select random value NOT in array

Question

How would I select a random value (0 to 30) that is not in this array?

var list = new Array(1,3,4,7,8,9);

My question was slightly open ended, and I apologize. I am looking for different options, leads. — Sparkup
– Sparkup, Commented Aug 8, 2011 at 8:24

slaphappy · Accepted Answer · 2011-08-08 08:15:31Z

8

Build the complementary array and pick random values from it.

var list2 = new Array();
for(var i=0; i<30; i++)
  if(!list.contains(i))
    list2.push(i);

Then:

var rand = list2[Math.floor(Math.random() * list2.length)];

answered Aug 8, 2011 at 8:15

slaphappy

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Comments

Marianissimus · Accepted Answer · 2019-05-21 18:02:00Z

3

A little recursive function:

getNum() {
  let randomNum = Math.floor(Math.random() * (30 - 1)) + 1
  if (list.includes(randomNum)) {
    return getNum()
  }
   return randomNum
}

Might be a little faster, since it first tries to return a random number, and then checks if it's in the array.

edited May 21, 2019 at 18:02

answered May 21, 2019 at 17:39

Marianissimus

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2 Comments

Jason Aller Over a year ago

What is the performance like as the number of elements in the exclusion array grows? How does this strategy perform when the possible range grows from 30 to 30,000, and then to 30,000,000?

Marianissimus Over a year ago

Thanks for asking, ran some tests. Used increasing arrays of random numbers and compared recursion to @slaphappy's complementary arr (most voted solution).

Parameters(how many generated / how many in array): (1 in 1000) Recursion ~ 0.015ms / Complementary: ~ 1.25, 1.26 ms (1 in 10000) Rec ~ 0.030ms / Com ~ 115ms (1 in 100.000) Rec ~ 0.44ms / Com ~ 11573ms (or crash) (1000 in 1000) Rec ~ 4 ms / Com ~ 1182 ms (10.000 in 10.000) Rec ~ 317ms / Com ~ crash  (1000.000 in 1000.000) Rec ~ 31422ms / Com ~ no way

Note that recursion can sometimes double its runtime (when it calls itself)!

foxy · Accepted Answer · 2011-08-08 08:17:21Z

2

function RandomValueNotInArray(array)
{
    var e;
    do
    {
        e = Math.random() * 31; // n + 1
    } while (array.contains(e))
    return e;
}

answered Aug 8, 2011 at 8:17

foxy

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3 Comments

wizztjh Over a year ago

but Math.random will generate float and It should be Math.random()*31?

T J Over a year ago

The code speaks for itself, but for the sake of completeness consider adding a description

OOPS Studio Over a year ago

Math.random() returns a decimal value. You need to either Math.trunc() or Math.floor() the return value before comparing it to a list of integers, otherwise it will never match.

Shamim Hafiz - MSFT · Accepted Answer · 2011-08-08 08:16:59Z

1

Assuming your list is reasonable small in size, create a list of numbers that are not in the array and then select a number from that array at random.

answered Aug 8, 2011 at 8:16

Shamim Hafiz - MSFT

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Comments

Blazemonger · Accepted Answer · 2013-05-14 20:21:28Z

1

You need a while loop that tests if rand is in your restricted array and, if so, re-generate a new random number:

var rand;
do {
    rand = Math.floor(Math.random() * 31); // re-randomize, 0 to 30 inclusive
} while ($.inArray(rand, restricted) > -1);
return rand;

http://jsfiddle.net/mblase75/dAN8R/

Don't want jQuery? You can replace $.inArray(rand, restricted) with restricted.indexOf(rand) if you use this polyfill for old browsers.

edited May 14, 2013 at 20:21

answered May 14, 2013 at 16:22

Blazemonger

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2 Comments

T J Over a year ago

Isn't this same as this answer stackoverflow.com/a/6979340/2333214 posted almost 2 years before this..

Blazemonger Over a year ago

@TJ It might be, if array.contains() was a built-in method or Math.random() * 31 wasn't a floating-point number.

Pocket Universe · Accepted Answer · 2011-08-08 08:15:35Z

0

I would probably make an array or linked list from which I would subtract the unwanted items. That way I could keep removing items and just randomly select items from position 0 to the array's length - 1 without having to select the same thing twice.

Another way of doing it is to randomize a number between 0 and 30 and to keep doing it while it is found in the array. The only problems with that is knowing when the array is full (to get rid of infinite loops) and that it is a whole lot more processor intensive.

answered Aug 8, 2011 at 8:15

Pocket Universe

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Comments

wizztjh · Accepted Answer · 2011-08-08 08:28:49Z

0

you can use filter .

var filteredArray = list.filter(function(e){
  return e!= Math.floor(Math.random() * (31));

});

edited Aug 8, 2011 at 8:28

answered Aug 8, 2011 at 8:15

wizztjh

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1 Comment

Shamim Hafiz - MSFT Over a year ago

Your code will create a list containing some numbers that are present in the array. The OP wants something quite opposite, only a single number not in the array.

MPaulo · Accepted Answer · 2012-11-13 05:03:37Z

-1

PHP in 2 lines:

$result = array_diff(range(1,30), array(1,3,4,7,8,9)); 
echo $result[array_rand($result)];

answered Nov 13, 2012 at 5:03

MPaulo

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2 Comments

Blazemonger Over a year ago

If this question was tagged as PHP before, it isn't now.

MPaulo Over a year ago

Just trying to help people arriving here from Google.

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