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JavaScript JavaScript Functions Arrow Functions Function Challenge Solution

Posted by Dinu Comendant
Dinu Comendant
6,049 Points

What's the point?

I don't understand what's the point to write

Math.floor(Math.random() * (upper - lower + 1)) + 1;

When I get the same results with:

Math.floor(Math.random() * upper) + lower;

2 Answers

Peter Vann
Peter Vann
36,425 Points

Hi Dinu!

The actual formula is:

const rand = Math.floor(Math.random() * (upper - lower + 1)) + lower;

Which give you random numbers between and including the upper and lower values.

Your way:

const rand = Math.floor(Math.random() * upper) + lower;

Produces some values above/higher/larger than upper.

Try it with different upper and lower values, such as 17 and 13, or 20 and 15.

I hope that helps.

Stay safe and happy coding!

ja5on
ja5on
10,338 Points

why is there a + lower at the end?
const rand = Math.floor(Math.random() * (upper - lower + 1)) + lower;

In all my time here it still doesn't make sense! upper say is 6, - lower say 3 = 3. + 1 = 4 + lower again!! which is now 4+3 = 7??

Thurston Daly
seal-mask
.a{fill-rule:evenodd;}techdegree
Thurston Daly
Full Stack JavaScript Techdegree Student 11,810 Points
Thurston Daly
.a{fill-rule:evenodd;}techdegree
Thurston Daly
Full Stack JavaScript Techdegree Student 11,810 Points

Hey jas0n,

You're missing the order of operations and that the formula is multiplying against Math.random() first, which generates a random number using a floating point number. Math.floor() then rounds down the product of Math.random() and (upper - lower + 1) to either the closest lower number or equal to the generated number (if its whole). Lastly, the final + lower is increasing the produced number to be inside the range your upper and lower numbers.