freeCodeCamp/curriculum/challenges/english/02-javascript-algorithms-and-data-structures/intermediate-algorithm-scripting/sum-all-primes.english.md

id, title, isRequired, challengeType, forumTopicId

id	title	isRequired	challengeType	forumTopicId
a3bfc1673c0526e06d3ac698	Sum All Primes	true	5	16085

Description

A prime number is a whole number greater than 1 with exactly two divisors: 1 and itself. For example, 2 is a prime number because it is only divisible by 1 and 2. In contrast, 4 is not prime since it is divisible by 1, 2 and 4.

Rewrite sumPrimes so it returns the sum of all prime numbers that are less than or equal to num.

Remember to use Read-Search-Ask if you get stuck. Try to pair program. Write your own code.

Instructions

Tests

tests:
  - text: <code>sumPrimes(10)</code> should return a number.
    testString: assert.deepEqual(typeof sumPrimes(10), 'number');
  - text: <code>sumPrimes(10)</code> should return 17.
    testString: assert.deepEqual(sumPrimes(10), 17);
  - text: <code>sumPrimes(977)</code> should return 73156.
    testString: assert.deepEqual(sumPrimes(977), 73156);

Challenge Seed

function sumPrimes(num) {
  return num;
}

sumPrimes(10);

Solution

function eratosthenesArray(n) {
    var primes = [];
    if (n > 2) {
        var half = n>>1;
        var sieve = Array(half);
        for (var i = 1, limit = Math.sqrt(n)>>1; i <= limit; i++) {
            if (!sieve[i]) {
                for (var step = 2*i+1, j = (step*step)>>1; j < half; j+=step) {
                    sieve[j] = true;
                }
            }
        }
        primes.push(2);
        for (var p = 1; p < half; p++) {
            if (!sieve[p]) primes.push(2*p+1);
        }
    }
    return primes;
}

function sumPrimes(num) {
  return eratosthenesArray(num+1).reduce(function(a,b) {return a+b;}, 0);
}

sumPrimes(10);