Missing last element in array

Question

I'm trying to sum all primes up to a number.

First I removed all evens and pushed the rest to an odd-only array.

Then I'm going over the array and splicing all the numbers that divide by other numbers but 1 and themselves, and replacing them with zero.

Everything seemed to go okay, however, notice in my first console.log that the last element is 977 (which is the num passed).

A line later I forEach the array and print all numbers over 970 but 977 isn't there.

Any ideas on how this happened? (I ruled out voodoo..)

function sumPrimes(num) {
  
  var arr = [2];
  for (var i = 3; i <= num; i++) {
    if (i % 2 !== 0) {
      arr.push(i);
    }
  }

  console.log(arr);
  arr.forEach(function(x) {
    
    if(x > 970){
      console.log(x);
    }
    
    if (x > 3) {
      for (var j = 3; j < x; j += 2) {
        if (x % j == 0) {
          arr.splice(arr.indexOf(x), 1, 0);
        }
      }
    }
  })

  // console.log(arr);
  var res = arr.reduce(function(acc, val) {
    return acc + val;
  }, 0)
  console.log(res);
}

sumPrimes(977);

Answer 1

The issue is how you're manipulating the array and the array never actually ends up containing 977.

What happens is you find a non-prime and alter the array in place so that it's now 0 but you continue to run checks on that prime.

This causes any following indexOf lookup for that value to return -1 and hence you get unexpected results.

if (x % j == 0) {
   arr.splice(arr.indexOf(x), 1, 0); // This won't work after the first assign to 0
}

You can easily fix this by inserting a 'break;' immediately following the splice to short circuit the loop, however you can easily simplify & dramatically speed this up using the index parameter available in a forEach to remove the array position lookup entirely.

function sumPrimes(num) {

  var arr = [2];
  for (var i = 3; i <= num; i++) {
    if (i % 2 !== 0) {
      arr.push(i);
    }
  }

  console.log(arr);
  arr.forEach(function(x, idx) {

    if(x > 970){
      console.log(x);
    }

    if (x > 3) {
      for (var j = 3; j < x; j += 2) {
        if (x % j == 0) {
          arr[idx] = 0;
          break;
        }
      }
    }
  })

  // console.log(arr);
  var res = arr.reduce(function(acc, val) {
    return acc + val;
  }, 0)
  console.log(res);
}

sumPrimes(977);

---

EDIT: If anyone wants a simple, more efficient method of determining whether a number is prime or not, I suggest using the Trial Division method, not the one above.

You can find a succinct, well tested example on Rosetta code:

function isPrime(n) {
  if (n == 2 || n == 3 || n == 5 || n == 7) {
    return true;
  } else if ((n < 2) || (n % 2 == 0)) {
    return false;
  } else {
    for (var i = 3; i <= Math.sqrt(n); i += 2) {
      if (n % i == 0)
        return false;
    }
    return true;
  }
}

console.log(isPrime(977));