随机数组重复索引项

Question

This code works fine in their example but repeats some of my index items when I try and use it.

var lastloaded = 0;
window.onload = loadPages;

    Array.prototype.knuthShuffle = function()
    {
        var i = this.length, j, temp;
        while ( --i )
        {
            j = Math.floor( Math.random() * (i - 1) );
            temp = this[i];
            this[i] = this[j];
            this[j] = temp;
        }
    };

var arr = ["aCard.html", "bCard.html", "cCard.html", "dCard.html"];

function loadPages () {
     arr.knuthShuffle();
    var frame = document.getElementById("frameWrap");
    if (lastloaded+1>arr.length){
        lastloaded = window.location = "greatJob.html";
    }
    frame.src = arr[lastloaded];
    lastloaded++;
};
document.getElementById('tom').onclick = loadPages;

Can anyone tell me what I am missing from this code to keep it from repeating items in my array?

Answer 1

I'm not sure I completely understand how your page works, but it seems like it is shuffling the array in order to figure out the next page to go to. This means that it is newly shuffled with every page load, and you therefore have no guarantee as to the uniqueness of the pages - in fact, it would be extremely unlikely for you to get all the unique pages (1 in n!, to be precise)

In order to ensure uniqueness, you MUST save the generated order, not just the index you were at.

Answer 2

You have a problem with your declaration of j and temp which might add up on multiple shuffles and give you some strange behavior.

specifically this line:

 var i = this.length, j, temp; 

and these lines:

 j = Math.floor( Math.random() * (i - 1) ); temp = this[i]; 

The problem here is that you didn't actually declare the j and temp variables, that's an invalid syntax. Then when declaring them within your loop without the var keyword they are treated as global variables. Solvable by modifying the first line into:

 var j, temp; var i = this.length; 

Edit : Actually that isn't it, as already pointed out by t.niese every time you click tom you're re-shuffling.

What you want to do is shuffle once and use the newly shuffled array every time. So decouple your shuffling from your page loading by taking arr.knuthShuffle(); out of the loadPages() function.