Pushing unique numbers within a range to an array in Javascript

Question

I am trying to insert numbers from 1 to 15 into an array. And here is the code:

<html>
    <head></head>
    <body>
        <button id="myBtn" value="HELLO">HELLO</button>
        <script type="text/javascript">
            var btn = document.getElementById("myBtn");
            var num = null;
            btn.addEventListener('click', function() {
                var questions = new Array();
                num = Math.floor(Math.random() * 14 + 2);
                questions.push(num);

                for (var i = 1; i <= 15; i++) {
                    num = Math.floor(Math.random() * 14 + 2);
                    if (questions.indexOf(num) !== -1) {
                        alert(num + " Exists in array. So not pushing it");
                    } else {
                        alert(num + " is not found. So pushing it");
                        questions.push(num);
                    }
                    console.log(questions);
                }
                alert(questions);
            })
        </script>
    </body>
</html>

If you run this with the console open. You will notice that though a number is not in the array the in operator still discards the number without pushing. Can I know why and how to correct this?

And also is there any better way to insert x number of numbers in random order each time.

score 1 · Accepted Answer · edited May 23 '17 at 11:49

1

You should not use in operator with Arrays. What you should have done is

if (questions.indexOf(num) !== -1) {

When you use in with an Array, it will not check the values but the indices of the Array. That is why your code fails.

Please check this answer of mine to know more about, why you should not use in operator with Arrays.

The best way to generate N unique random numbers is to, generate the list of numbers and then shuffle them, like this

function getRandomNumbers() {
    var rand, index = 0,
        array = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15];

    array.forEach(function(value) {
        rand = Math.floor(Math.random() * ++index);
        array[index - 1] = array[rand];
        array[rand] = value;
    });

    return array;
}

console.log(getRandomNumbers());

This is adopted from the _.shuffle function of Underscore.js library, which shuffles the list of data with Fisher-Yates Shuffle algorithm.

edited May 23 '17 at 11:49

Community

1
1

answered Apr 04 '14 at 05:11

thefourtheye

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Yeah ok got it. Why shouldnt I use that? It works perfectly with lists in python. OK Is there any other way to insert 15 numbers to an array that are in unique order each time? – user3283104 Apr 04 '14 at 05:15
@user3283104 Regarding the `why` part, please check the linked question in my answer. I have explained extensively. – thefourtheye Apr 04 '14 at 05:17
@user3283104: Python and JavaScript are different languages, and `in` works differently in Python. – Felix Kling Apr 04 '14 at 05:30
@FelixKling Look at the last part of my question. Any answers for that? – user3283104 Apr 04 '14 at 05:31
@thefourtheye Thanks I almost did something similar to this but was a bit buggy. Thanks for this. If you agree I will let you know for what I was using this. – user3283104 Apr 04 '14 at 05:41
@user3283104 If it is not much of a secret, I am all ears :) – thefourtheye Apr 04 '14 at 05:45
@thefourtheye Ok will show it. Give me a week or two to complete it. :) – user3283104 Apr 04 '14 at 06:14

score 1 · Answer 2 · answered Apr 04 '14 at 05:12

1

the in operator works on objects, so you're really checking to see if your array has an index, not a value.

answered Apr 04 '14 at 05:12

mgilson

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Pushing unique numbers within a range to an array in Javascript

2 Answers2