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hash tables in javascript?


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  #1  
Old August 31st, 2002, 01:17 PM
Matthew Doucette Matthew Doucette is offline
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hash tables in javascript?
Can you do hash tables in javascript?
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  #2  
Old August 31st, 2002, 02:29 PM
Tuxie Tuxie is offline
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Yes,read the Objects section at http://www.crockford.com/javascript/survey.html
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Old August 31st, 2002, 02:39 PM
Matthew Doucette Matthew Doucette is offline
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Thanks. Now, can you show me how to define multiple hashes all at once instead of one at a time?
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Old August 31st, 2002, 03:33 PM
Tuxie Tuxie is offline
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Yeah,heres an example:
Code:
var myHashtable = {name: "Someone",age: "2"};


So a list of <key>:<value> delimited by commas,in a block.
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Old September 1st, 2002, 10:09 AM
Matthew Doucette Matthew Doucette is offline
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I should have guessed that but I figured I would ask anyway. It's hard to tell when you have proper formatting, even when it works, with client-side languages.

Thanks!
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Old September 1st, 2002, 10:29 AM
Matthew Doucette Matthew Doucette is offline
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One more thing... is there any way to calculate the length of the hash table instead of going through each entry in a loop and incrementing a counter? Also, once you figure out the length, is there anyway to grab a random hash entry? For example, if the hash is 100 entries in lenght and I want to grab #25, can I do this?
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Old September 1st, 2002, 09:31 PM
adios adios is offline
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Using a for/in loop on a hash returns the entries in the order in which they were defined; so...

<html>
<head>
<title>untitled</title>
<script type="text/javascript" language="javascript">

var hashObj = Object;
hashObj['1'] = 'hash1';
hashObj['2'] = 'hash2';
hashObj['3'] = 'hash3';
hashObj['4'] = 'hash4';
hashObj['5'] = 'hash5';

function gethashObj(which) {
var count = 1;
for (key in hashObj) if (count++ == which) return hashObj[key];
}

function go() {
x=Math.ceil(Math.random()*5);
alert('random number = ' + x);
alert('hashObj = ' + gethashObj(x));
}

</script>
</head>
<body>
<a href="#" onclick="go()">go</a>
</body>
</html>

As for the other...JS hashes are unordered, unlike arrays, so additional coding is needed (I believe) to get 'length'.
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Old September 1st, 2002, 09:50 PM
Matthew Doucette Matthew Doucette is offline
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Ok. Without being able to find the length 'easily' I am just going to use 2 arrays instead of 1 hash table. I think that is the easiest and most compact method. Thanks for the help!
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Old September 2nd, 2002, 07:03 PM
MJEggertson MJEggertson is offline
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Yup, you're going to need to use two arrays.

If you want to go all out, best create a class with the arrays as memebers, that way you can add functions and handlers for your hash tables and create an all-inclusive map class that will keep an internal count of everything. That's on my list of things to do a project of mine.
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  #10  
Old September 3rd, 2002, 05:04 PM
adios adios is offline
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<html>
<head>
<title>untitled</title>
<script type="text/javascript" language="javascript">

Object.prototype.getLength = function() {
len = 0;
for (prop in this) if (this[prop].constructor != Function) ++len;
return len;
}

Object.prototype.output = function (isHTML) {
var hStr = '';
for (prop in window) if (window[prop] == this) hStr += 'Obj: ' + prop + '\n';
for (prop in this) if (this[prop].constructor != Function)
hStr += prop + ' : ' + this[prop] + ((isHTML) ? '<br>' : '\n');
return hStr;
}
///////////////////////
var myHash = {
prop1: 'thingy' ,
prop2: 'anotherthingy' ,
prop3: 'yetanotherthingy' ,
prop4: 'non-thingy'
}
///////////////////////

</script>
</head>
<body text="darkred"><pre>
var myHash = {
prop1: 'thingy' ,
prop2: 'anotherthingy' ,
prop3: 'yetanotherthingy' ,
prop4: 'non-thingy'
}
</pre><br><br>

<a href="#"
onclick="if(myHash)alert(myHash.getLength())">myHash.getLength()</a><br><br>
<a href="#"
onclick="if(myHash)alert(myHash.output())">myHash.output()</a><br><br>
<a href="#"
onclick="if(myHash)this.innerHTML=myHash.output(true)">myHash.output(true)</a>
</body>
</html>
Last edited by adios : September 3rd, 2002 at 05:32 PM.
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  #11  
Old September 3rd, 2002, 06:19 PM
MJEggertson MJEggertson is offline
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Damn, adios, your example finally made me notice something. I just caught on that if you assign an array element like myHash['foo']='bar', you can actually get back what the key is. I thought the 'bar' string was hashed, translated to an index, and lost forever after the assignment. Suddenly, an Array object is almost a true hash table. All that I need to do is generate a simple function for searching, which can be done by extending the Array object with a prototype.

It couldn't get much easier.
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