When you have Math.floor(Math.random()*10)+1 its supposed to pick a random number between 1-10 from what I understand.

However, when I change the +1 to any number higher or lower then 1 I get the same result. Why is this? What does the +1 mean exactly?

When you have Math.floor(Math.random()*10)+1 its supposed to pick a random number between 1-10 from what I understand.

However, when I change the +1 to any number higher or lower then 1 I get the same result. Why is this? What does the +1 mean exactly?

• javascript

• 2 If you add something more than +1 and you still end up with numbers between 1-10 something is very wrong. – Dave Newton Commented Jun 27, 2012 at 14:10
• What exactly do you mean by "same result"? can you clarify or post some output? – Salman Arshad Commented Jun 27, 2012 at 14:11
• (To clarify--if you always and only get numbers between 1-10. Obviously any given run might only produce 1-10, but over time, this will almost certainly not be the case.) – Dave Newton Commented Jun 27, 2012 at 14:17
• "when I change the +1... I get the same result." This question is based on a false premise. I think you know what adding 1 does, but you have some other issue in your code. If you have such an issue, please post your actual code. – user1106925 Commented Jun 27, 2012 at 14:17
• @DaveNewton: dilbert.com/strips/comic/2001-10-25 – user1106925 Commented Jun 27, 2012 at 14:22

4 Answers 4

The random number generator produces a value in the range of 0.0 <= n < 1.0. If you want a number between 1 and something you'll need to apply a +1 offset.

Generally you can use:

Math.floor(Math.random() * N) + M

This will generate values between M and M + N - 1.

demo Fiddle

Math.random() generates a random number between 0 and 1.

Therefore Math.random()*10 generates a random number between 0 and 10, and (Math.random()*10)+1 a number between 1 and 11.

Math.floor() drops the decimal of this number, and makes it an integer from 0 to 10.

You can see a sequential progression of the logic here

Whole number is between 1 and 10

• Math.random() generates number between 0 and 1 (with many decimal places).

etc: math.random() [randomly return : 0.19157057767733932]
(this number will still have many decimal places)

To get a random whole number, you need to multiply random generated number by 10.

etc
math.random()*10 =[randomly return : 2.9757621488533914]

• Since the number will still have many decimal places, use floor() method to round a number down/nearest integer, this will give u a value between 0 and 9. Then, u can add 1 to make it a number between 1 and 10.

etc:
math.floor(0.6)   [return 0 ]
math.floor(0.6)+ 1   [return 1]

Basic:

(random() >= 0) always true

(random() < 1) always true

(Math.floor(random()) == 0) always true

Maximum:

(Math.floor(random() * 10) >= 0) always true

(Math.floor(random() * 10) < 10) always true

Minimum:

(Math.floor(random() * 10) + 1 >= 1) always true

(Math.floor(random() * 10) + 1 < 11) always true

Max Round:

(Math.round(random() * 10, 0) >= 0) always true

(Math.round(random() * 10, 0) <= 10) always true