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JavaScript JavaScript Functions Arrow Functions Function Challenge Solution

Posted by Darrin Fagley
Darrin Fagley
12,432 Points

Where is the lower number?

Okay so I thought about this equation a little differently in my head. I am not fully understanding the explanations in some of the other questions about how the range is determined, but the math made sense to me if I thought about it like this:

For example, if we use 1 as the lower and 10 as the upper: Math.floor(Math.random() * (upper - lower) + 1) + lower;

So if we have 10 - 1 that equals 9. Math.random() will go up to but not include 9 so we add plus 1 giving us 10 in which case Math.random() gives us up to but not including 10. Then we add lower, which then allows the upper number to be included inclusively. I understood and got all that. However, while experimenting with the full code and different value sets, I ran across something I did not understand.

const getRandomNumber = (lower, upper) => { return Math.floor(Math.random() * (upper - lower) + 1) + lower; };

// Call the function and pass it different values console.log (getRandomNumber(1, 10)); console.log(getRandomNumber(1, 3));

Using the code above, which as far as I can tell is the same as in the video's except it is an arrow function, I ran the console.log(getRandomNumber(1, 10)); probably 50 times and got every number but the lowest number of 1, it never came up even once. This was further proven when I ran console.log(getRandomNumber(1,2); which should be either a 1 or a 2 and every single time I run it I only get 2. So I am confused as to how we are determining the lower number and why it is not being included in my tests?

Darrin Fagley
Darrin Fagley
12,432 Points

Blaize Pennington, Ah!!!!! I see it now. Thank you for clearing that up, it was definitely causing some confusion on my end.

1 Answer

Blaize Pennington
Blaize Pennington
13,879 Points
Blaize Pennington
Blaize Pennington
13,879 Points 
const getRandomNumber = (lower, upper) => { return Math.floor(Math.random() * (upper - lower) + 1) + lower; };

The problem you are having is with order of operations. Let's look at your final code without variables.

Math.floor(Math.random() * (50 - 1) + 1) + 1;
//First thing is the parentheses, so 50 - 1 = 49;
Math.floor(Math.random() * 49 + 1) + 1;
//This is where the error is, because of order of operations, you Multiply and Divide before you Add and Subtract. 
//So the next thing you do is calculate the Math.random(); And let's say you hit the lowest number possible.
Math.floor(0 + 1) + 1;
//This as you can see results in 2 being your answer, not 1

If you add the +1 inside the parentheses, it changes everything:

Math.floor(Math.random() * (50 - 1 + 1)) + 1;
//First thing is the parentheses, so 50 - 1 + 1 = 50;
Math.floor(Math.random() * 50) + 1;
//The lowest possible result
Math.floor(0) + 1;
0 + 1 = 1;
//The highest possible result
Math.floor(49.99999) + 1;
49 + 1 = 50