how to exclude individual numbers from math.random

Question

I need jquery to make an array, which includes only the numbers, that are selected with checkboxes. In other words I need math.random to exclude answers that arent checked. There will be more checkboxes, when I get over this problem

if ($('#check1') .is(':checked')) {
    var nrI = "1";
} else {
    var nrI = "";
}
if ($('#check2').is(':checked')) {
    var nrII = "2";
} else {
    var nrII = "";
} 

var numbrid = [nrI,nrII];

if (nrI + nrII == 0) {
    var numRand = Math.floor((Math.random()*2)+1);
} else {
    var numRand = Math.floor((Math.random()*numbrid.length)+1);
}   
$("#number") .text(numRand);

Answer 1

Try

var numbrids = $('input[id^="check"]:checked').map(function(index, el) {
    return $(el).attr('id').substring(5);
});

if (numbrids.length) {
    var numRand = Math.floor((Math.random() * numbrids.length) + 1);
} else {
    var numRand = Math.floor((Math.random() * 2) + 1);
}
$("#number").text(numRand);

I'm not sure about the second part of the question ie what you want to do with numbrids

Demo: Fiddle

If your requirement is same as @AndyRay comment then

var numbrids = $('input[id^="check"]:checked').map(function(index, el) {
    return $(el).attr('id').substring(5);
}), numRand;

if (numbrids.length) {
    numRand = numbrids [Math.floor((Math.random() * numbrids.length))];
} else {
    numRand = 0;
}
$("#number").text(numRand);

Demo: Fiddle