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Joe Williams
4,014 PointsIf I can't teach it, I know I don't really grasp it. What is actually happening in this function?
I understand that this function is creating a random number between 2 numbers -- but when I actually look at the formula, I don't understand what's happening. Can anyone explain this in a way I can grasp?
function getRandomNumber ( lower, upper ) {
return Math.floor(Math.random() * (upper - lower + 1)) + lower ;
}
console.log( getRandomNumber ( 1, 6 ));
console.log( getRandomNumber ( 100, 1000));
1 Answer
Ken Alger
Treehouse TeacherJoe;
Let's see if I can be of some help, hopefully more help than confusion anyway. We'll try to look at this step by step mathematically.
Given:
return Math.floor(Math.random() * (upper - lower + 1)) + lower ;
Let's take a look at the second example, 100 upper, 10 lower
- Start inside the first set of parenthesis
(upper - lower + 1), (100-10+1) = 91. - Multiply that by the
Math.randomfunction, which returns a floating-point, pseudo-random number in the range [0, 1) that is, from 0 (inclusive) up to but not including 1 (exclusive). For the sake of our discussion, let's say that it returns 0.2028994421681174. Times our "randomly" generated number and our range we get approximately 18.46. - Apply
Math.floorto that and we get 18. - Add 10 and we get 28.
So why go through all of that is your question, right? The upper and lower variables set the range, multiplying it by random will generate a random number, which multiplied by our range calculation puts it above 0. We add that to our lower variable to make sure that it falls within our range.
Let's look at another example, if our generated number is significantly small, like .00202899. We would get approx 0.1846. Round that down we're at 0, add in our lower variable and our getRandomNumber function would return our lower variable.
Hopefully that helped more than it hindered.
Ken
Jim Withington
12,025 PointsHey: did you mean "4. Add 10 and you get 28" ?
Otherwise, this helped me make sense of it, thanks!
Ken Alger
Treehouse TeacherJim;
Yes, I just edited my post to fix that error. Not sure how I got 19 from 10+18 originally. Nice catch.
Ken
Joe Williams
4,014 PointsJoe Williams
4,014 PointsWhere I'm getting confused is, I understand what's happening operationally. Lower and upper are being passed into the function, and console.log is showing the range between 1-6, and 100-1000. But where I get confused is looking at the number. Subtracting the lower number from the higher number, adding +1 to it, multiplying that with a random number, rounding it down ... this is where I'm getting lost.