Need an explanation!!

Question

Here is my random number solution:
 var input = prompt("Insert starting number");
            var max = parseInt(input);
            var input1 = prompt("Second number");
            var min = parseInt(input1);
            var randomNumber = Math.floor(Math.random() * (max - min + 1)) + min;
            var res = randomNumber + " is between " + max + " and " + min;
            document.write(res);
but i didn't understand  this variable
var randomNumber = Math.floor(Math.random() * (max - min + 1)) + min;
can you explain me how it's work?

Joel Feaster · Accepted Answer

Math.random() returns a number from 0 to 1, not including one. Flooring this number will always result in a zero.
If you multiply this float by the max prior to flooring, you will get numbers all the way up to the maximum, including those below the minimum, but not the maximum itself. Say you have 10 as the maximum, you cannot get a 10. 
In order to rule out those below the minimum, you subtract the minimum from the maximum. In order to include the maximum, you add one. This is all multiplied by the random floating point number between 0 and 1.
At that point, we aren't quite there. You will be receiving numbers based on the range between the minimum and maximum as explained above. Adding the minimum back in, post-randomizing, ensure that you have numbers that are between the minimum and the maximum.
TLDR: The range of numbers (i.e. the number of numbers between min and max) is defined by those within the floor method's parentheses. The minimum added at the end ensures these numbers are between the desired minimum and maximum.

mhammad hamza · Answer

Thanks Joel. for this good explanation. Really thankful :)

Joel Feaster · Answer

This is similar to what we did before for the dice roll, only applying lower bounds to the number. 
By subtracting the minimum number from the maximum, you define the range. Say you have minimum of 5 and a maximum of 20, you will get the range of 15. There are 15 possible numbers that you could select at random between these bounds.
This is where you get your random number. Adding the minimum back on, post-randomizing, ensures that you don't need to mess with conditionals. It's much easier as such. Let me know if you need any clarifications!

Jason Anders · Answer

Hey Mhammad, 
Please see a previous answer of mine regarding random number generation in this post (https://teamtreehouse.com/community/will-this-randomnumber-generation-do-the-same-thing-without-future-problems-or-am-i-missing-something).
:)

mhammad hamza · Answer

I didnt understand why i am adding min in the last of this variable:
var randomNumber = Math.floor(Math.random() * (max - min + 1)) +min;

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mhammad hamza

mhammad hamza

Need an explanation!!

5 Answers

Joel Feaster

Joel Feaster

Joel Feaster

Joel Feaster

mhammad hamza

mhammad hamza

mhammad hamza

mhammad hamza

Jason Anders

Jason Anders