Math.random() explanation

Question

This is a pretty simple Java (though probably applicable to all programming) question:

Math.random() returns a number between zero and one.

If I want to return an integer between zero and hundred, I would do:

(int) Math.floor(Math.random() * 101)

Between one and hundred, I would do:

(int) Math.ceil(Math.random() * 100)

But what if I wanted to get a number between three and five? Will it be like following statement:

(int) Math.random() * 5 + 3

I know about nextInt() in java.lang.util.Random. But I want to learn how to do this with Math.random().

BTW: the range is from 0.0 inclusive to 1.0 exclusive (you won't actaully get 1.0 ever) Using nextInt() is a far better choice, not only is it simpler but also much faster. — Peter Lawrey, Nov 01 '11 at 08:23
Using `Math.ceil` is wrong, it gives the wrong result when `Math.random()` returns `0`. — starblue, Nov 02 '11 at 07:26
What if Math.floor returns 0.029? How to always get a two digit number with a single statement? — Naisheel Verdhan, Oct 03 '15 at 14:33

AusCBloke · Accepted Answer · 2011-11-01T02:42:03.290

164

int randomWithRange(int min, int max)
{
   int range = (max - min) + 1;     
   return (int)(Math.random() * range) + min;
}

Output of randomWithRange(2, 5) 10 times:

The bounds are inclusive, ie [2,5], and min must be less than max in the above example.

EDIT: If someone was going to try and be stupid and reverse min and max, you could change the code to:

int randomWithRange(int min, int max)
{
   int range = Math.abs(max - min) + 1;     
   return (int)(Math.random() * range) + (min <= max ? min : max);
}

EDIT2: For your question about doubles, it's just:

double randomWithRange(double min, double max)
{
   double range = (max - min);     
   return (Math.random() * range) + min;
}

And again if you want to idiot-proof it it's just:

double randomWithRange(double min, double max)
{
   double range = Math.abs(max - min);     
   return (Math.random() * range) + (min <= max ? min : max);
}

edited Nov 01 '11 at 02:42

answered Nov 01 '11 at 02:29

AusCBloke

18,014
6
40
44

1

What if you don't use `(int)` and want it to return a `double`? – switz Nov 01 '11 at 02:36
If you want double then just replace the `int`s with `double`s (and the typecast is unnecessary). I assumed you wanted `int`s but I'll add to my post. – AusCBloke Nov 01 '11 at 02:40
2

Actually with `double`s remove the `+ 1` also since `Math.random()` isn't being truncated. However, the range will be [min, max) since `Math.random` "Returns a double value with a positive sign, greater than or equal to 0.0 and less than 1.0." There'd be a very minimal chance of the number being exactly `max` anyway even if it was possible. – AusCBloke Nov 01 '11 at 02:47

Teezy7 · Answer 2 · 2019-04-15T13:32:20.077

45

If you want to generate a number from 0 to 100, then your code would look like this:

(int)(Math.random() * 101);

To generate a number from 10 to 20 :

(int)(Math.random() * 11 + 10);

In the general case:

(int)(Math.random() * ((upperbound - lowerbound) + 1) + lowerbound);

(where lowerbound is inclusive and upperbound exclusive).

The inclusion or exclusion of upperbound depends on your choice. Let's say range = (upperbound - lowerbound) + 1 then upperbound is inclusive, but if range = (upperbound - lowerbound) then upperbound is exclusive.

Example: If I want an integer between 3-5, then if range is (5-3)+1 then 5 is inclusive, but if range is just (5-3) then 5 is exclusive.

edited Apr 15 '19 at 13:32

answered May 18 '13 at 13:33

Teezy7

515
5
9

2

`upperbound` exclusive? I think it'll be inclusive. – vidit Sep 07 '16 at 14:55
1

@vidit it's exclusive – Jan 20 '17 at 22:49
1

don't think you're gettin the big picture upperbound can be either, as provided in the example – Teezy7 Nov 13 '18 at 10:19

score 19 · Answer 3 · edited Nov 01 '11 at 02:29

19

The Random class of Java located in the java.util package will serve your purpose better. It has some nextInt() methods that return an integer. The one taking an int argument will generate a number between 0 and that int, the latter not inclusive.

edited Nov 01 '11 at 02:29

trashgod

203,806
29
246
1,045

answered Nov 01 '11 at 02:15

G_H

11,739
3
38
82

I updated my question, I'd like to know how to do it with `Math.random()`. Thanks. – switz Nov 01 '11 at 02:17

score 1 · Answer 4 · answered Jun 21 '18 at 19:19

1

To generate a number between 10 to 20 inclusive, you can use java.util.Random

int myNumber = new Random().nextInt(11) + 10

answered Jun 21 '18 at 19:19

Ilya Gazman

31,250
24
137
216

score 0 · Answer 5 · answered Jun 12 '18 at 16:58

Here's a method which receives boundaries and returns a random integer. It is slightly more advanced (completely universal): boundaries can be both positive and negative, and minimum/maximum boundaries can come in any order.

int myRand(int i_from, int i_to) {
  return (int)(Math.random() * (Math.abs(i_from - i_to) + 1)) + Math.min(i_from, i_to);
}

In general, it finds the absolute distance between the borders, gets relevant random value, and then shifts the answer based on the bottom border.

Math.random() explanation

5 Answers5

Linked

Related