Could anyone explain to me the 2nd and 3rd statement in the for loop
What is the 1st and 2nd i in the middle part and the last one (i = i + 6)
function prime(n) {
if (n <= 1) return false;
if (n <= 3) return true;
if (n % 2 == 0 || n % 3 == 0) return false
for (let i = 5; i * i <= n; i = i + 6) {
if (n % i == 0 || n % (i + 2) == 0)
return false;
}
return true
}
console.log(prime(11))
console.log(prime(25))
console.log(prime(29))There are a couple of hacks in this function to determine primality of a number n.
n, we can safely say numbers like 9,15,21... will not divide n.The if inside the loop checks for divisibility of n by i or i + 2. Since we know, 3 | i+4 (-- from 2 above), we don't check divisibility there as it is not required. And since this loop runs from i = 5 upto i = sqrt(n), if we find no factors, we can exit the loop and return true for number's primality (-- from 1 above)