Javascript Random Integer Between two Numbers

Question

I'm trying to create a random number generator that generates random numbers between two numbers. For instance, let's say I want to generate a random number between 4 and 10, I would want it to be able to choose from any number from 4 - 10. Here is what I tried:

var randNumMin = 4;
var randNumMax = 10;
var randInt = (Math.floor(Math.random() * (randNumMax - randNumMin + 1)) + randNumMin);

However, that didn't seem to work and generated weird random numbers that weren't between 4 and 10, and some began with 0. What would be the correct algorithm to do this?

Here is the code I'm implementing the algorithm in:

$(function() {
    $('#generateRandNums').click(function() {

        var numCount = document.getElementById("randNumCount").value;
        var randNumMin = document.getElementById("randNumMin").value;
        var randNumMax = document.getElementById("randNumMax").value;

        if (numCount.match(/^[\d]*$/ ) && randNumMin.match(/^[\d]*$/ ) && randNumMax.match(/^[\d]*$/ )) {       
            if (numCount == "" || randNumMin == "" || randNumMax == "")  {
                alert ("Please fill out all forms then try again.");

            } else {
                if (randNumMin>randNumMax) {
                    alert ("Please make sure your first number is smaller than the second, then try again.");
                } else {
                    if (randNumMin<0) {
                        alert ("Please make sure that you generate a positive number of random numbers, then try again.");
                    } else {

                        if (numCount>1) {
                            var randResult = ("You generated " + numCount + " random numbers between " + randNumMin + " and " + randNumMax + " and got the numbers ")
                            oneNumber = 0;
                        } else {
                            var randResult = ("You generated a random number between " + randNumMin + " and " + randNumMax + " and got the number ");
                            oneNumber = 1;
                        }
                        for (i=0;i<numCount;i++) {
                        //Get a random number between randNumMin and randNumMax
                        var randInt = (Math.floor(Math.random() * (randNumMax - randNumMin + 1)) + randNumMin);
                            if (i == numCount-1) {
                                if (oneNumber == 0) {
                                    randResult = (randResult + "and " + randInt + ".");
                                } else {
                                    randResult = (randResult + randInt + ".");
                                }
                            } else {
                                randResult = (randResult + randInt + ", ");
                            }
                        }
                        $("#randNumResults").val(randResult);
                    }
                }
            }
        } else {
            alert ("Make sure you only enter numbers and no spaces, then try again.");
        }

    });
});

I also tried replacing the randInt line with this:

var randInt = Math.floor((Math.random() * ((randNumMax + 1) - randNumMin)) + randNumMin);

It still didn't work. I'm not sure if the algorithm is wrong or I'm incorporating it wrong into the function. An answer is appreciated, thanks.

Possible duplicate of Random between two numbers in Javascript — hippietrail
– hippietrail, Commented Aug 5, 2012 at 20:03
Does this answer your question? Generate random number between two numbers in JavaScript — Heretic Monkey
– Heretic Monkey, Commented Mar 26, 2020 at 19:30

Community · Accepted Answer · 2017-05-23 11:47:26Z

57

Generating random whole numbers in JavaScript in a specific range?

/**
 * Returns a random number between min and max
 */
function getRandomArbitary (min, max) {
    return Math.random() * (max - min) + min;
}

/**
 * Returns a random integer between min and max
 * Using Math.round() will give you a non-uniform distribution!
 */
function getRandomInt (min, max) {
    return Math.floor(Math.random() * (max - min + 1)) + min;
}

http://roshanbh.com.np/2008/09/get-random-number-range-two-numbers-javascript.html

//function to get random number upto m
function randomXToY(minVal,maxVal,floatVal)
{
    var randVal = minVal+(Math.random()*(maxVal-minVal));
    return typeof floatVal=='undefined'?Math.round(randVal):randVal.toFixed(floatVal);
}

or

Generate random number between two numbers in JavaScript

edited May 23, 2017 at 11:47

CommunityBot

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answered Apr 13, 2012 at 1:46

Ian

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6 Comments

Jack Davis Over a year ago

I tried the equation in the link in the function in the question, and it didn't work.

Ian Over a year ago

Not sure if you saw my updated answer or not, but there are multiple sources now. The first link/code is supposedly from the Mozilla Developer Center.

Ian Over a year ago

I have a feeling that the problem also is because you aren't converting the strings to ints. BUT this can be avoided by using the getRandomInt() function I provided above because of the order of operations. Since it completes "max - min" before adding "1", the result is an int (as long as max & min are valid numbers)...then the rest of the calculation is done as integers and should be accurate.

Jack Davis Over a year ago

Sorry, I don't understand the code above. I'm fairly new to javascript. Could you explain the process of converting it into a integer a little more simple for me?

Ian Over a year ago

The "+" operator works different for strings and integers. When you use "str" + "str", it concatenates them, combining them into "strstr" technically. When you use int + int, it adds them mathematically. When you use "str" + int, or int + "str", it concatenates them as a string because at least one of the arguments is a string. If you want to add a string and an int, you have to parseInt(Int). When you use the "-" operator, the behavior is to subtract numbers. It seems that by default, Jscript attempts to convert all arguments to numbers and if that fails, produces NaN, otherwise the result.

|

qw3n · Accepted Answer · 2012-04-13 02:59:40Z

2

Your problem is you never converted your string to numbers try adding this

if (  numCount.match(/^[\d]*$/ ) && 
    randNumMin.match(/^[\d]*$/ ) && 
    randNumMax.match(/^[\d]*$/ )){
  if (numCount === "" || randNumMin === "" || randNumMax === "")  {
    alert ("Please fill out all forms then try again.");
  } else {
    numCount=numCount-0;randNumMin=randNumMin-0;randNumMax=randNumMax-0;

Another note you need to change your checking if the value is an empty string to strict equality. To see what I mean try using zero for one of the values. 0 == ""//returns true because both are falsy 0 === ""//returns false.

answered Apr 13, 2012 at 2:59

qw3n

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4 Comments

Jack Davis Over a year ago

Would you know why my "Please make sure the first number is smaller than the second number" Not working when I make the first number anything but zero?

qw3n Over a year ago

@JackDavis I'm not sure what is wrong since it seems to be working fine for me. Here is a fiddle of what I have jsfiddle.net/YLxPr.

Jack Davis Over a year ago

Nevermind, I just had to convert to integers before all the ifs. Also, why weren't they integers in the first place?

qw3n Over a year ago

@JackDavis you were getting them from input boxes which means they are strings so you have to convert them to numbers.

rhigdon · Accepted Answer · 2014-04-07 20:46:48Z

2

Underscore.js has a nice utility for this _.random(min,max)

answered Apr 7, 2014 at 20:46

rhigdon

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Comments

mccambridge · Accepted Answer · 2012-04-13 01:52:26Z

1

How about this?

var max = 10;
var min = 4;
var random = Math.floor((Math.random() * ((max + 1) - min)) + min);

answered Apr 13, 2012 at 1:52

mccambridge

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3 Comments

Jack Davis Over a year ago

I implemented that in the function I posted in the question, and it still seemed to not work and give me numbers that were way to big.

mccambridge Over a year ago

I reread your issue, and this code is basically the same as what you've got -- just with some extra parenthesis for readability. Don't think the issue lies in the random generator.

Jack Davis Over a year ago

Right, I've learned that from the other comments. It's that I'm not converting into an integer.

Aaron Plocharczyk · Accepted Answer · 2019-12-02 03:04:35Z

0

randojs.com makes this a simple one-liner:

rando(4, 10)

You just need to add this to the head of your html document, and you can do pretty much whatever you want with randomness easily. Random values from arrays, random jquery elements, random properties from objects, and even preventing repetitions if needed.

<script src="https://randojs.com/1.0.0.js"></script>

edited Dec 2, 2019 at 3:04

answered Dec 2, 2019 at 2:44

Aaron Plocharczyk

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