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Joseph Puchalik
Courses Plus Student 4,029 PointsSlight differences in JS from the "official" answer... can someone review and explain if mine is flawed and why?
This is JS I came up with on my own (which I was quite happy with). It SEEMS to work. But I haven't had time to enter a couple million different numbers into it to see if it generates a number outside the range...
var rangeFloor = parseInt( prompt('Enter the low number in your range:') );
var rangeCeiling = parseInt( prompt('Enter the high number in your range:') );
var numGen = Math.ceil( Math.random() * (rangeCeiling - rangeFloor) ) + rangeFloor;
document.write('Your random number is: ' + numGen);
2 Answers
Colin Bell
29,679 PointsVery solid. However, you're going to want to use Math.floor instead of Math.ceil() in your numGen variable. While Math.random() can technically generate a 0, the actual chances of that happening are extraordinarily slim.
So your random number generation is for all intents and purposes generating a number between 1 to rangeCeiling instead of 0 to rangeCeiling and then adding that to your rangeFloor. If you're trying to find a random number within a large range you'll probably never notice, but it becomes easily noticeable in small ranges. For example, if you're trying to find a random number between 1 and 2 would always be 2, a random number between 4 and 5 would always be 5, etc.
var rangeFloor = 4;
var rangeCeiling = 5;
Math.ceil ( Math.random() * (rangeCeiling - rangeFloor) ) + rangeFloor;
// Math.ceil ( Math.random() * (5 - 4) ) + 4;
// Doesn't matter if Math.random() * ( 5 - 4 ) is equal to .00001 or .999999,
// Math.ceil() will always push that up to 1
// So that plus the rangeFloor will always === 5 in this case.
Math.random() will never return 1, so getting the Math.floor on the range plus 1 then adding the rangeFloor to that will find a true random number between your rangeFloor and your rangeCeiling
var rangeFloor = 1;
var rangeCeiling = 2;
Math.floor ( Math.random() * (rangeCeiling - rangeFloor + 1) ) + rangeFloor;
So if Math.random() generates 0.49 then 0.49 * ( 2 - 1 + 1)) === 0.98. The floor of that is 0. Add 1 and you get 1.
if Math.random() generates 0.99 then 0.5 * ( 2 - 1 + 1)) === 1.98. The floor of that is 1. Add 1 and you get 2.
Joseph Puchalik
Courses Plus Student 4,029 PointsOK, thank you for your help. I had considered small ranges but I never tested or considered ranges where the rangeFloor and rangeCeiling were consecutive numbers.
Colin Bell
29,679 PointsI just used consecutive numbers to easily show it. If you were to use 4 as the rangeFloor and 6 as the rangeCeiling you'd only get 5 or 6, never 4. With a rangeFloor and rangeCeiling 2 and 7, you'd only ever get 3, 4, 5, 6, or 7. With Math.ceil() you'd (almost) never get the number you put in as the rangeFloor - unless Math.random() returned its mythical 0.