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JavaScript JavaScript Foundations Numbers The Math Object

Posted by Juan Sosa
Juan Sosa
4,572 Points

To generate a random number between 0 and 20.

The real answer must be Math.floor(Math.random()*21)

index.html 
<!DOCTYPE html>
<html lang="en">
  <head>
    <title>JavaScript Foundations: The Math Object</title>
    <link rel="stylesheet" href="core.css">
    <script>
      var diameter = 5.75,
          a = 17,
          b = 42,
          c = 1337,
          ageIfILiveToYear2100 = (new Date(2100, 0, 1) - new Date(1995, 5, 16, 7, 20)) / (1000 * 60 * 60 * 24 * 365.242);

      var circumference = Math.PI * diameter;
      var chance = Math.floor(Math.random()*20);

      /*
      The JavaScript Math object has the following
      constant (among others):
      Math.PI
          Approximately 3.14159. Pi (π) is used
          to determine the circumference of a circle
          given its diameter by multiplying the diameter
          by π. Geometery refresher: the circumference
          of a circle is the distance around the outside
          edge. The diameter is the width of the circle.

      The JavaScript Math object also has the following
      methods (among others):
      Math.random()
          Returns a random number between 0 and 1
      Math.max(a, b, ...)
          Returns the largest value of the passed
          arguments
      Math.floor(a)
          Returns the value of a number rounded down to
          the nearest integer
      */



    </script>
  </head>
  <body>
    <h1>JavaScript Foundations</h1>
    <h2>The Math Object</h2>
    <script src="viewer.js"></script>
  </body>
</html>
Juan Sosa
Juan Sosa
4,572 Points

Math.floor(Math.random()*20), generate numbers between 0 and 19, not 20. Math.round(Math.random()*20), generate numbers between 0 and 20, but the numbers 0 and 20 have half of probability than the others.

The best respond must be Math.floor(Math.random()*21).

2 Answers

Marcus Parsons
Marcus Parsons
15,719 Points
Marcus Parsons
Marcus Parsons
15,719 Points

The problem with the MDN implementation is that you would still have to use 21 in order to get a value from 0-20, because just as the article states: Returns a random number between min (inclusive) and max (exclusive).

The easy solution to make the max inclusive is just to add 1 inside of the Math.floor statement like so:

function getRandomInt(min, max) {
//Will return a number inside the given range, inclusive of both minimum and maximum
//i.e. if min=0, max=20, returns a number from 0-20
  return Math.floor(Math.random() * (max - min + 1)) + min;
}
Cena Mayo
Cena Mayo
55,236 Points
Cena Mayo
Cena Mayo
55,236 Points

This code covers all possible ranges, without requiring you to hard code a value (such as 21):

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

This is from the MDN page on Math.random.