JavaScript Random Number [duplicate]

Question

This maybe more mathematical question than programming. In JS I wanted to a function that returns a random integer number in an interval lets say 1-6 and this is what I found:

// Returns a random integer between min and max
// Using Math.round() will give you a non-uniform distribution!
function getRandomInt(min, max) {
  return Math.floor(Math.random() * (max - min + 1)) + min;
}

I feel guilty if I copy and paste this in my code. I don't understand this : Why we subtract min from max, add 1, multiply the answer by Math.random() and then add the min. I tired with several numbers manually on paper and it work just fine ! But I don't understand why !

Answer 1

Assuming you already understand the behaviour of Math.floor and Math.random, here's the rest step by step:

• Math.random() ↝ a random number between 0 (inclusive) and 1 (exclusive)
• Math.random() * max ↝ a random number between 0 (inclusive) and max (exclusive)
• Math.floor(Math.random() * max) ↝ a random integer between 0 (incl.) and max (excl.)
• Math.floor(Math.random() * (max - min)) + min ↝ a random integer between min (incl.) and max (excl.)
• Math.floor(Math.random() * ((max + 1) - min)) + min ↝ a random integer between min (incl.) and max+1 (excl.) (OR between min and max both inclusive)