tech.thingoid

var primeMinister = new Object();

primeMinister["barton"] = "Sir Edmund BARTON";
primeMinister["bruce"] = "Stanley Melbourne BRUCE";
primeMinister["chifley"] = "Joseph Benedict CHIFLEY";
primeMinister["cook"] = "Joseph COOK";
primeMinister["curtin"] = "John Joseph CURTIN";
primeMinister["deakin"] = "Alfred DEAKIN";
primeMinister["fadden"] = "Arthur William FADDEN";
primeMinister["fisher"] = "Andrew FISHER";
primeMinister["forde"] = "Francis Michael FORDE";
primeMinister["fraser"] = "John Malcolm FRASER";
primeMinister["gorton"] = "John Grey GORTON";
primeMinister["hawke"] = "Robert James Lee HAWKE";
primeMinister["holt"] = "Harold Edward HOLT";
primeMinister["howard"] = "John Winston HOWARD";
primeMinister["hughes"] = "William Morris HUGHES";
primeMinister["keating"] = "Paul John KEATING";
primeMinister["lyons"] = "Joseph Aloysius LYONS";
primeMinister["mcewen"] = "John McEWEN";
primeMinister["mcmahon"] = "William McMAHON";
primeMinister["menzies"] = "Robert Gordon MENZIES";
primeMinister["page"] = "Earle Christmas Grafton PAGE";
primeMinister["reid"] = "George Houstoun REID";
primeMinister["scullin"] = "James Henry SCULLIN";
primeMinister["watson"] = "John Christian WATSON";
primeMinister["whitlam"] = "Edward Gough WHITLAM";

var selectedPM = prompt( "Which PM?","" );

var outputPM = primeMinister[ selectedPM ];

alert( outputPM );

var primeMinister = new Object();

primeMinister["barton"] = "Sir Edmund BARTON";
...
primeMinister["whitlam"] = "Edward Gough WHITLAM";

var selectedPM = prompt( "Which PM?","" );

var outputPM = primeMinister[ selectedPM ];

alert( outputPM );

var outputPM = primeMinister[ selectedPM ];

var outputPM = primeMinister[ selectedPM.toLowerCase() ];

primeMinister = new Object();

primeMinister["barton"] = ["Sir Edmund BARTON","1901-03"];
primeMinister["bruce"] = ["Stanley Melbourne BRUCE","1923-29"];
primeMinister["chifley"] = ["Joseph Benedict CHIFLEY","1945-49"];
primeMinister["cook"] = ["Joseph COOK","1913-14"];
primeMinister["curtin"] = ["John Joseph CURTIN","1941-45"];
primeMinister["deakin"] = ["Alfred DEAKIN","1903-04","1905-08","1909-10"];
primeMinister["fadden"] = ["Arthur William FADDEN","1941-41"];
primeMinister["fisher"] = ["Andrew FISHER","1908-09","1910-13","1914-15"];
primeMinister["forde"] = ["Francis Michael FORDE","1945-45"];
primeMinister["fraser"] = ["John Malcolm FRASER","1975-83"];
primeMinister["gorton"] = ["John Grey GORTON","1968-71"];
primeMinister["hawke"] = ["Robert James Lee HAWKE","1983-91"];
primeMinister["holt"] = ["Harold Edward HOLT","1966-67"];
primeMinister["howard"] = ["John Winston HOWARD","1996-"];
primeMinister["hughes"] = ["William Morris HUGHES","1915-23"];
primeMinister["keating"] = ["Paul John KEATING","1991-96"];
primeMinister["lyons"] = ["Joseph Aloysius LYONS","1932-39"];
primeMinister["mcewen"] = ["John McEWEN","1967-68"];
primeMinister["mcmahon"] = ["William McMAHON","1971-72"];
primeMinister["menzies"] = ["Robert Gordon MENZIES","1939-41","1949-66"];
primeMinister["page"] = ["Earle Christmas Grafton PAGE","1939-39"];
primeMinister["reid"] = ["George Houstoun REID","1904-05"];
primeMinister["scullin"] = ["James Henry SCULLIN","1929-32"];
primeMinister["watson"] = ["John Christian WATSON","1904-04"];
primeMinister["whitlam"] = ["Edward Gough WHITLAM","1972-75"];

var selectedPM = prompt( "Which PM?","" );

var outputPM = primeMinister[ selectedPM.toLowerCase() ];

alert( outputPM.join("\n") );

alert( outputPM.join("\n") );

primeMinister = new Object();

primeMinister["barton"] = ["Sir Edmund BARTON","1901-03"];
...  as in listing 2 ...
primeMinister["whitlam"] = ["Edward Gough WHITLAM","1972-75"];

var numberTerms = numberPrompt( "How many terms served?", 1 );

var recordsFound = "";

for ( testKey in primeMinister )
{
    if ( primeMinister[ testKey ].length > numberTerms )
    {
    recordsFound += primeMinister[ testKey ][0] 
    + "\nTerm " 
    + numberTerms 
    + ": " 
    + primeMinister[ testKey ][ numberTerms ] 
    + "\n";
    }
}

if ( recordsFound == "" )
{
    alert( "No prime ministers have served for " 
           + numberTerms 
           + " separate terms." );
}
else
{
    alert( recordsFound );
}

var numberTerms = numberPrompt( "How many terms served?", 1 );

for ( testKey in primeMinister )

   if ( primeMinister[ testKey ].length > numberTerms )

recordsFound += primeMinister[ testKey ][0] 
+ "\nTerm " 
+ numberTerms 
+ ": " 
+ primeMinister[ testKey ][ numberTerms ] 
+ "\n";

if ( recordsFound == "" )

12 Responses to “Real programming with JavaScript - Associative Arrays”

1. tech.thingoid Says:
   November 7th, 2005 at 7:04 am

   Q. When is an array not an Array()?

   A. When it’s an associative array.

   In JavaScript, an associative array does not actually need to be an Array() type object. In fact, it could (and should) be any type of object - even the most basic Object() type - rather than an Array().

   Although it’s perfectly legal to do so it would be a bit misleading of me to use Array() here. Even if the primeMinister object is of the Array() type, all of the specialised Array() functions like length, join(), and sort() do not work for associative array data. Data for arrays and associative arrays are stored completely separately in an Array() object, which makes it particularly unsuitable for the purpose.

   I was aware of the above when, in the published version of this article, I used the Array() type rather than Object() type in the examples in the mistaken belief that it would simplify the conceptual connection between Array() objects and associative arrays. Of course there is no actual connection between the two and in making such a link this was more confusing that it need have been.

2. jehiah Says:
   December 30th, 2005 at 12:07 pm

   I think an easier way to write/store that data is like this

   primeMinister = {”barton” : [”Sir Edmund BARTON”,”1901-03″],
   “bruce”:[”Stanley Melbourne BRUCE”,”1923-29″],
   “chifley”:”Joseph Benedict CHIFLEY”,”1945-49″],
   “cook”:”Joseph COOK”,”1913-14″}

   Then you use it like

   alert(primeMinister[”barton”]);

3. tech.thingoid Says:
   December 30th, 2005 at 3:36 pm

   Hi Jehiah,

   I agree that the “Perl-style” usage that you suggest here is both easier typing and reading, indeed that is my preferred style. However I don’t think it’s particularly clear what’s going on for people who are just learning about associative arrays. As this series of articles is targeted at beginning programmers I deliberately chose to be more verbose than I normally would be in my own programming.

   A further advantage in using the
   primeMinister["barton"] = "Sir Edmund BARTON";
   format is that without any further information the reader knows how to add more entries to a hash, since that’s exactly what this does (an empty hash is first created then each successive line adds one element).

   On the other hand, in the format you recommend:
   primeMinister = {"barton" : ["Sir Edmund BARTON". "1901-03", ... "whitlam" : ["Edward Gough WHITLAM","1972-75"]}
   the hash is created and populated effectively simultaneously. This isn’t “complete” because it leaves the reader uninformed about how to extend the hash later in the program.

   But I consider that a side-effect, my main intention is clarity for the learner programmer. That said, I appreciate your comment because it helps to remind people that there are often several ways to achieve the same result in programming.

   Cheers,

   tech.thingoid.

4. Beren Says:
   December 31st, 2005 at 5:21 am

   One thing that I am unclear on - if none of the specialised Array() functions work for associative arrays, how do you remove an item? Simply saying primeMinister[”barton”] = null will not remove that key from the list, so when you iterate through the keys, there is an empty element!

   Cheers,

   Beren

5. tech.thingoid Says:
   December 31st, 2005 at 8:20 am

   Removing elements from a hash or array

   Hi Beren,

   Thanks for your question.

   The command you’re looking for is delete. See the Core JavaScript Guide for a complete rundown, but basically what you need to do is something like the following. If you wamted to remove Prime Minister Keating (as some did at the time!):

   delete primeMinister["keating"];

   And that’s it.

   This also works for array elements of Array() objects too.

   Cheers,

   tech.thingoid.

6. Bruno Says:
   January 12th, 2006 at 5:49 am

   Hi!

   What about getting an associative array’s size, without using a loop to count all possible values?

7. tech.thingoid Says:
   January 12th, 2006 at 11:21 am

   Getting the size of a JavaScript associative array

   Hi Bruno,

   The Array() object provides the length property, which you use like so:

   someArray.length

   But to the best of my knowledge that option isn’t available when using associative arrays. The elements in associative arrays don’t have a logical sequence, so I guess counting them doesn’t necessarily make sense.

   I’m afraid you’re stuck with running a counter inside a loop, something like this:

   
var counter = 0;
for (var i in primeMinister)
{
counter++;
}


   Sorry ’bout that.

   tech.thingoid.

8. Eugene Says:
   February 3rd, 2006 at 12:17 am

   well … how can i create multidimensional hash ?
   eg:

   varName[1]["z"][33] = 1;
varName[1]["y"][34] = 2;
varName[2]["aa"]['a'] = 3;
varName[2]["aa"]['b'] = 4;
varName[2]["aa"]['c'] = 5;

   without varName[1] = new Object();

9. tech.thingoid Says:
   February 3rd, 2006 at 1:02 pm

   Faking a multidimensional hash

   Hi Eugene,

   As you suggest, a multidimensional hash can be made by making a series of objects inside the first hash.

   varName[1] = new Object();
varName[2] = new Object();
... and so on

   That’s a perfectly acceptable way to do it.

   However, if you find that’s a bit of overkill you can also fake it just by concatenating the key values:

   var varName = new Object()

   var i = 1 + "z" + 33;

   varName[i] = 1;
varName[1 + "y" + 34] = 2;
varName[2 + "aa" + 'a'] = 3;
varName[2 + "aa" + 'b'] = 4;
varName[2 + "aa" + 'c'] = 5;

   i = "2aaa";
alert(varName["1z33"]);
alert(varName[i]);
alert(varName[2 + "aa" + 'c']);

   If you prefer your multiple dimensions separated, say by a comma, just include it in the concatenation:

   varName[1 + "," + "z" + "," + 33] = 1;

   alert(varName["1,z,33"]);

   I’d recommend this both for readability and especially if you’re using numeric keys (remember that 1 + 2 + 3 is not the concatenated value “123″, it’s the numeric value 6).

   Sure it’s a hack, but it’s legit: it doesn’t matter how you make your hash indexes as long as each one is unique.

   Cheers,

   tech.thingoid.

10. Ed Says:
    April 3rd, 2006 at 11:12 am

    Hey tech.thingoid, thanks for the helpful articles. You said, “The elements in associative arrays don’t have a logical sequence, so I guess counting them doesn’t necessarily make sense.”

    What if you WANT your elements to have a logical sequence (say, alphabetical). For example,

    var map = new Object();

    var foo = [x, y, z];
    var bar = [v, w, z];
    var baz = [a, b, c];

    I want to be able to add foo to map, and then bar, and have bar be “before” foo (because it comes first alphabetically). If I later add baz, I want it to be “between” bar and foo for the same reason.

    I get why associative arrays don’t have any logical sequence, and so adding ‘in sequence’ doesn’t really make sense. What I’m describing (and wanting!) is more like the Java TreeMap-like objects, which maintains the keys in order and guarantees quick access (I need both).

    What would solve this for me is a way to sort() the keys of an associative array/object (maintaining the associated element relationships of course).

    I’ve searched for both these solutions but haven’t found one yet. I thought I’d give you a try and see if you had any ideas.

    Thanks in advance!

    e.

11. tech.thingoid Says:
    April 5th, 2006 at 8:03 am

    Hi Ed,

    An intriguing problem, although I’m not sure I understand what you’re trying to do so I hope my answer makes some sense.

    I don’t think JavaScript has this functionality built-in. It may be possible to build a user-defined function that would do this but the simplest solution I can think of is to keep an ordinary array of the hash table keys. Something like this:

    var keyIndex = new Array();
var hashTable = new Object();

    hashTable[”z”] = “the last item”;
    keyIndex.push(”z”);

    hashTable[”x”] = “the first item”;
    keyIndex.push(”x”);

    hashTable[”y”] = “the middle item”;
    keyIndex.push(”y”);

    You can then use the Array() object’s built-in sort() function to order the keys and loop through the keyIndex array to give you your keys in order:

    keyIndex.sort();

    var i;
    for (i in keyIndex)
    {
    document.write(hashTable[ keyIndex[i] ] + “<br />”);
    }

    This should produce the output:

    the first item
    the middle item
    the last item
    

    There’s a bit of overhead in managing both the keyIndex and hashTable objects so I don’t think it will be particularly fast. And I don’t think it’s a very elegant solution but at least it’s an option for you. I’m sorry if it’s not really what you’re looking for.

    Please note: I haven’t tested this example code, so no guarantees that it’ll work!

    Cheers,

    tech.thingoid.

12. Simon Norris Says:
    February 20th, 2008 at 9:26 pm

    This may just be a lazy hack but it works for me. I’ve been unable to sort an associative array or find a script which works so…

    Lazy, lazy, lazy: concatenate it in an ordinary array, sort that and split the values into a new associative array. Probably not recommended for the phone book but like a say - works for me.

    Also removes duplicates

    function sortAssoc(inputArray) {
    var tempArray = new Array();
    var outputArray = new Array();

    i=0;
    
    for (keyVar in inputArray) {
        tempArray[i] = inputArray[keyVar] + '//--string_divider--//' + keyVar;
        i++;
    }
    
    tempArray = tempArray.sort();
    
    for (i=0; i < tempArray.length; i++) {
        if (tempArray[i]!=tempArray[i-1]) {
            temp = tempArray[i].split('//--string_divider--//');
            tempIndex = temp[1];
            outputArray[tempIndex] = temp[0];
        }
    }
    
    return outputArray;
    

    }