Question

I have a question regarding javascript Math.random():

I have (for a game I’m building) to randomly generate every number from a given set (i.e. from 0 to 1000) and every time I have to generate a number, I have to check if that number has already been “generated”.

The solution is pretty easy thinking about a simple algorithm that checks if the random integer is already present in the generated set. It loops generating numbers until it can’t find it.
A snippet below:

/* ... */
for(var i = 0; i<upperBound; i++){
    var randN = Math.floor(Math.random()*upperBound);
    while(myRandomNumbers.contains(randN)){
        loops++;
        randN = Math.floor(Math.random()*upperBound);
    }
    myRandomNumbers.push(randN);
}
/* ... */

running example here

I’d like to know: is this the best way to achieve this? or are there any ways, instead of looping until it generates a “good” number, to exclude a particular set in the random generation?

Thanks a lot everyone!

Answer 1

1. Generate the set of numbers in order.
2. Sort the list randomly.

Here’s an example using a naive, biased sort:

for (var nums=[],i=0;i<1000;++i) nums[i]=i+1;
nums.sort(function(){ return Math.random()-0.5 });

Then you can just pop() numbers off of nums to get the next ‘random’ number, guaranteed to never have been used before.