Create an object/Native demonstration

Task:

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 follow difference:

No new item can be added;
Item cannot be deleted, (but native delete method may used to reset the item's value to default) ;

Objective:

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.

J

Given a list of keys and an associated list of values, the idiomatic way of expressing this concept in J would be:

<lang j>lookup=: values {~ keys&i.</lang>

For example:

<lang j>lookup=: 10 20 30 40 50 {~ (;:'this is a test')&i.

  lookup ;:'a test'

30 40</lang>

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.

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.

Works with: JavaScript version 1.7

<lang javascript>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 + "}" ;
   } ;

}</lang>

Test run:

<lang javascript> const BR = "
\n"

var pl = function(s) {

   document.write(s + BR) ;

} ;

pl("

") ;

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) ;
}
</lang>

output :

<pre>
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)