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Start your free trial###### Chelsea Crisp

4,824 Points# Math.floor( Math.random () * (max - min + 1)) + min) Explanation

Can someone just explain the logic behind this math/function? What does the asterisk signify? Why the + 1?

###### Chelsea Crisp

4,824 PointsI understand the asterisk means multiply but what is being multiplied?

###### Jason Anello

Courses Plus Student 94,610 PointsThe return value of Math.random() is being multiplied with the result of (max - min + 1)

Let us know if Chris's update clears it up for you.

## 6 Answers

###### Jason Anello

Courses Plus Student 94,610 PointsHi Chelsea,

Maybe it will help to see how the formula can be derived starting only with Math.random() and Math.floor(). That way you'll know that it's correct and more importantly why it's correct.

I'll try to provide a conceptual understanding of what's going on with the formula.

We know that Math.random() returns a value in the range [0, 1). 0 is included but 1 is excluded. You can think of it as the range 0 to 0.999999...

Now let's pass that result to the Math.floor() function.

```
Math.floor(Math.random());
```

Since `floor`

will truncate the decimal and give you back the whole number part we're always going to get 0 here. Make sure you understand that or the rest isn't going to make any sense. Ask questions if you need to.

We can only get 1 integer out of this. It's not so much important that the integer is 0 but that we can only get 1 integer out.

If we are going to be able to get more numbers out of this then we need to make that range bigger. When you multiply something by 2 it becomes twice as big as it was before. It scales up by a factor of 2. Let's see what happens when we multiply Math.random() by 2

```
Math.floor(Math.random() * 2);
```

That will give us a new range of 0 to 1.999999... which is twice as big as the range we started with.

What happens when those numbers are passed into Math.floor? All the numbers generated from 0 to 0.9999... will be truncated to 0 and all the numbers from 1 to 1.9999... will be truncated to 1

Now we're able to get 2 different integers out of this. If we multiply by 2 we can get 2 numbers back. It stands to reason that if we multiply by 6 then we will be able to get 6 numbers out.

That will give us a range that's 6 times as big, 0 to 5.99999.....

I won't write it all out but after passing through the floor function you would get

```
0 to .99999... -> 0
1 to 1.99999... -> 1
...
5 to 5.99999... -> 5
```

In general, whatever you multiply Math.random() by is how many integers you'll be able to generate.

Now we can start deriving the formula and I'll use a specific example to help.

Let's say we want to generate numbers from 5 to 10 inclusive. We need to know how many numbers are there.

Setting up the variables -

```
var max = 10;
var min = 5;
```

If we list them out, 5, 6, 7, 8, 9, 10 and count them we see that there are 6 total numbers. We know from before that we're going to have to multiply by 6 in order to get 6 numbers out.

How can we come up with the 6 using our max and min variables?? If I do `max - min`

I get 5 which is 1 short. max - min gives you the distance from 5 to 10. You always have to add 1 to that if you want the total amount of numbers.

That gives us the expression `max - min + 1`

Putting that into the formula,

```
Math.floor(Math.random() * (max - min + 1));
```

It's important that `max - min + 1`

is enclosed in parentheses so that all of that happens before the multiplication.

At this point the formula can generate the correct amount of numbers but they always start at 0 because the range from Math.random starts from 0.

```
0, 1, 2, 3, 4, 5 // What we have
5, 6, 7, 8, 9, 10 // What we want
```

Notice that if we add 5 to all the numbers in the first row, we'll get the second row. 5 is what our min value is in the example.

So if we add the min value onto the end of our formula, it will shift all the numbers over to the ones we want.

```
Math.floor(Math.random() * (max - min + 1)) + min;
```

You can think of it as a 2 step operation, you scale up the range, and then you shift it.

I meant for this to be shorter but I hope it helps you out.

###### Chelsea Crisp

4,824 PointsThis was the perfect explanation. I totally get it now, thank you so much!

###### Gerard Dlima

11,908 PointsGreat Explanation.Thanks!

###### Niklas Albinsson

6,045 PointsThank you for this answer, perfectly explained.

###### Meli N.H.

Courses Plus Student 1,656 PointsPerfect explanation.

###### Andrew Denson

4,544 PointsI just posted a question about this. Guess i should have looked for the answer first. Glad I found it though.

Thanks for answer Jason. Big help.

###### jennyellis

12,231 PointsYour explanation was awesome. Thank you so much for taking the time to describe this so well!

###### Chris Davis

16,280 PointsMath.random() generates a random floating point number (decimal) from 0 up to but not including 1 - Example - .25

```
var myRand = Math.random();
console.log(myRand);
```

When you combine Math.random() * 10 you are saying, give me a random floating point number from 0 up to but not including 10 - Example - 4.25

```
var myRand = Math.random() * 10;
console.log(myRand);
```

When you combine Math.floor(Math.random() * 10) you are asking for a random floating point number from 0 up to but not including 10 then Math.floor() will round down to a whole number - Example - 4

```
var myRand = Math.floor(Math.random() * 10);
console.log(myRand);
```

When you combine Math.floor(Math.random() * 10) + 1 you are saying give me a random whole number from 0 up to but not including 10, then the +1 ensures that you receive a random number from 1 - 10

```
var myRand = Math.floor(Math.random() * 10) + 1;
console.log(myRand);
```

Max and min is your range, say you want to generate a random whole number from 2 (min) to 10 (max). Let me see if i can explain this! Math.floor(Math.random() * (max - min + 1)) is generating a whole number between the range of 0 to 8. The (max - min + 1) is the logic that creates the range. When you add the + min at the end you are adding the +2 to your range and end up with a random number from 2 to 10. It took me a little while to wrap my head around this logic concept, however it becomes easier to understand the more you use it.

```
var myRand = Math.floor(Math.random () * (max - min + 1) + min);
console.log(myRand);
```

I hope this helps a little bit. Here is an example I have updated the fiddle to include the final logic

###### Jason Anello

Courses Plus Student 94,610 Points*changed from comment to answer*

###### Chelsea Crisp

4,824 PointsThanks! That was very helpful. Although I still don't understand the "max - min" before the + 1 or the "+ min" at the end.

This was a formula given to me during an random number generator exercise by treehouse.

###### Chelsea Crisp

4,824 Points"Math.floor(Math.random() * (max - min + 1)) is generating a whole number between the range of 0 to 8."

How are you getting 8? 10 - 2 + 1 = 9? Are you adding 2 to 9 and 0 to get a range between 2-10? The math adds up to 11 but that way 10 would be included because it's up to but not including 11?

Also, why can't you just say something like Math.floor(Math.random() * (max - 1) + min)?

###### Chris Davis

16,280 PointsI have updated my explanation to hopefully help you understand generating a random whole number within a range :)

###### Adam Maley

5,946 PointsThis is confusing...

var myRand = Math.floor(Math.random () * (max - min + 1) + min); If I insert Math.random= 0.5 as the number generated. Max=10 Min=1 Is this the correct thinking?

```
var myRand = Math.floor(0.5 * (10 - 1 + 1) + min);
var myRand = Math.floor(0.5 * (9 + 1) + min);
var myRand = Math.floor(0.5 * (10) + min);
var myRand = Math.floor(5 + min);
var myRand = Math.floor(6); <-------- In my case because of the 0.5 it happened to be already at an Integer of 6?
var myRand = 6
```

###### Jason Anello

Courses Plus Student 94,610 PointsHi Adam,

Yes, that's correct. If Math.random() generated 0.5 in this scenario then you would get a 6.

All the numbers that Math.random() generates from 0.5 to 0.59999... would result in a final value of 6

###### Lucas Guimarães

**Front End Web Development**Techdegree Student 3,900 Points

This kind of explanation you don`t see everywhere .... well done

###### Louise Iyengar

**Full Stack JavaScript**Techdegree Graduate 21,991 Points

So glad I found this! Excellent explanation.

## Jason Anello

Courses Plus Student 94,610 Points## Jason Anello

Courses Plus Student 94,610 PointsThe asterisk means multiplication.