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{{draft task}}
 
Create a Hash/Associative Array/Dictionary-like object that can be initialized with key/value pairs. The object should behave like a native Hash/Associative Array/Dictionary of the language, but with the following differences:
'''Task:'''
 
* No new keys can be added;
Create a Hash/Associative Array/Dictionary-like object, which is initialized with some default key/value pairs. The object should behave like a native Hash/Associative Array/Dictionary of the language, if any, but with the following differences:
* Keys cannot be removed;
* Attempting to delete a key should set that keys value back to that used during initialisation.
(The value assigned to keys may be changed by normal assignment however).
 
If the language supports '''Magic Methods''', then show how these work.
# No new item can be added;
# Item cannot be deleted, (but native delete method may used to reset the item's value to default) ;
 
=={{header|BASIC}}==
'''Objective:'''
==={{header|BASIC256}}===
<syntaxhighlight lang="basic256">map mapa
mapa["A"] = 65
mapa["B"] = 66
mapa["C"] = 67
 
foreach valor in mapa
The objective is not just create such object, but to demonstarion the language's native way of object creation. For some language, the task should show how the so-called '''Magic Methods''' work.
print valor
print mapa[valor]
next valor</syntaxhighlight>
{{out}}
<pre>A
65
B
66
C
67</pre>
 
==={{header|FreeBASIC}}===
FB doesn't have Dict natively, but we can implement them via Type
<syntaxhighlight lang="freebasic">Type dict
m1 As String*1
m2 As Integer
End Type
 
Dim mapOf(1 To 3) As dict => {("A", 65), ("B", 66), ("C", 67)}
 
For i As Integer = 1 To Ubound(mapOf)
Print mapOf(i).m1
Print mapOf(i).m2
Next i</syntaxhighlight>
{{out}}
<pre>A
65
B
66
C
67</pre>
 
=={{header|C++}}==
<syntaxhighlight lang="cpp">#include <iostream>
#include <map>
#include <utility>
 
using namespace std;
 
template<typename T>
class FixedMap : private T
{
// Two standard maps are used to implement FixedMap. One as a private
// base class which will allow the values (but not the keys) to be modified.
// Members of a private base class are not exposed to the derived class which will
// prevent keys from being added or deleted. Another map will hold copies of
// the initial values.
T m_defaultValues;
public:
FixedMap(T map)
: T(map), m_defaultValues(move(map)){}
// Expose members of the base class that do not modify the map.
using T::cbegin;
using T::cend;
using T::empty;
using T::find;
using T::size;
 
// Also expose members that can modify values but not add or remove keys.
using T::at;
using T::begin;
using T::end;
// The [] operator will normally add a new key if the key is not already in the
// map. Instead, throw an error if the key is missing.
auto& operator[](typename T::key_type&& key)
{
// Make it behave like at()
return this->at(forward<typename T::key_type>(key));
}
// Instead of removing a key, change the sematics of erase() to restore
// the original value of the key.
void erase(typename T::key_type&& key)
{
T::operator[](key) = m_defaultValues.at(key);
}
 
// Also change the sematics of clear() to restore all keys
void clear()
{
// Reset the base class using the defaults
T::operator=(m_defaultValues);
}
};
 
// Print the contents of a map
auto PrintMap = [](const auto &map)
{
for(auto &[key, value] : map)
{
cout << "{" << key << " : " << value << "} ";
}
cout << "\n\n";
};
 
int main(void)
{
// Create a fixed map based on the standard map
cout << "Map intialized with values\n";
FixedMap<map<string, int>> fixedMap ({
{"a", 1},
{"b", 2}});
PrintMap(fixedMap);
cout << "Change the values of the keys\n";
fixedMap["a"] = 55;
fixedMap["b"] = 56;
PrintMap(fixedMap);
cout << "Reset the 'a' key\n";
fixedMap.erase("a");
PrintMap(fixedMap);
cout << "Change the values the again\n";
fixedMap["a"] = 88;
fixedMap["b"] = 99;
PrintMap(fixedMap);
cout << "Reset all keys\n";
fixedMap.clear();
PrintMap(fixedMap);
try
{
// Adding or retrieving a missing key is a run time error
cout << "Try to add a new key\n";
fixedMap["newKey"] = 99;
}
catch (exception &ex)
{
cout << "error: " << ex.what();
}
}
</syntaxhighlight>
{{out}}
<pre>Map intialized with values
{a : 1} {b : 2}
 
Change the values of the keys
{a : 55} {b : 56}
 
Reset the 'a' key
{a : 1} {b : 56}
 
Change the values the again
{a : 88} {b : 99}
 
Reset all keys
{a : 1} {b : 2}
 
Try to add a new key
error: map::at</pre>
 
=={{header|D}}==
{{trans|Python}}
<syntaxhighlight lang="d">struct DefaultAA(TK, TV) {
TV[TK] standard, current;
 
this(TV[TK] default_) pure /*nothrow*/ @safe {
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*/ @safe {
current = standard.dup;
}
}
 
void main() {
import std.stdio;
auto d = ["a": 1, "b": 2].DefaultAA!(string, int);
 
d.writeln; // ["a":1, "b":2]
d["a"] = 55; d["b"] = 66;
d.writeln; // ["a":55, "b":66]
d.clear;
d.writeln; // ["a":1, "b":2]
d["a"] = 55; d["b"] = 66;
d["a"].writeln; // 55
d.remove("a");
d.writeln; // ["a":1, "b":66]
}</syntaxhighlight>
{{out}}
<pre>["a":1, "b":2]
["a":55, "b":66]
["a":1, "b":2]
55
["a":1, "b":66]</pre>
 
=={{header|Go}}==
Go's built-in map type is mutable and so, to complete this task, we need to create a read-only wrapper for it which doesn't permit further items to be added or existing items to be deleted though does allow them to be reset to their default value.
 
First create a sub-directory, romap, of the project directory and place the following package in it:
<syntaxhighlight lang="go">package romap
 
type Romap struct{ imap map[byte]int }
 
// Create new read-only wrapper for the given map.
func New(m map[byte]int) *Romap {
if m == nil {
return nil
}
return &Romap{m}
}
 
// Retrieve value for a given key, if it exists.
func (rom *Romap) Get(key byte) (int, bool) {
i, ok := rom.imap[key]
return i, ok
}
 
// Reset value for a given key, if it exists.
func (rom *Romap) Reset(key byte) {
_, ok := rom.imap[key]
if ok {
rom.imap[key] = 0 // default value of int
}
}</syntaxhighlight>
 
This package can now be imported and used within the main package as follows:
<syntaxhighlight lang="go">package main
 
import (
"./romap"
"fmt"
)
 
func main() {
// create a normal map
m := map[byte]int{'A': 65, 'B': 66, 'C': 67}
 
// place it in a read-only wrapper so no new item can be added or item deleted.
rom := romap.New(m)
 
// retrieve value represented by 'C' say
i, _ := rom.Get('C')
fmt.Println("'C' maps to", i)
 
// reset this to default value (doesn't actually delete the key)
rom.Reset('C')
i, _ = rom.Get('C')
fmt.Println("'C' now maps to", i)
}</syntaxhighlight>
 
{{out}}
<pre>
'C' maps to 67
'C' now maps to 0
</pre>
 
=={{header|J}}==
Line 16 ⟶ 282:
Given a list of keys and an associated list of values, the idiomatic way of expressing this concept in J would be:
 
<langsyntaxhighlight lang="j">lookup=: values {~ keys&i.</langsyntaxhighlight>
 
For example:
 
<langsyntaxhighlight lang="j"> lookup=: 10 20 30 40 50 {~ (;:'this is a test')&i.
lookup ;:'a test'
30 40</langsyntaxhighlight>
 
Notes:
Line 31 ⟶ 297:
 
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.
 
=={{header|Java}}==
 
Java supports unmodifiable maps, sets, lists, and other more specialized unmodifiable collections. In this example, we have a unmodifiable map. We first create an ordinary map, modify as needed, then call the <code>Collections.unmodifiableMap</code>. We can subsequently read the map, but modification is not permitted. The returned map will subsequently throw a <code>UnsupportedOperationException</code> exception if a mutation operator is called. Several are demonstrated below.
 
<syntaxhighlight lang="java">
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
 
// Title: Create an object/Native demonstration
 
public class ImmutableMap {
 
public static void main(String[] args) {
Map<String,Integer> hashMap = getImmutableMap();
try {
hashMap.put("Test", 23);
}
catch (UnsupportedOperationException e) {
System.out.println("ERROR: Unable to put new value.");
}
try {
hashMap.clear();
}
catch (UnsupportedOperationException e) {
System.out.println("ERROR: Unable to clear map.");
}
try {
hashMap.putIfAbsent("Test", 23);
}
catch (UnsupportedOperationException e) {
System.out.println("ERROR: Unable to put if absent.");
}
for ( String key : hashMap.keySet() ) {
System.out.printf("key = %s, value = %s%n", key, hashMap.get(key));
}
}
private static Map<String,Integer> getImmutableMap() {
Map<String,Integer> hashMap = new HashMap<>();
hashMap.put("Key 1", 34);
hashMap.put("Key 2", 105);
hashMap.put("Key 3", 144);
 
return Collections.unmodifiableMap(hashMap);
}
}
</syntaxhighlight>
 
{out}}
<pre>
ERROR: Unable to put new value.
ERROR: Unable to clear map.
ERROR: Unable to put if absent.
key = Key 1, value = 34
key = Key 2, value = 105
key = Key 3, value = 144
</pre>
 
=={{header|JavaScript}}==
Line 37 ⟶ 364:
{{works with|JavaScript|1.7}}
 
<langsyntaxhighlight lang="javascript">var keyError = new Error("Invalid Key Error (FixedKeyDict)") ;
 
function FixedKeyDict(obj)
Line 99 ⟶ 426:
return "FixedKeyDict{" + s + "}" ;
} ;
}</langsyntaxhighlight>
 
Test run:
 
<langsyntaxhighlight lang="javascript">
const BR = "<BR>\n"
 
Line 137 ⟶ 464:
pl("error test : " + e.message) ;
}
</syntaxhighlight>
</lang>
 
output :
Line 159 ⟶ 486:
</pre>
 
=={{header|Rubyjq}}==
jq objects are JSON objects and can be created using JSON syntax, e.g. <syntaxhighlight lang="jq">{"language": "jq"}</syntaxhighlight>
{{in progress|lang=Ruby|day=15|month=February|year=2011}}
Objects can also be created programmatically, e.g. <syntaxhighlight lang="jq">{"one": 1} + {"two": 2}</syntaxhighlight>
 
jq objects, however, are really just values: they are immutable, and cannot be "deleted" any more than the number 1 can be deleted.
TODO: Inherit from Hash (class FencedHash < Hash). Add missing methods. Add test case.
 
=={{header|Julia}}==
<lang ruby>require 'forwardable'
<syntaxhighlight lang="julia">
using BackedUpImmutable
 
function testBackedUpImmutableDict()
# A FencedHash acts like a Hash, but with a fence around its keys.
fibr = BackedUpImmutableDict{String,Int64}(["a" => 0, "b" => 1, "c" => 1, "d" => 2,
# After the creation of a FencedHash, one cannot add nor remove keys.
"e" => 3, "f" => 5, "g" => 8, "h" => 13, "i" => 21, "j" => 34, "extra" => -1])
# Any attempt to insert a new key will raise KeyError. Any attempt to
 
# delete a key-value pair will keep the key but revert the value to
x = fibr["extra"]
# the default value.
@test x == -1
fibr["extra"] = 0
y = fibr["extra"]
@test y == 0
restore!(fibr, "extra")
z = fibr["extra"]
@test z == -1
@test_throws String begin fibr["k"] = 55 end
fibr["a"] = 9
fibr["b"] = 7
# test restore all to default
restoreall!(fibr)
@test fibr["a"] == 0
end
</syntaxhighlight>
All tests pass.
 
=={{header|Kotlin}}==
<syntaxhighlight lang="scala">// version 1.1.2
 
fun main(args: Array<String>) {
// This line creates a read-only map which cannot be changed in any way nor cleared
val map = mapOf('A' to 65, 'B' to 66, 'C' to 67)
println(map)
}</syntaxhighlight>
 
{{out}}
<pre>
{A=65, B=66, C=67}
</pre>
 
=={{header|M2000 Interpreter}}==
{{trans|C sharp}}
<syntaxhighlight lang="m2000 interpreter">
Module CheckIt {
Class LockedHash {
Private:
inventory Vars ' no same keys
unlock
module nosuchvariable {
Error "No such value:"+letter$
}
module NoNewItem {
Error "No new item, use unlock method before"
}
module NoRemoveItem {
Error "Can't remove item, use unlock method before"
}
Public:
module Unlock {
.unlock<=True
}
module Writeln {
m=each(.Vars)
while m {
Print Quote$(Eval$(m, m^));",";Eval(m),
}
Print
}
Value (st$){
st$=Ucase$(st$)
if exist(.Vars, st$) then =Eval(.Vars) : Exit
.nosuchvariable st$
}
Set (st$){
st$=Ucase$(st$)
Read val
if exist(.Vars, st$) then Return .Vars, st$:=val : Exit
If .unlock then { Append .Vars, st$:=val} Else .NoNewItem
}
module Remove (st$) {
if not .unlock then .NoRemoveItem
st$=Ucase$(st$)
Try {
delete .Vars, st$
}
}
module Clear {
Clear .Vars
}
Class: ' this part exist only at construction
module LockedHash {
While match("SN") {
read st$, val
st$=ucase$(st$)
\\ if we append key which exist we get error
Append .Vars, st$:=val
}
}
}
d=LockedHash("a", 1, "b", 2)
d.writeln
d("a")=55 : d("b")=66
d.writeln
d.clear
d.writeln
d.unlock
d("a")=55 : d("b")=66
Print d("a")=55, d("a")/d("b")<1
d.remove "a"
d.writeln
}
Checkit
</syntaxhighlight>
 
=={{header|Mathematica}} / {{header|Wolfram Language}}==
<syntaxhighlight lang="mathematica">a[1] = "Do not modify after creation";
a[2] = "Native demonstration";
Protect[a];</syntaxhighlight>
Example usage:
<pre>a[3] = 2
->Set::write: Tag a in a[1] is Protected. >>
</pre>
 
=={{header|Nim}}==
We leverage native stdlib table as our own object by implementing limited actual native table functionalities.
<syntaxhighlight lang="nim">import tables, options
 
type
MyTable = object
table: TableRef[string, int]
 
# return empty if the key is not available
proc `[]`(m: MyTable, key: string): Option[int] =
if key in m.table: result = some m.table[key]
else: result = none int
 
# update an item, doing nothing if the key is available during first initialization
proc `[]=`(m: var MyTable, key: string, val: int) =
if key notin m.table: return
m.table[key] = val
 
proc reset(m: var MyTable) =
for _, v in m.table.mpairs: v = 0
 
# sugar for defining MyTable object
proc toTable(vals: openarray[(string, int)]): MyTable =
result.table = newTable vals
 
proc main =
# MyTable construction
var myobj = {"key1": 1, "key2": 2, "key3": 3}.toTable
# test getting existing key
let val1 = myobj["key1"]
if val1.isSome: echo "val1: ", val1.get
 
# test adding new key
myobj["key4"] = 4
let val4 = myobj["key4"]
if val4.isSome: echo val4.get
else: echo "val4 is empty"
 
# test reset and test whether its value is zero-ed
reset myobj
doAssert myobj["key3"].get == 0
 
main()
</syntaxhighlight>
{{output}}
<pre>
val1: 1
val4 is empty</pre>
 
=={{header|Perl}}==
<syntaxhighlight lang="perl">use strict;
 
package LockedHash;
use parent 'Tie::Hash';
use Carp;
 
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 :prototype(\%) {
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: $@" }</syntaxhighlight>output:<syntaxhighlight lang="text">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</syntaxhighlight>
 
=={{header|Phix}}==
There is no native "read-only" setting on phix dictionaries, so the following wraps a pair of them to
provide the requested functionality.
<!--<syntaxhighlight lang="phix">(phixonline)-->
<span style="color: #008080;">with</span> <span style="color: #008080;">javascript_semantics</span>
<span style="color: #008080;">enum</span> <span style="color: #000000;">STD</span><span style="color: #0000FF;">,</span> <span style="color: #000000;">CUR</span>
<span style="color: #004080;">sequence</span> <span style="color: #000000;">fkds</span> <span style="color: #0000FF;">=</span> <span style="color: #0000FF;">{}</span> <span style="color: #000080;font-style:italic;">-- fixed key dictionaries ;-)</span>
<span style="color: #004080;">integer</span> <span style="color: #000000;">freelist</span> <span style="color: #0000FF;">=</span> <span style="color: #000000;">0</span>
<span style="color: #008080;">procedure</span> <span style="color: #000000;">fkd_destroy</span><span style="color: #0000FF;">(</span><span style="color: #004080;">integer</span> <span style="color: #000000;">id</span><span style="color: #0000FF;">)</span>
<span style="color: #004080;">integer</span> <span style="color: #0000FF;">{</span><span style="color: #000000;">std</span><span style="color: #0000FF;">,</span><span style="color: #000000;">cur</span><span style="color: #0000FF;">}</span> <span style="color: #0000FF;">=</span> <span style="color: #000000;">fkds</span><span style="color: #0000FF;">[</span><span style="color: #000000;">id</span><span style="color: #0000FF;">]</span>
<span style="color: #7060A8;">destroy_dict</span><span style="color: #0000FF;">(</span><span style="color: #000000;">std</span><span style="color: #0000FF;">)</span>
<span style="color: #7060A8;">destroy_dict</span><span style="color: #0000FF;">(</span><span style="color: #000000;">cur</span><span style="color: #0000FF;">)</span>
<span style="color: #000000;">fkds</span><span style="color: #0000FF;">[</span><span style="color: #000000;">id</span><span style="color: #0000FF;">]</span> <span style="color: #0000FF;">=</span> <span style="color: #000000;">freelist</span>
<span style="color: #000000;">freelist</span> <span style="color: #0000FF;">=</span> <span style="color: #000000;">id</span>
<span style="color: #008080;">end</span> <span style="color: #008080;">procedure</span>
<span style="color: #008080;">function</span> <span style="color: #000000;">fkd_new</span><span style="color: #0000FF;">(</span><span style="color: #004080;">sequence</span> <span style="color: #000000;">key_pairs</span><span style="color: #0000FF;">)</span>
<span style="color: #004080;">integer</span> <span style="color: #000000;">std</span> <span style="color: #0000FF;">=</span> <span style="color: #7060A8;">new_dict</span><span style="color: #0000FF;">(</span><span style="color: #000000;">key_pairs</span><span style="color: #0000FF;">),</span>
<span style="color: #000000;">cur</span> <span style="color: #0000FF;">=</span> <span style="color: #7060A8;">new_dict</span><span style="color: #0000FF;">(</span><span style="color: #000000;">std</span><span style="color: #0000FF;">),</span>
<span style="color: #000000;">id</span> <span style="color: #0000FF;">=</span> <span style="color: #000000;">freelist</span>
<span style="color: #008080;">if</span> <span style="color: #000000;">id</span><span style="color: #0000FF;">=</span><span style="color: #000000;">0</span> <span style="color: #008080;">then</span>
<span style="color: #000000;">fkds</span> <span style="color: #0000FF;">=</span> <span style="color: #7060A8;">append</span><span style="color: #0000FF;">(</span><span style="color: #000000;">fkds</span><span style="color: #0000FF;">,{</span><span style="color: #000000;">std</span><span style="color: #0000FF;">,</span><span style="color: #000000;">cur</span><span style="color: #0000FF;">})</span>
<span style="color: #000000;">id</span> <span style="color: #0000FF;">=</span> <span style="color: #7060A8;">length</span><span style="color: #0000FF;">(</span><span style="color: #000000;">fkds</span><span style="color: #0000FF;">)</span>
<span style="color: #008080;">else</span>
<span style="color: #000000;">freelist</span> <span style="color: #0000FF;">=</span> <span style="color: #000000;">fkds</span><span style="color: #0000FF;">[</span><span style="color: #000000;">id</span><span style="color: #0000FF;">]</span>
<span style="color: #000000;">fkds</span><span style="color: #0000FF;">[</span><span style="color: #000000;">id</span><span style="color: #0000FF;">]</span> <span style="color: #0000FF;">=</span> <span style="color: #0000FF;">{</span><span style="color: #000000;">std</span><span style="color: #0000FF;">,</span><span style="color: #000000;">cur</span><span style="color: #0000FF;">}</span>
<span style="color: #008080;">end</span> <span style="color: #008080;">if</span>
<span style="color: #008080;">return</span> <span style="color: #000000;">id</span>
<span style="color: #008080;">end</span> <span style="color: #008080;">function</span>
<span style="color: #008080;">procedure</span> <span style="color: #000000;">fkd_clear</span><span style="color: #0000FF;">(</span><span style="color: #004080;">integer</span> <span style="color: #000000;">id</span><span style="color: #0000FF;">)</span>
<span style="color: #004080;">integer</span> <span style="color: #0000FF;">{</span><span style="color: #000000;">std</span><span style="color: #0000FF;">,</span><span style="color: #000000;">cur</span><span style="color: #0000FF;">}</span> <span style="color: #0000FF;">=</span> <span style="color: #000000;">fkds</span><span style="color: #0000FF;">[</span><span style="color: #000000;">id</span><span style="color: #0000FF;">]</span>
<span style="color: #7060A8;">destroy_dict</span><span style="color: #0000FF;">(</span><span style="color: #000000;">cur</span><span style="color: #0000FF;">)</span>
<span style="color: #000000;">fkds</span><span style="color: #0000FF;">[</span><span style="color: #000000;">id</span><span style="color: #0000FF;">][</span><span style="color: #000000;">CUR</span><span style="color: #0000FF;">]</span> <span style="color: #0000FF;">=</span> <span style="color: #7060A8;">new_dict</span><span style="color: #0000FF;">(</span><span style="color: #000000;">std</span><span style="color: #0000FF;">)</span>
<span style="color: #008080;">end</span> <span style="color: #008080;">procedure</span>
<span style="color: #008080;">function</span> <span style="color: #000000;">fkd_get</span><span style="color: #0000FF;">(</span><span style="color: #004080;">integer</span> <span style="color: #000000;">id</span><span style="color: #0000FF;">,</span> <span style="color: #004080;">object</span> <span style="color: #000000;">key</span><span style="color: #0000FF;">)</span>
<span style="color: #008080;">return</span> <span style="color: #7060A8;">getd</span><span style="color: #0000FF;">(</span><span style="color: #000000;">key</span><span style="color: #0000FF;">,</span><span style="color: #000000;">fkds</span><span style="color: #0000FF;">[</span><span style="color: #000000;">id</span><span style="color: #0000FF;">][</span><span style="color: #000000;">CUR</span><span style="color: #0000FF;">])</span>
<span style="color: #008080;">end</span> <span style="color: #008080;">function</span>
<span style="color: #008080;">procedure</span> <span style="color: #000000;">fkd_set</span><span style="color: #0000FF;">(</span><span style="color: #004080;">integer</span> <span style="color: #000000;">id</span><span style="color: #0000FF;">,</span> <span style="color: #004080;">object</span> <span style="color: #000000;">key</span><span style="color: #0000FF;">,</span> <span style="color: #000000;">data</span><span style="color: #0000FF;">)</span>
<span style="color: #004080;">integer</span> <span style="color: #000000;">node</span> <span style="color: #0000FF;">=</span> <span style="color: #7060A8;">getd_index</span><span style="color: #0000FF;">(</span><span style="color: #000000;">key</span><span style="color: #0000FF;">,</span><span style="color: #000000;">fkds</span><span style="color: #0000FF;">[</span><span style="color: #000000;">id</span><span style="color: #0000FF;">][</span><span style="color: #000000;">CUR</span><span style="color: #0000FF;">])</span>
<span style="color: #008080;">if</span> <span style="color: #000000;">node</span><span style="color: #0000FF;">=</span><span style="color: #004600;">NULL</span> <span style="color: #008080;">then</span> <span style="color: #008080;">throw</span><span style="color: #0000FF;">(</span><span style="color: #008000;">"invalid/new key"</span><span style="color: #0000FF;">)</span> <span style="color: #008080;">end</span> <span style="color: #008080;">if</span>
<span style="color: #7060A8;">setd</span><span style="color: #0000FF;">(</span><span style="color: #000000;">key</span><span style="color: #0000FF;">,</span><span style="color: #000000;">data</span><span style="color: #0000FF;">,</span><span style="color: #000000;">fkds</span><span style="color: #0000FF;">[</span><span style="color: #000000;">id</span><span style="color: #0000FF;">][</span><span style="color: #000000;">CUR</span><span style="color: #0000FF;">])</span>
<span style="color: #008080;">end</span> <span style="color: #008080;">procedure</span>
<span style="color: #008080;">procedure</span> <span style="color: #000000;">fkd_remove</span><span style="color: #0000FF;">(</span><span style="color: #004080;">integer</span> <span style="color: #000000;">id</span><span style="color: #0000FF;">,</span> <span style="color: #004080;">object</span> <span style="color: #000000;">key</span><span style="color: #0000FF;">)</span>
<span style="color: #004080;">integer</span> <span style="color: #0000FF;">{</span><span style="color: #000000;">std</span><span style="color: #0000FF;">,</span><span style="color: #000000;">cur</span><span style="color: #0000FF;">}</span> <span style="color: #0000FF;">=</span> <span style="color: #000000;">fkds</span><span style="color: #0000FF;">[</span><span style="color: #000000;">id</span><span style="color: #0000FF;">],</span>
<span style="color: #000000;">node</span> <span style="color: #0000FF;">=</span> <span style="color: #7060A8;">getd_index</span><span style="color: #0000FF;">(</span><span style="color: #000000;">key</span><span style="color: #0000FF;">,</span><span style="color: #000000;">std</span><span style="color: #0000FF;">)</span>
<span style="color: #008080;">if</span> <span style="color: #000000;">node</span><span style="color: #0000FF;">=</span><span style="color: #004600;">NULL</span> <span style="color: #008080;">then</span> <span style="color: #008080;">throw</span><span style="color: #0000FF;">(</span><span style="color: #008000;">"invalid key"</span><span style="color: #0000FF;">)</span> <span style="color: #008080;">end</span> <span style="color: #008080;">if</span>
<span style="color: #7060A8;">setd</span><span style="color: #0000FF;">(</span><span style="color: #000000;">key</span><span style="color: #0000FF;">,</span><span style="color: #7060A8;">getd_by_index</span><span style="color: #0000FF;">(</span><span style="color: #000000;">node</span><span style="color: #0000FF;">,</span><span style="color: #000000;">std</span><span style="color: #0000FF;">),</span><span style="color: #000000;">cur</span><span style="color: #0000FF;">)</span>
<span style="color: #008080;">end</span> <span style="color: #008080;">procedure</span>
<span style="color: #008080;">function</span> <span style="color: #000000;">fkd_sprint</span><span style="color: #0000FF;">(</span><span style="color: #004080;">integer</span> <span style="color: #000000;">id</span><span style="color: #0000FF;">)</span>
<span style="color: #004080;">integer</span> <span style="color: #000000;">cur</span> <span style="color: #0000FF;">=</span> <span style="color: #000000;">fkds</span><span style="color: #0000FF;">[</span><span style="color: #000000;">id</span><span style="color: #0000FF;">][</span><span style="color: #000000;">CUR</span><span style="color: #0000FF;">]</span>
<span style="color: #004080;">sequence</span> <span style="color: #000000;">res</span> <span style="color: #0000FF;">=</span> <span style="color: #7060A8;">getd_all_keys</span><span style="color: #0000FF;">(</span><span style="color: #000000;">cur</span><span style="color: #0000FF;">)</span>
<span style="color: #008080;">for</span> <span style="color: #000000;">i</span><span style="color: #0000FF;">=</span><span style="color: #000000;">1</span> <span style="color: #008080;">to</span> <span style="color: #7060A8;">length</span><span style="color: #0000FF;">(</span><span style="color: #000000;">res</span><span style="color: #0000FF;">)</span> <span style="color: #008080;">do</span>
<span style="color: #004080;">object</span> <span style="color: #000000;">ri</span> <span style="color: #0000FF;">=</span> <span style="color: #000000;">res</span><span style="color: #0000FF;">[</span><span style="color: #000000;">i</span><span style="color: #0000FF;">]</span>
<span style="color: #000000;">res</span><span style="color: #0000FF;">[</span><span style="color: #000000;">i</span><span style="color: #0000FF;">]</span> <span style="color: #0000FF;">=</span> <span style="color: #0000FF;">{</span><span style="color: #000000;">ri</span><span style="color: #0000FF;">,</span><span style="color: #7060A8;">getd</span><span style="color: #0000FF;">(</span><span style="color: #000000;">ri</span><span style="color: #0000FF;">,</span><span style="color: #000000;">cur</span><span style="color: #0000FF;">)}</span>
<span style="color: #008080;">end</span> <span style="color: #008080;">for</span>
<span style="color: #008080;">return</span> <span style="color: #000000;">res</span>
<span style="color: #008080;">end</span> <span style="color: #008080;">function</span>
<span style="color: #008080;">procedure</span> <span style="color: #000000;">main</span><span style="color: #0000FF;">()</span>
<span style="color: #004080;">integer</span> <span style="color: #000000;">id</span> <span style="color: #0000FF;">=</span> <span style="color: #000000;">fkd_new</span><span style="color: #0000FF;">({{</span><span style="color: #008000;">"a"</span><span style="color: #0000FF;">,</span><span style="color: #000000;">1</span><span style="color: #0000FF;">},{</span><span style="color: #008000;">"b"</span><span style="color: #0000FF;">,</span><span style="color: #000000;">2</span><span style="color: #0000FF;">}})</span>
<span style="color: #0000FF;">?</span><span style="color: #000000;">fkd_sprint</span><span style="color: #0000FF;">(</span><span style="color: #000000;">id</span><span style="color: #0000FF;">)</span> <span style="color: #000080;font-style:italic;">-- {{"a",1},{"b",2}}</span>
<span style="color: #000000;">fkd_set</span><span style="color: #0000FF;">(</span><span style="color: #000000;">id</span><span style="color: #0000FF;">,</span><span style="color: #008000;">"a"</span><span style="color: #0000FF;">,</span><span style="color: #000000;">55</span><span style="color: #0000FF;">)</span>
<span style="color: #000000;">fkd_set</span><span style="color: #0000FF;">(</span><span style="color: #000000;">id</span><span style="color: #0000FF;">,</span><span style="color: #008000;">"b"</span><span style="color: #0000FF;">,</span><span style="color: #000000;">66</span><span style="color: #0000FF;">)</span>
<span style="color: #0000FF;">?</span><span style="color: #000000;">fkd_sprint</span><span style="color: #0000FF;">(</span><span style="color: #000000;">id</span><span style="color: #0000FF;">)</span> <span style="color: #000080;font-style:italic;">-- {{"a",55},{"b",66}}</span>
<span style="color: #000000;">fkd_clear</span><span style="color: #0000FF;">(</span><span style="color: #000000;">id</span><span style="color: #0000FF;">)</span>
<span style="color: #0000FF;">?</span><span style="color: #000000;">fkd_sprint</span><span style="color: #0000FF;">(</span><span style="color: #000000;">id</span><span style="color: #0000FF;">)</span> <span style="color: #000080;font-style:italic;">-- {{"a",1},{"b",2}}</span>
<span style="color: #000000;">fkd_set</span><span style="color: #0000FF;">(</span><span style="color: #000000;">id</span><span style="color: #0000FF;">,</span><span style="color: #008000;">"a"</span><span style="color: #0000FF;">,</span><span style="color: #000000;">55</span><span style="color: #0000FF;">)</span>
<span style="color: #000000;">fkd_set</span><span style="color: #0000FF;">(</span><span style="color: #000000;">id</span><span style="color: #0000FF;">,</span><span style="color: #008000;">"b"</span><span style="color: #0000FF;">,</span><span style="color: #000000;">66</span><span style="color: #0000FF;">)</span>
<span style="color: #0000FF;">?</span><span style="color: #000000;">fkd_get</span><span style="color: #0000FF;">(</span><span style="color: #000000;">id</span><span style="color: #0000FF;">,</span><span style="color: #008000;">"a"</span><span style="color: #0000FF;">)</span> <span style="color: #000080;font-style:italic;">-- 55</span>
<span style="color: #000000;">fkd_remove</span><span style="color: #0000FF;">(</span><span style="color: #000000;">id</span><span style="color: #0000FF;">,</span><span style="color: #008000;">"a"</span><span style="color: #0000FF;">)</span>
<span style="color: #008080;">try</span>
<span style="color: #000000;">fkd_set</span><span style="color: #0000FF;">(</span><span style="color: #000000;">id</span><span style="color: #0000FF;">,</span><span style="color: #008000;">"NoNewKey"</span><span style="color: #0000FF;">,</span><span style="color: #000000;">77</span><span style="color: #0000FF;">)</span>
<span style="color: #008080;">catch</span> <span style="color: #000000;">e</span>
<span style="color: #0000FF;">?</span><span style="color: #000000;">e</span><span style="color: #0000FF;">[</span><span style="color: #004600;">E_USER</span><span style="color: #0000FF;">]</span> <span style="color: #000080;font-style:italic;">-- "invalid/new key"</span>
<span style="color: #008080;">end</span> <span style="color: #008080;">try</span>
<span style="color: #0000FF;">?</span><span style="color: #000000;">fkd_sprint</span><span style="color: #0000FF;">(</span><span style="color: #000000;">id</span><span style="color: #0000FF;">)</span> <span style="color: #000080;font-style:italic;">-- {{"a",1},{"b",66}}</span>
<span style="color: #000000;">fkd_destroy</span><span style="color: #0000FF;">(</span><span style="color: #000000;">id</span><span style="color: #0000FF;">)</span>
<span style="color: #008080;">end</span> <span style="color: #008080;">procedure</span>
<span style="color: #000000;">main</span><span style="color: #0000FF;">()</span>
<!--</syntaxhighlight>-->
 
=={{header|Python}}==
<syntaxhighlight lang="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)</syntaxhighlight>
 
=={{header|Racket}}==
{{trans|D}}
This task is implemented as a new fenced-hash time with an interface similar to the native hash. Also it can be used a native dict.
 
Implementation of functions that handle fenced-hash:
<syntaxhighlight lang="racket">
;(struct fenced-hash (actual original) ...)
 
(define (fenced-hash-ref dict
key
[default (lambda () (error "key not found" key))])
(hash-ref (fenced-hash-actual dict) key default))
(define (fenced-hash-set! dict key val)
(unless (hash-has-key? (fenced-hash-actual dict) key)
(error "unable to add key" key))
(hash-set! (fenced-hash-actual dict) key val))
(define (fenced-hash-remove! dict key) ;reset the value!
(unless (hash-has-key? (fenced-hash-actual dict) key)
(error "key not found" key))
(hash-set! (fenced-hash-actual dict)
key
(hash-ref (fenced-hash-original dict) key)))
(define (fenced-hash-clear! dict) ;reset all values!
(hash-for-each (fenced-hash-original dict)
(lambda (key val) (hash-set! (fenced-hash-actual dict) key val))))
 
(define (fenced-hash-has-key? dict key)
(hash-has-key? (fenced-hash-actual dict) key))
(define (fenced-hash-count dict)
(hash-count (fenced-hash-actual dict)))
 
(define (fenced-hash-iterate-first dict)
(hash-iterate-first (fenced-hash-actual dict)))
(define (fenced-hash-iterate-next dict pos)
(hash-iterate-next (fenced-hash-actual dict) pos))
(define (fenced-hash-iterate-key dict pos)
(hash-iterate-key (fenced-hash-actual dict) pos))
(define (fenced-hash-iterate-value dict pos)
(hash-iterate-value (fenced-hash-actual dict) pos))
 
(define (*fenced-hash-print dict port mode)
;private custom-write ;mode is ignored
(write-string "#fenced-hash" port)
(write (hash->list (fenced-hash-actual dict)) port))</syntaxhighlight>
 
Definition of the actual structure and a “public” creator:
<syntaxhighlight lang="racket">(struct fenced-hash (actual original)
#:extra-constructor-name *fenced-hash ;private constructor
#:omit-define-syntaxes ;not sure this is a good idea
#:methods gen:custom-write
[(define write-proc *fenced-hash-print)]
 
#:methods gen:dict
[(define dict-ref fenced-hash-ref)
(define dict-set! fenced-hash-set!)
(define dict-remove! fenced-hash-remove!)
(define dict-has-key? fenced-hash-has-key?) ;unused in 5.6.3
(define dict-count fenced-hash-count)
(define dict-iterate-first fenced-hash-iterate-first)
(define dict-iterate-next fenced-hash-iterate-next)
(define dict-iterate-key fenced-hash-iterate-key)
(define dict-iterate-value fenced-hash-iterate-value)])
 
 
(define (fenced-hash . args) ; public constructor
(define original (apply hash args))
(*fenced-hash (hash-copy original) original))</syntaxhighlight>
 
'''Example:''' Use the fenced-hash functions:
<syntaxhighlight lang="racket">(define d (fenced-hash "a" 1 "b" 2))
 
(displayln d)
(fenced-hash-set! d "a" 55)
(fenced-hash-set! d "b" 66)
(displayln d)
(fenced-hash-clear! d)
(displayln d)
(fenced-hash-set! d "a" 55)
(fenced-hash-set! d "b" 66)
(displayln d)
(fenced-hash-remove! d "a")
(displayln d)</syntaxhighlight>
{{out}}
<pre>#fenced-hash(("b" . 2) ("a" . 1))
#fenced-hash(("b" . 66) ("a" . 55))
#fenced-hash(("b" . 2) ("a" . 1))
#fenced-hash(("b" . 66) ("a" . 55))
#fenced-hash(("b" . 66) ("a" . 1))</pre>
 
'''Example (continued):''' Use the same object as a dict. The dict-clear! method is not defined, so we must call fenced-hash-clear! instead.
<syntaxhighlight lang="racket">(fenced-hash-clear! d)
(displayln d)
(dict-set! d "a" 55)
(dict-set! d "b" 66)
(displayln d)
(fenced-hash-clear! d) ;dict-clear is not defined
(displayln d)
(dict-set! d "a" 55)
(dict-set! d "b" 66)
(displayln d)
(dict-remove! d "a")
(displayln d)</syntaxhighlight>
{{out}}
<pre>#fenced-hash(("b" . 2) ("a" . 1))
#fenced-hash(("b" . 66) ("a" . 55))
#fenced-hash(("b" . 2) ("a" . 1))
#fenced-hash(("b" . 66) ("a" . 55))
#fenced-hash(("b" . 66) ("a" . 1))</pre>
 
=={{header|Raku}}==
(formerly Perl 6)
{{Works with|rakudo|2016.08}}
Here we use delegation to handle all the normal hash methods that we don't need to override to define our new class.
<syntaxhighlight lang="raku" line>class FixedHash {
has $.hash handles *;
method new(*@args) { self.bless: hash => Hash.new: @args }
method AT-KEY(FixedHash:D: $key is copy) is rw {
$!hash.EXISTS-KEY($key) ?? $!hash.AT-KEY($key) !! Failure.new(q{can't store value for unknown key});
}
method DELETE-KEY($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
</syntaxhighlight>
{{out}}
<pre>(1 2)
(1 (Any))
(1 42)
can't store value for unknown key
in block <unit> at native-demonstration.p6:17
 
Actually thrown at:
in block <unit> at native-demonstration.p6:17</pre>
 
By defining [http://design.raku.org/S12.html#FALLBACK_methods FALLBACK] any class can handle undefined method calls. Since any class inherits plenty of methods from <tt>Any</tt> our magic object will be more of a novice conjurer then a master wizard proper.
 
<syntaxhighlight lang="raku" line>class Magic {
has %.hash;
multi method FALLBACK($name, |c) is rw { # this will eat any extra parameters
%.hash{$name}
}
 
multi method FALLBACK($name) is rw {
%.hash{$name}
}
}
 
my $magic = Magic.new;
$magic.foo = 10;
say $magic.foo;
$magic.defined = False; # error</syntaxhighlight>
 
{{output}}
<pre>10
Cannot modify an immutable Bool
in block <unit> at native-demonstration.p6:15</pre>
 
=={{header|Ring}}==
<syntaxhighlight lang="ring">
# Project : Create an object/Native demonstration
 
map = []
map["A"] = 65
map["B"] = 66
map["C"] = 67
see map + nl
</syntaxhighlight>
Output:
<pre>
A
65
B
66
C
67
</pre>
 
=={{header|Ruby}}==
{{works with|Ruby|1.9}}
<syntaxhighlight lang="ruby"># A FencedHash acts like a Hash, but with a fence around its keys.
# One may change its values, but not its keys. Any attempt to insert
# a new key raises KeyError. One may delete a key, but this only
# restores its original value.
#
# FencedHash reimplements these Hash methods: #[] #[]= #clear #delete
# #delete_if #default #default= #each_key #each_pair #each_value
# #fetch #has_key? #keep_if #keys #length #values #values_at
class FencedHash
extend Enumerable, Forwardable
 
# call--seq:
# FencedHash.new(hash, obj=nil) -> fh
# @hash: our Hash inside the fence
# @keys: set of all keys in the fence (really another Hash)
# @default_proc: passes self, not @hash
#
# Creates a FencedHash that takes its keys and original values from
# When x is in @keys but not in @hash, then @hash[x] gives the
# a source _hash_. The source _hash_ can be any object that
# correct default value. Allows FencedHash#default_proc= to work.
# responds to each_pair. Sets the default value for missing keys to
# _obj_, so FencedHash#[] returns _obj_ when a key is not in fence.
def initialize(hash, obj=nil)
@default = obj
@hash = {}
hash.each_pair do |key, value|
# @hash[key][0] = current value
# @hash[key][1] = original value
@hash[key] = [value, value]
end
end
 
def initialize_clone(orig)
# Object#clone calls here in Ruby 2.0. If _orig_ was frozen, then
# each array of _values_ is frozen, so make frozen clones.
super
copy = {}
@hash.each_pair {|key, values| copy[key] = values.clone }
@hash = copy
end
 
def initialize_dup(orig)
# Object#dup calls here in Ruby 2.0. If _orig_ was frozen, then
# make duplicates that are not frozen.
super
copy = {}
@hash.each_pair {|key, values| copy[key] = values.dup }
@hash = copy
end
 
# Retrieves current value for _key_, like Hash#[]. If _key_ is not
# in fence, returns default object.
def [](key)
values = @hash[key]
if values
values[0]
else
@default
end
end
 
# call-seq:
# fh[key] = value -> value
# fh.store(key, value) -> value
#
# Sets _value_ for a _key_. Returns _value. If _key_ is not in
# @keys is not an Array because Hash#include? is faster than
# fence, raises KeyError.
# Array#include?. @keys is not a Set (require 'set') because there
def []=(key, value)
# was no Set#compare_by_identity method.
values = @hash[key]
#++
if values
attr_reader :default_proc
values[0] = value
def_delegators :@hash, :[], :compare_by_identity?
else
def_delegators :@keys, :has_key?, :include?, :key?, :keys, :member?
raise KeyError, "fence prevents adding new key: #{key.inspect}"
end
end
alias store []=
 
# Resets all keys to their original values. Returns self.
# Acts like Hash::[] but creates a FencedHash.
def self.[](*args)clear
@hash.each_value {|values| values[0] = values[1]}
fh = FencedHash.allocate
self
fh.instance_eval do
end
@hash = Hash[*args]
 
@keys = {}
# Resets _key_ to its original value. Returns old value before
@hash.each_key { |key| @keys[key] = true }
# reset. If _key_ is not in fence, returns +nil+.
def delete(key)
values = @hash[key]
if values
old = values[0]
values[0] = values[1]
old # return old
end # else return nil
end
 
# call-seq:
# fh.delete_if {|key, value| block } -> fh
# fh.delete_if -> enumerator
#
# Yields each _key_ with current _value_ to _block_. Resets _key_
# to its original value when block evaluates to true.
def delete_if
if block_given?
@hash.each_pair do |key, values|
yield(key, values[0]) and values[0] = values[1]
end
self
else
enum_for(:delete_if) { @hash.size }
end
end
 
# The default value for keys not in fence.
attr_accessor :default
 
# call-seq:
# FencedHashfh.new(obj, *keys) each_key {|key| block} -> fh
# FencedHashfh.new(*keys)each_key { |fh, key| block } -> fhenumerator
#
# Yields each key in fence to the block.
# Creates a FencedHash...
def initializeeach_key(*args, &block)
if block
@hash.each_key(&block)
self
else
enum_for(:each_key) { @hash.size }
end
end
 
# call-seq:
# fh.each_pair {|key, value| block} -> fh
# fh.each_pair -> enumerator
#
# Yields each key-value pair to the block, like Hash#each_pair.
# This yields each [key, value] as an array of 2 elements.
def each_pair
if block_given?
@hash.each_pair {|key, values| yield [key, values[0]] }
@default_proc = block
self
@hash = Hash.new { |hash, key| yield self, key }
else
enum_for(:each_pair) { @hash.size }
# FencedHash.new() acts like FencedHash.new(nil) because
# if args.empty?, then args.shift returns nil.
@hash = Hash.new(args.shift)
end
end
 
# call-seq
@keys = {}
# @hash fh.each_keyeach_value { |keyvalue| @keys[key]block} =-> true }fh
# fh.each_value -> enumerator
#
# Yields current value of each key-value pair to the block.
def each_value
if block_given?
@hash.each_value {|values| yield values[0] }
else
enum_for(:each_value) { @hash.size }
end
end
 
# call-seq:
# Acts like Hash#clear.
# fenhsh.fetch(key [,default])
def clear
# fenhsh.fetch(key) {|key| block }
@hash.clear
self#
# Fetches value for _key_. Takes same arguments as Hash#fetch.
def fetch(*argv)
argc = argv.length
unless argc.between?(1, 2)
raise(ArgumentError,
"wrong number of arguments (#{argc} for 1..2)")
end
if argc == 2 and block_given?
warn("#{caller[0]}: warning: " +
"block supersedes default value argument")
end
 
key, default = argv
values = @hash[key]
if values
values[0]
elsif block_given?
yield key
elsif argc == 2
default
else
raise KeyError, "key not found: #{key.inspect}"
end
end
 
# Freezes this FencedHash.
# Acts like Hash#compare_by_identity.
def compare_by_identityfreeze
@hash.compare_by_identityeach_value {|values| values.freeze }
super
@keys.compare_by_identity
self
end
 
end</lang>
# Returns true if _key_ is in fence.
def has_key?(key)
@hash.has_key?(key)
end
alias include? has_key?
alias member? has_key?
 
# call-seq:
# fh.keep_if {|key, value| block } -> fh
# fh.keep_if -> enumerator
#
# Yields each _key_ with current _value_ to _block_. Resets _key_
# to its original value when block evaluates to false.
def keep_if
if block_given?
@hash.each_pair do |key, values|
yield(key, values[0]) or values[0] = values[1]
end
self
else
enum_for(:keep_if) { @hash.size }
end
end
 
# Returns array of keys in fence.
def keys
@hash.keys
end
 
# Returns number of key-value pairs.
def length
@hash.length
end
alias size length
 
# Converts self to a regular Hash.
def to_h
result = Hash.new(@default)
@hash.each_pair {|key, values| result[key] = values[0]}
result
end
 
# Converts self to a String.
def to_s
"#<#{self.class}: #{to_h}>"
end
alias inspect to_s
 
# Returns array of current values.
def values
@hash.each_value.map {|values| values[0]}
end
 
# Returns array of current values for keys, like Hash#values_at.
def values_at(*keys)
keys.map {|key| self[key]}
end
end</syntaxhighlight>
 
=={{header|Scala}}==
{{Out}}Best seen running in your browser either by [https://scalafiddle.io/sf/OuVZ3bT/0 ScalaFiddle (ES aka JavaScript, non JVM)] or [https://scastie.scala-lang.org/qW5qzmdKSZSyAbZEqDROoA Scastie (remote JVM)].
<syntaxhighlight lang="scala">object CreateMapObject extends App {
val map = Map('A' -> 65, 'B' -> 66, 'C' -> 67)
 
println(map)
}</syntaxhighlight>
 
=={{header|Tcl}}==
This solution uses a dict(ionary), so requires Tcl 8.5 or better. Variable traces are used to detect write or unset access to such a protected variable, restore it to the backup value at protection time, and throw an exception
 
<syntaxhighlight lang="tcl">proc protect _var {
upvar 1 $_var var
trace add variable var {write unset} [list protect0 $var]
}
proc protect0 {backup name1 name2 op} {
upvar 1 $name1 var
trace remove variable var {write unset} [list protect 0 $backup]
set var $backup
trace add variable var {write unset} [list protect0 $backup]
return -code error "$name1 is protected"
}
proc trying cmd { #-- convenience function for demo
puts "trying: $cmd"
if [catch {uplevel 1 $cmd} msg] {puts $msg}
}</syntaxhighlight>
Testing:
dict set dic 1 one
dict set dic 2 two
puts dic:$dic
protect dic
trying "dict set dic 3 three"
puts dic:$dic
trying "dict unset dic 1"
trying "unset dic"
puts dic:$dic
 
displays on stdout:
dic:1 one 2 two
trying: dict set dic 3 three
can't set "dic": dic is protected
dic:1 one 2 two
trying: dict unset dic 1
can't set "dic": dic is protected
trying: unset dic
dic:1 one 2 two
 
=={{header|Wren}}==
<syntaxhighlight lang="wren">class FixedSizeMap {
construct new(map) {
// copy the map so it cannot be mutated from the original reference
_map = {}
for (me in map.toList) _map[me.key] = me.value
}
 
containsKey(key) { _map[key] != null }
 
count { _map.count }
 
keys { _map.keys }
 
values { _map.values }
 
[key] { _map[key] }
[key] =(value) {
// do nothing if key doesn't already exist
if (_map[key] != null) _map[key] = value
}
 
reset(key) {
var t = _map[key].type
// leave unaltered if no suitable default value
_map[key] = (t == Num) ? 0 :
(t == String) ? "":
(t == Bool) ? false :
(t == List) ? [] :
(t == Map) ? {} : _map[key]
}
 
iterate(iterator) { _map.iterate(iterator) }
iteratorValue(iterator) { _map.iteratorValue(iterator) }
 
toString { _map.toString }
}
 
var map = { "a": 1, "b": 2 }
var fsm = FixedSizeMap.new(map)
System.print(fsm)
System.print(fsm.count)
fsm["a"] = 3
fsm["b"] = 4
System.print(fsm)
System.print(fsm.containsKey("c"))
fsm["c"] = 5 // attempt to add a new key/value pair
System.print(fsm) // ignored
fsm.reset("a")
System.print(fsm)
System.print(fsm.keys.toList)
System.print(fsm.values.toList)
for (me in fsm) System.print([me.key, me.value])
</syntaxhighlight>
 
{{out}}
<pre>
{b: 2, a: 1}
2
{b: 4, a: 3}
false
{b: 4, a: 3}
{b: 4, a: 0}
[b, a]
[4, 0]
[b, 4]
[a, 0]
</pre>
 
=={{header|zkl}}==
zkl has two dictionary objects: SD, a small dictionary that is created immutable and the "regular" dictionary has has a makeReadOnly method. They both behave the same when locked down.
<syntaxhighlight lang="zkl">d:=SD("one",1,"two",2);
d.keys; //-->L("one","two")
d["one"]; //-->1
d.add("three",3); // error thrown
d.pop("one") // error thrown</syntaxhighlight>
 
 
 
 
{{omit from|AWK|Cannot tie the []= operator nor the delete command to custom functions.}}
Line 237 ⟶ 1,489:
{{omit from|C|No associative arrays.}}
{{omit from|dc|No associative arrays.}}
{{omit from|GUISS}}
{{omit from|PureBasic}}
{{omit from|sed|No associative arrays.}}
{{omit from|UNIX Shell|Bourne shell does not have these, but bashisms may be available on some shells}}
{{omit from|Tcl|The value system and object system do not interact in a way that is necessary to support the prerequisites of this task. In particular, native dicts and arrays are not objects, and objects formally occupy part of the space of functions and not values or variables.}}
{{omit from|PicoLisp|Can't change the behavior of the property system without changing the language around it.}}
PureFox
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