Shuffling array properties in JavaScript

Question

I have a data dictionary like this:

var data = {
    'text1': 1,
    'text2': 2,
    'text3': 3,
    ...
    'text20': 20
];

I need to pick a random selection of those keys and then shuffle it's values. In the example, it should write something like this:

> console.log(choose(data, 5));
[ { key: 'text15', value: 8 },
{ key: 'text6', value: 3 },
{ key: 'text3', value: 15 },
{ key: 'text19', value: 6 },
{ key: 'text8', value: 19 } ]

For now I'm extracting the keys into another array and sorting by Math.random() but I'm stuck at swaping the values because no key should have the same value it initially had.

How would you swap key/values here?

Thanks

is it with or without replacement? e.g. can the value with 'text8' appear twice? — ninjagecko
– ninjagecko, Commented May 22, 2011 at 20:46
What do you mean by "no key should have the same value it initially had"? Also sorting by a key function Math.random() does not give you a permutation with equal probability; it depends on the sorting algorithm used. You have to get N keys via Math.random(), store those, then sort by them. — ninjagecko
– ninjagecko, Commented May 22, 2011 at 20:57
Because I tried to swap the values of the first with the last, second with before last, but of course that leaves the one in the middle with the same value. — John Crichton
– John Crichton, Commented May 22, 2011 at 21:01
Well if you don't need randomization, just shuffling, shift each value one step in either direction, then unshift/push — Adam Bergmark
– Adam Bergmark, Commented May 22, 2011 at 21:07

vitch · Accepted Answer · 2011-05-22 21:07:15Z

1

I put together a possible solution using underscore.js to simplify traversing the object and arrays in a cross browser manner:

var data = {
    text1: 1,
    text2: 2,
    text3: 3,
    text4: 4,
    text5: 5,
    text6: 6,
    text7: 7,
    text8: 8,
    text9: 9,
    text10: 10
};

function choose(data, num)
{
    var keys = _.sortBy(
                    _.keys(data),
                    function(k)
                    {
                        return (Math.random() * 3) - 1;
                    }
                ),
        results = [],
        k1, k2;
    if (num > keys.length) {
        throw new Error('Impossible to retrieve more values than exist');
    }
    while (results.length < num) {
        k1 = k2 || keys.pop();
        k2 = keys.pop();
        results.push({key:k1, value: data[k2]});
    }
    return results;
}

console.log(choose(data, 5));

This isn't necessarily an optimal approach but it seems to meet your requirements. I first grab all of the keys and sort them randomly. I then loop through the random keys creating a new object with one key and the following keys value. That way you'll always end up with a different value associated with each key. If you need it to work when the value of num passed in to the function == the number of keys in the data then you'll have to add a little more code - I'll leave that as an exercise for the reader :)

You can have a play with this code on jsfiddle:

http://jsfiddle.net/zVyQW/1/

answered May 22, 2011 at 21:07

vitch

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4 Comments

John Crichton Over a year ago

Very similar to my solution but the || trick is better that incrementing two counters. I'm giving you the points.

Gumbo Over a year ago

This fails if you want exactly n items as you would need n+1 keys to get the n-th value.

vitch Over a year ago

@Gumbo - as you can see if you read what I wrote under the code, that was left as a (fairly trivial) exercise for the reader...

Gumbo Over a year ago

Ok, I didn’t read that. But it can be fixed quite easily: k2 = keys.pop() || results[0].key;

Gumbo · Accepted Answer · 2011-05-23 07:26:39Z

You could do this:

collect names and corresponding values in two arrays names and values
shuffle both arrays independently of each other
take the first n items of both arrays and combine them

Here’s an example implementation:

Array.prototype.shuffle = function() {
    for (var i=this.length-1, j, tmp; i>0; i--) {
        j = Math.round(Math.random()*i);
        tmp = this[i], this[i] = this[j], this[j] = tmp;
    }
    return this;
};

function choose(data, number) {
    var names = [], values = [], pick = [];
    for (var name in data) {
        if (data.hasOwnProperty(name)) {
            names.push(name);
            values.push(data[name]);
        }
    }
    names = names.shuffle(), values = values.shuffle();
    for (var i=Math.min(number >>> 0, names.length-1); i>=0; i--) {
        pick.push({key: names[i], value: values[i]});
    }
    return pick;
}

RobG · Accepted Answer · 2011-06-09 00:45:12Z

0

Been a while since this was answered, but I was working on shuffling and found the following to be by far the fastest implementation with an evenly random distribution.

It's fast because it only makes one call to Math.random on each iteration, all the rest is done by property access. It doesn't modify the array, just reassigns values.

  function shuffle(a) {
    var t, j, i=a.length, rand=Math.random;

    // For each element in the array, swap it with a random
    // element (which might be itself)
    while (i--) {
      k = rand()*(i+1)|0;
      t = a[k];
      a[k]=a[i];
      a[i]=t;
    }
    return a;
  }

answered Jun 9, 2011 at 0:45

RobG

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Comments

Tabares · Accepted Answer · 2013-09-03 19:20:48Z

It uses a combination of three functions (including the Array shuffle prototype method).

Here is the complete code:

var obj = {
    "red":"RED",
    "blue":"BLUE",
    "green":"GREEN",
    "yellow":"YELLOW",
    "purple":"PURPLE"
};

Array.prototype.shuffle = function(){
    for (var i = 0; i < this.length; i++){
        var a = this[i];
        var b = Math.floor(Math.random() * this.length);
        this[i] = this[b];
        this[b] = a;
    }
}

obj = shuffleProperties(obj); // run shuffle

function shuffleProperties(obj) {
    var new_obj = {};
    var keys = getKeys(obj);
    keys.shuffle();
    for (var key in keys){
        if (key == "shuffle") continue; // skip our prototype method
        new_obj[keys[key]] = obj[keys[key]];
    }
    return new_obj;
}

function getKeys(obj){
    var arr = new Array();
    for (var key in obj)
        arr.push(key);
    return arr;
}


for(key in obj){
   alert(key);
}

Check all post, Best Regards.

Giacomo1968 · Accepted Answer · 2015-11-28 07:00:21Z

0

Use an implementation of random that randomizes a discrete set of values, such as Math.rand seen here. For each index, randomize Math.rand(index, length-1) to get a list of random indexes, the location off all indices will change.

edited Nov 28, 2015 at 7:00

Giacomo1968

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answered May 22, 2011 at 21:10

Adam Bergmark

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