Sum all primes...Challenge

JavaScript
1

Hey, everything went nearly smooth, till last few challenges im dealing with.

function sumPrimes(num) {
  var numbers = [];

 
    for (var i = 2; i <= num; i++) {
      if (isPrimeNumber(i)) {
        
        numbers.push(i);
      }
    }

 return numbers.reduce(function(a, b) {
  return a + b;

});
    
  }

    function isPrimeNumber(num) { 

    for (var x = 2; x < num; x++) {
    if(num % x === 0) {

     return false;
    }


  }

  return true;
}


sumPrimes(10);

Can anyone PLEASE explain the logic behind this? how those two function match each other? how the loop passes results…I really dont get this one

2

For the IsPrime function
The fundamental thing is

if(num % x === 0)

this checks if the paramter num is evenly divisible by x. I.e. if the remainder when dividing is 0. If this is the case then by the definition of a prime number it can’t be a prime number.

The loop checks if this condition is true for any number from 2 to num-1.
If no such number divides the argument num, then it is a prime.

N.b. that this is a horribly inefficient prime checker.
https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes is much much faster.

3

okay, so we have a num = 10.

if(10 % 2 === 0) remainder = 2…So this is true… What now? this 2 goes to the first function like this:
if(isPrimeNumber(i) === if(2(2)…This is the same? so now the 2 goes to numbers… And now we have

if(10 % 3 === 0) remained = 1…
if(isPrimerNumber(3) === if(1(3)…Please explain a bit further of hove those functions match and which number they pass each other…I understand the rest

4

Ok, sumPrimes(10)

First thing that happens is the for loop
for(var i = 2; i <= 10; i++)

So inside this loop the calls are
isPrime(2)
isPrime(3)
isPrime(4)
isPrime(5)
isPrime(6)
isPrime(7)
isPrime(8)
isPrime(9)
isPrime(10)

isPrime(2) results in the for loop
for(var x = 2; x < 2 x++) {
} i.e. it never runs
and thus isPrime will return true.

Therefore 2 will be pushed to the array numbers

isPrime(3)
will result in the prime
for(var x=2; x<3; x++) {

}
so it will check if 3 is divisible by 2, which it isn’t (the remainder is 1), therefore isPrime(3) will return true and 3 will be pushed (added) to the array numbers.

isPrime(4) results in the for loop
for(var x=2; x <4;x++) {
}
i.e it will check if 4 is divisible by 2 or 3.
Since 4 is divisible by 2 (remainder 0) the function will return false, and 4 will not be pushed to the array numbers.

This will continue on.

1 Like
5

Thank you very much.