Why is this function creating an infinite looop?

Question

The first function determines if a number is prime. The second function is supposed to create an array with all prime numbers up to and including the max value, but it gives me an infinite loop for some reason.

function isPrime(num) {
  for (i = 2; i < num; i++) {
    if (num % i === 0) {
      return false
    }
  }
  if (num <= 1) {
    return false;
  }
  return true;
}

function primes(max) {
  var all = [];
  for (i = 2; i <= max; i++) {
    if (isPrime(i)) {
      all.push(i);
    }
  }
}

primes(17);

Related: [What is the purpose of the var keyword and when to use it (or omit it)?](https://stackoverflow.com/questions/1470488/what-is-the-purpose-of-the-var-keyword-and-when-to-use-it-or-omit-it) — Jonathan Lonowski, Jan 26 '17 at 05:53
Use strict mode and you won't have these problems because it will complain fast and loud. — Oriol, Jan 26 '17 at 05:59
If you want to get a list of primes, use a [sieve of Eratosthenes](https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes), will be faster. — Oriol, Jan 26 '17 at 06:00

score 3 · Answer 1 · answered Jan 26 '17 at 05:53

Your i variable is global, so both functions use the same i. This means the first function changes it while the second one is looping.

As the first function will have set i to num-1 when it finishes, and num was the value of i before executing it, it effectively decrements i with one. And so i will get the same value in the next iteration of the loop in the second function, never getting forward.

Solve this by putting the var keyword in both functions.

for(var i=2; // ...etc)

score 0 · Answer 2 · answered Jan 26 '17 at 05:58

0

The variable i in your two loops are global variables and they overwrite each other so the first loop never ends.

answered Jan 26 '17 at 05:58

Coding Pig

66
5

score 0 · Answer 3 · answered Jan 26 '17 at 06:10

In ur code problem is with the scope of variable i, In prime no calculation, u can check upto the sqrt of no, if no is not divisible by any no upto its sqrt, then it will a prime no, Try this:

function isPrime(num) {
  let k = Math.sqrt(num);
  for (let i = 2; i <= k; i++) {
    if (num % i === 0) {
      return false
    }
  }
  return true;
}

function primes(max) {
  let all = [];
  for (let i = 2; i <= max; i++) {
    if (isPrime(i)) {
      all.push(i);
    }
  }
  console.log(all)
}

primes(17);
primes(25);

Why is this function creating an infinite looop?

3 Answers3