Question

I want to create an array of random/pseudo-random numbers using a seed. I want the very same array to be created when the same seed is used and I want to have little or no visible pattern in the array. I’m working in JavaScript.

This is the random function I’m using, which I’m quite happy with (sorry, I forgot who the original author is):

function random(seed) {
    if (!seed) seed = new Date().getTime();
    seed = (seed*9301+49297) % 233280;
    return seed/(233280.0);
}

This is the array generation:

var superSeed = random();
var nRandom = 100;
var randomArray = new Array();
for (var i = 0 ; i < nRandom ; i++){
    randomArray.push(random((superSeed*10)+ (i)));
}

Somehow the pattern seems to be quite similar, no matter how often I run it. This question seems to be similar, but since it’s about matrixes, I don’t understand what’s being said.

Thanks!

Answer 1

Having worked on similar things before I think we can use a fairly simple series, which takes two initial values and then you can get a lot more.

var a1,b1;

function InitSequence(v1, v2) {
    a1 = Math.pow(v1, 5) / Math.pow(v1, 3);
    b1 = Math.pow(v2, 8);
    lastrand = (a1 + b1) & 0x7fffffff;
}

function SequenceNext() {
    var alast = a1;
    var nextVal = (a1 + b1) & 0x7fffffff;
    b1 = alast;
    a1 = nextVal;
    return nextVal;
}

Then use it like this:

InitSequence(75, 21);
for (var i = 0; i < 99; i++) {
    v = SequenceNext();
}

I tested it like this:

var used = new Array();

InitSequence(75, 21); // any two values will do.

// fill 10k into array.

for (var i = 0; i < 9999; i++) {
    v = SequenceNext();
    if (undefined != used[v]) {
        used[v]++;
    } else used[v] = 1;

    //document.write(i+": "+v+"<br>");
}

// see if there any duplicates.
var tdup = 0;
for (xx in used) {
    ndup = used[xx];
    if (ndup > 1) {
        document.write("duplicated " + xx + " :" + ndup + "<br>");
        tdup += ndup;
    }
}
document.write("Total dups " + tdup + "<br>");

---

This is using the Fibonacci series, which in mathematical terms, the sequence Fn of Fibonacci numbers is defined by the recurrence relation
. I’m starting with different values – (v1^5) / (v1^3) and v2 ^ 8; otherwise it would only ever be identical.