proc
# Acts like Hash::[] but creates a FencedHash.
def self.[](*args)
allocate.instance_eval do
@hash = Hash[*args]
self
end
end
# call-seq:
# FencedHash.new(obj=nil [,keys]) -> fh
# FencedHash.new([keys]) { |fh, key| block } -> fh
#
# Creates a FencedHash.....
def initialize(*args, &block)
n = args.length
if block_given?
raise ArgumentError, "wrong number of arguments" if n > 1
@default_proc = block
@hash = Hash.new { |hash, key| block[self, key] }
if n > 0
args[0].each { |key| @hash[key] = nil }
clear
end
else
raise ArgumentError, "wrong number of arguments" if n > 2
default = if n > 0 then n[0] else nil end
@hash = Hash.new(default)
if n > 1
args[1].each { |key| @hash[key] = default }
end
end
end
def initialize_copy(orig)
super
@hash = @hash.dup
end
# Clears all values. For each key-value pair, this retains the key
# but resets the value to default.
#--
# The line "@hash = @hash" checks that _self_ is not frozen, because
# Object#freeze only freezes _self_ and not @hash.
#++
def clear
@hash = @hash
@hash.each_key { |key| delete key }
self
end
# .....
def default=(obj)
@default_proc = nil
@hash.default = obj
end
# .....
def default_proc=(proc_obj)
# Convert _proc_obj_ to a block parameter.
proc_obj = proc &proc_obj
@hash.default_proc = proc { |hash, key| proc_obj[self, key] }
@default_proc = proc_obj
end
# Deletes the value of the key-value pair for _key_.
#
# If _key_ is in the fence.....
def delete(key)
@hash = @hash
begin
original_value = @hash.fetch(key)
rescue IndexError
# _key_ is not in the fence.
if block_given?
yield key
else
nil
end
else
# _key_ is in the fence.
if @default_proc
@default_proc[self, key]
else
@hash[key] = @hash.default
end
original_value
end
end
# .....
def delete_if
return enum_for(:delete_if) unless block_given?
@hash = @hash
@hash.each { |key, value| delete key if yield key, value }
self
end
# Yields each key-value pair to the block, or returns an enumerator.
# Acts like Hash#each.
def each &block # :yields: key, value
return enum_for(:each) unless block
@hash.each &block
end
alias each_pair each
# Yields each key to the block, or returns an enumerator.
# Acts like Hash#each_key.
def each_key &block # :yields: key
return enum_for(:each_key) unless block
@hash.each_key &block
end
# Yields each value to the block, or returns an enumerator.
# Acts like Hash#each_value.
def each_value &block # :yields: value
return enum_for(:each_value) unless block
@hash.each_value &block
end
# Returns true if _other_ is a FencedHash and has the same key-value
# pairs as _self_. Acts like Hash#eql?.
#--
# Consistent with FencedHash#hash because it delegates to @hash.hash.
#++
def eql?(other)
FencedHash === other and
@hash.eql?(other.instance_eval { @hash })
end
# Returns true if _other_ is a FencedHash and if the key-value pairs
# of _self_ equal those of _other_. Acts like Hash#==.
def ==(other)
FencedHash === other and
@hash == (other.instance_eval { @hash })
end
# .....
def keep_if
return enum_for(:keep_if) unless block_given?
@hash = @hash
@hash.each { |key, value| delete key unless yield key, value }
self
end
# Stores a _value_ for a _key_. This only works if _key_ is in the
# fence; FencedHash prevents the insertion of new keys. If _key_ is
# not in the fence, then this method raises KeyError.
def store(key, value)
@hash = @hash
if @hash.has_key? key
@hash.store(key, value)
else
c = if defined? KeyError then KeyError else IndexError end
raise c, "fence prevents new key: #{key}"
end
end
alias []= store
# Converts _self_ to a regular Hash. Returns a new Hash that has the
# same key-value pairs as _self_.
def to_hash
@hash.dup
end
# Converts _self_ to a String.
def to_s
"#<#{self.class}: #{@hash.inspect}>"
end
alias inspect to_s
end
# fh-test.rb
require 'fencedhash'
require 'test/unit'
class TestFencedHash < Test::Unit::TestCase
if RUBY_VERSION >= "1.9"
KeyEx = KeyError
FrozenEx = RuntimeError
else
KeyEx = IndexError
FrozenEx = TypeError
end
def setup
@fh = FencedHash[:q => 11, :w => 22, :e => 33,
:r => 44, :t => 55, :y => 66]
end
def test_bracket_operator
assert_equal 11, @fh[:q]
assert_equal 22, @fh[:w]
assert_equal 33, @fh[:e]
assert_equal 44, @fh[:r]
assert_equal 55, @fh[:t]
assert_equal 66, @fh[:y]
assert_nil @fh[:u]
end
def test_delete
assert_equal 44, (@fh.delete :r)
assert_nil @fh.fetch(:r)
assert_nil @fh.delete(:r)
assert_nil @fh.delete(:u)
@fh[:r] = "replacement"
assert_equal "replacement", (@fh.delete :r)
end
def test_delete_if
a = @fh.delete_if { |key, value| key == :t || value == 66 }
assert_same @fh, a
assert_equal 2, @fh.values.grep(nil).length
@fh[:y] = "why?"
@fh[:t] = "tea!"
assert_equal 0, @fh.values.grep(nil).length
end
def test_default
fruit = FencedHash.new(0, [:apple, :banana, :cranberry])
assert_equal [0, 0, 0], fruit.values
fruit[:apple] += 1
fruit[:banana] += 5
fruit[:cranberry] *= 5
assert_equal 1, fruit[:apple]
assert_equal 5, fruit[:banana]
assert_equal 0, fruit[:cranberry]
assert_equal 0, fruit.default
end
def test_default_assign
assert_nil @fh.default
@fh.delete :w
@fh.default = -1
assert_equal -1, @fh.default
@fh.delete :e
assert_nil @fh[:w]
assert_equal -1, @fh[:e]
end
def test_default_proc
count = 0
fruit = FencedHash.new([:apple, :banana, :cranberry]) do |h, k|
if h.key? k then h[k] = [] else count += 1 end
end
fruit[:apple].push :red
fruit[:banana].concat [:green, :yellow]
fruit[:cranberry].push :red
assert_equal 1, fruit[:orange]
assert_equal [:red], fruit[:apple]
assert_equal [:green, :yellow], fruit[:banana]
assert_equal [:red], fruit.delete(:cranberry)
assert_equal 2, fruit[:orange]
assert_equal [], fruit[:cranberry]
assert_nil fruit.delete(:orange)
assert_equal 3, fruit[:orange]
assert_equal [], fruit.default_proc[FencedHash[1 => 2], 1]
end
def test_each
count = 0
@fh.each do |key, value|
assert_kind_of Symbol, key
assert_kind_of Integer, value
assert_equal true, (@fh.has_key? key)
assert_equal true, (@fh.has_value? value)
count += 1
end
assert_equal 6, count
end
def test_eql?
other = FencedHash[:r, 44, :t, 55, :y, 66,
:q, 11, :w, 22, :e, 33]
float = FencedHash[:y, 66.0, :t, 55.0, :r, 44.0,
:e, 33.0, :w, 22.0, :q, 11.0]
tt = [true, true]
ff = [false, false]
if RUBY_VERSION >= "1.9"
assert_equal tt, [(@fh.eql? other), (other.eql? @fh)]
assert_equal ff, [(@fh.eql? float), (float.eql? @fh)]
assert_equal ff, [(other.eql? float), (float.eql? other)]
end
assert_equal tt, [@fh == other, other == @fh]
assert_equal tt, [@fh == float, float == @fh]
assert_equal tt, [other == float, float == other]
h = @fh.to_hash
if RUBY_VERSION >= "1.9"
assert_equal ff, [(@fh.eql? h), (h.eql? @fh)]
end
assert_equal ff, [@fh == h, h == @fh]
end
def test_fetch
assert_equal 11, @fh.fetch(:q)
assert_equal 22, @fh.fetch(:w)
assert_equal 33, @fh.fetch(:e)
assert_equal 44, @fh.fetch(:r)
assert_equal 55, @fh.fetch(:t)
assert_equal 66, @fh.fetch(:y)
assert_raises(KeyEx) { @fh.fetch :u }
end
def test_freeze
assert_equal false, @fh.frozen?
@fh.freeze
2.times do
assert_equal true, @fh.frozen?
assert_raises(FrozenEx) { @fh.clear }
assert_raises(FrozenEx) { @fh.delete :q }
assert_raises(FrozenEx) { @fh.delete_if { true } }
assert_raises(FrozenEx) { @fh.keep_if { false } }
assert_raises(FrozenEx) { @fh.store :w, "different" }
assert_raises(FrozenEx) { @fh[:w] = "different" }
# Repeat the tests with a clone. The clone must be frozen.
@fh = @fh.clone
end
# A duplicate is not frozen.
@fh = @fh.dup
assert_equal false, @fh.frozen?
@fh[:w] = "different"
assert_equal "different", @fh[:w]
end
def test_has_key
2.times do |t|
assert_equal true, (@fh.has_key? :y)
assert_equal true, (@fh.include? :y)
assert_equal true, (@fh.key? :y)
assert_equal true, (@fh.member? :y)
assert_equal false, (@fh.has_key? :u)
assert_equal false, (@fh.include? :u)
assert_equal false, (@fh.key? :u)
assert_equal false, (@fh.member? :u)
# Repeat the tests.
# The fence must prevent any changes to the keys.
@fh.delete :y
(@fh[:u] = "value") rescue "ok"
end
end
def test_has_value
assert_equal true, (@fh.has_value? 22)
assert_equal true, (@fh.value? 22)
assert_equal false, (@fh.has_value? 4444)
assert_equal false, (@fh.value? 4444)
end
def test_inject
# To get an :inject method, FencedHash should mix in Enumerable.
assert_kind_of Enumerable, @fh
assert_equal 231, @fh.inject(0) { |sum, kv| sum + kv[1] }
end
def test_keep_if
a = @fh.keep_if { |key, value| key == :t || value == 66 }
assert_same @fh, a
assert_equal 4, @fh.values.grep(nil).length
@fh.delete :y
@fh.delete :t
assert_equal 6, @fh.values.grep(nil).length
end
def test_keys
assert_equal([:e, :q, :r, :t, :w, :y],
@fh.keys.sort_by { |o| o.to_s })
end
def test_length
assert_equal 6, @fh.length
assert_equal 6, @fh.size
end
def test_store
assert_raises(KeyEx) { @fh[:a] = 111 }
assert_equal 222, (@fh[:e] = 222)
assert_equal 222, (@fh.fetch :e)
assert_equal 333, @fh.store(:e, 333)
assert_equal 333, @fh[:e]
end
def test_values
assert_equal [11, 22, 33, 44, 55, 66], @fh.values.sort!
end
if RUBY_VERSION >= "1.8.7"
def test_delete_if_enum
a = @fh.delete_if.with_index { |kv, i| i >= 2 }
assert_same @fh, a
assert_equal 4, @fh.values.grep(nil).length
end
def test_keep_if_enum
a = @fh.keep_if.with_index { |kv, i| i >= 2 }
assert_same @fh, a
assert_equal 2, @fh.values.grep(nil).length
end
end
if RUBY_VERSION >= "1.9"
def test_class_bracket_operator
from_pairs = FencedHash[10, "ten", 20, "twenty", 30, "thirty"]
from_alist = FencedHash[ [ [10, "ten"], [20, "twenty"], [30, "thirty"] ] ]
from_hash = FencedHash[10 => "ten", 20 => "twenty", 30 => "thirty"]
from_fhash = FencedHash[from_pairs]
[from_pairs, from_alist, from_hash, from_fhash, from_pairs
].each_cons(2) do |a, b|
assert_equal a, b
assert_not_same a, b
end
end
def test_default_proc_assign
assert_nil @fh.default_proc
p = @fh.default_proc = proc { |h, k| h[k] = :deleted }
assert_same p, @fh.default_proc
assert_equal 11, @fh.delete(:q)
assert_equal :deleted, @fh[:q]
assert_raises(KeyEx) { @fh[:u] }
@fh.default = :value
assert_nil @fh.default_proc
@fh.default_proc = p
assert_nil @fh.default
end
def test_each_rewind
class << @fh
attr_reader :test_rewind
def rewind
@test_rewind = "correct"
end
end
assert_nil @fh.test_rewind
# @fh.each.rewind must call @fh.rewind. If @fh forwards :each
# to another object then this test fails.
@fh.each.rewind
assert_equal "correct", @fh.test_rewind
end
def test_insertion_order
assert_equal [:q, :w, :e, :r, :t, :y], @fh.keys
assert_equal [11, 22, 33, 44, 55, 66], @fh.values
end
def test_key
assert_equal :q, @fh.key(11)
assert_equal :w, @fh.key(22)
assert_equal :e, @fh.key(33)
assert_equal :r, @fh.key(44)
assert_equal :t, @fh.key(55)
assert_equal :y, @fh.key(66)
assert_nil @fh.key(77)
end
end
end
