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---
id: 598ee8b91b410510ae82efef
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title: Extensible prime generator
2018-10-10 22:03:03 +00:00
challengeType: 5
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forumTopicId: 302262
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dashedName: extensible-prime-generator
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---
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# --description--
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Write a generator of prime numbers, in order, that will automatically adjust to accommodate the generation of any reasonably high prime.
The generator should be able to:
< ul >
< li > Show the first < code > n< / code > prime numbers< / li >
< li > Show the prime numbers in a range< / li >
< li > Show the number of primes in a range< / li >
< li > Show the < code > n< sup > th< / sup > < / code > prime number< / li >
< / ul >
The function should have two parameters. The first will receive `n` or the range as an array. The second will receive a boolean, that specifies if the function returns the prime numbers as an array or a single number(the number of primes in the range or the < code > n< sup > th</ sup ></ code > prime). According to the parameters the function should return an array.
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# --hints--
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`primeGenerator` should be a function.
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```js
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assert(typeof primeGenerator === 'function');
```
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`primeGenerator(20, true)` should return `[2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71]` .
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```js
assert.deepEqual(primeGenerator(20, true), [
2,
3,
5,
7,
11,
13,
17,
19,
23,
29,
31,
37,
41,
43,
47,
53,
59,
61,
67,
71
]);
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```
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`primeGenerator([100, 150], true)` should return `[101, 103, 107, 109, 113, 127, 131, 137, 139, 149]` .
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```js
assert.deepEqual(primeGenerator([100, 150], true), [
101,
103,
107,
109,
113,
127,
131,
137,
139,
149
]);
```
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`primeGenerator([7700, 8000], false)` should return `30` .
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```js
assert.equal(primeGenerator([7700, 8000], false), 30);
```
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`primeGenerator(10000, false)` should return `104729` .
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```js
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assert.equal(primeGenerator(10000, false), 104729);
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```
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# --seed--
## --seed-contents--
```js
function primeGenerator(num, showPrimes) {
}
```
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# --solutions--
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```js
function primeGenerator(num, showPrimes) {
let i,
arr = [];
function isPrime(num) {
// try primes < = 16
if (num < = 16) { return (
num == 2 || num == 3 || num == 5 || num == 7 || num == 11 || num == 13
); }
// cull multiples of 2, 3, 5 or 7
if (num % 2 == 0 || num % 3 == 0 || num % 5 == 0 || num % 7 == 0)
{ return false; }
// cull square numbers ending in 1, 3, 7 or 9
for (let i = 10; i * i < = num; i += 10) {
if (num % (i + 1) == 0) return false;
if (num % (i + 3) == 0) return false;
if (num % (i + 7) == 0) return false;
if (num % (i + 9) == 0) return false;
}
return true;
}
if (typeof num === 'number') {
for (i = 0; arr.length < num ; i + + ) if ( isPrime ( i ) ) arr . push ( i ) ;
// first x primes
if (showPrimes) return arr;
// xth prime
return arr.pop();
}
if (Array.isArray(num)) {
for (i = num[0]; i < = num[1]; i++) if (isPrime(i)) arr.push(i);
// primes between x .. y
if (showPrimes) return arr;
// number of primes between x .. y
return arr.length;
}
}
```