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JavaScript JavaScript Loops, Arrays and Objects Simplify Repetitive Tasks with Loops A Closer Look at Loop Conditions

Richard Barkinskiy
Richard Barkinskiy
10,663 Points
Posted by Richard Barkinskiy
Richard Barkinskiy
10,663 Points

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

Just checking. Why is there a "+ 1" at the end of this function? Is it because we're looking for a number between 1 and 10, therefore without it, there's a chance the random number will be 0?

Thanks in advance.

3 Answers

lakindujayathilaka
lakindujayathilaka
15,244 Points

Exactly, if you want i will explain the entire line of code <blockquotes>

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

</blockquotes> First we have Math.random() the 'random()' method which acts on the 'Math' object creates a random number between zero and upto but not including one (example: 0.54864038856). Say if you have the number '6' stored in the variable 'upper', when you multiply the method with 'upper' like this Math.random() * upper you will generate a random number between zero and 6. but these are all decimal values (example: 5.464075) so inorder to round them down to the nearest integer we use Math.floor() method. Ok now we are done..but one small error is there...sometimes you will get zero as the random number to avoid this we add 1 at the end of the code like this Math.floor(Math.random() * upper) + 1; If i helped please rate my answer as the best

nelsongonzalezarango
nelsongonzalezarango
6,385 Points

This was a very good explanation, thank you!

Jason Anders
MOD
Jason Anders
Treehouse Moderator 145,858 Points
Jason Anders
Jason Anders
Treehouse Moderator 145,858 Points

Hi Richard,

That is correct. Without the `+1' there is a chance of it returning a zero.

For example (using 1 to 5), if the Math.random returns .1105 (as it will return a floating number from (and including 0) to (but NOT including) 1.

If you take that and multiply by 4 (Your high number minus your low number), you get 0.442. This is not between 1 and 5. But when you add 1, you now have 1.442. And, when you use .floor, it rounds down to 1 and not to ZERO.

I hope this makes sense. Keep Coding! :)

rdaniels
rdaniels
27,258 Points
rdaniels
rdaniels
27,258 Points

The math.random() function only gives a decimal value that is less than 1 (between 0 and .99999).
The Math.random() function returns a floating-point, pseudo-random number in the range [0, 1) that is, from 0 (inclusive) up to but not including 1 (exclusive), which you can then scale to your desired range. The implementation selects the initial seed to the random number generation algorithm; it cannot be chosen or reset by the user. The 1 being added makes sure that 0 is not a value being returned...