Checking the last digit of an integer value in JavaScript/p5

Question

So I'm coding something in p5.js to check whether a number is prime or not. Here's the function:

function prime(n) {   
let P = true;   
    for (let i = 2; i <= sqrt(n); i++) {
        if (n%i == 0) {
          P = false; break;
        } else {
          P = true; break;
        }   
    } return P; 
}

Everything works fine, except for the i <= sqrt(n) and if(n%i == 0) bits...
The % operator gives you the remainder of a/b, but when i type in:

prime(integer that ends with 5)

into the console, it returns true, when it should return false.
So I want to add a codtition that if the last digit of n is 5, return false. how do I do that?

Thanks!

Answer 1

Here is the working code for prime number

function prime(n) {
  var sqrtnum=Math.floor(Math.sqrt(n));
    var prime = n != 1;
    for(var i=2; i<sqrtnum+1; i++) { // sqrtnum+1
        if(n % i == 0) {
            prime = false;
            break;
        }
    }
    return prime;
}

Answer 2

Just remove break keyword in the else part of your code .

function prime(n) {   
let P = true;   
    for (let i = 2; i <= Math.sqrt(n); i++) {
        if (n%i == 0) {
          P = false; break;
        } else {
          P = true; 
        }   
    } return P; 
}
console.log(prime(25)?"prime":"not prime")

Answer 3

There are many optimisations you could implement here. For example you could exclude all even numbers from your testing loop. This is only a start. The sieve of Erastostenes provides guidance on further improvements.

function prime(n) {
 if (!n%2) return false;
 for (let i=3;i<=Math.sqrt(n); i+=2) 
  if (n%i == 0) return false
  return true
}
[47,48,1005,234,239].forEach(n=>
 console.log(n,prime(n)))

In this variation of the script the number is actually decomposed into its prime number divisors:

function decomp(n) {
 const comp=[];
 if (n%2==0) return [2,...decomp(n/2)];
 for (let i=3;i<=Math.sqrt(n); i+=2) 
  if (n%i==0) return [i,...decomp(n/i)];
 return [n];
}
function notPrime(n){
 const comps=decomp(n);
 return comps.length>1?comps:false;
}
[47,48,1005,1001,234,239,4968417].forEach(n=>
 console.log(n,notPrime(n)))