110 lines
3.2 KiB
Markdown
110 lines
3.2 KiB
Markdown
---
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title: Extensible prime generator
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id: 598ee8b91b410510ae82efef
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challengeType: 5
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forumTopicId: 302262
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---
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## Description
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<section id='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.
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The generator should be able to:
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<ul>
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<li>Show the first <code>n</code> prime numbers</li>
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<li>Show the prime numbers in a range</li>
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<li>Show the number of primes in a range</li>
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<li>Show the <code>n<sup>th</sup></code> prime number</li>
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</ul>
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The function should have two parameters. The first will receive <code>n</code> 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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</section>
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## Instructions
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<section id='instructions'>
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</section>
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## Tests
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<section id='tests'>
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```yml
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tests:
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- text: <code>primeGenerator</code> should be a function.
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testString: assert(typeof primeGenerator === 'function');
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- text: <code>primeGenerator(20, true)</code> should return <code>[2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71]</code>.
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testString: 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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- text: <code>primeGenerator([100, 150], true)</code> should return <code>[101, 103, 107, 109, 113, 127, 131, 137, 139, 149]</code>.
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testString: assert.deepEqual(primeGenerator([100, 150], true), [101, 103, 107, 109, 113, 127, 131, 137, 139, 149]);
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- text: <code>primeGenerator([7700, 8000], false)</code> should return <code>30</code>.
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testString: assert.equal(primeGenerator([7700, 8000], false), 30);
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- text: <code>primeGenerator(10000, false)</code> should return <code>104729</code>.
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testString: assert.equal(primeGenerator(10000, false), 104729);
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```
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</section>
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## Challenge Seed
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<section id='challengeSeed'>
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<div id='js-seed'>
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```js
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function primeGenerator(num, showPrimes) {
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}
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```
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</div>
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</section>
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## Solution
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<section id='solution'>
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```js
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function primeGenerator(num, showPrimes) {
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let i,
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arr = [];
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function isPrime(num) {
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// try primes <= 16
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if (num <= 16) { return (
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num == 2 || num == 3 || num == 5 || num == 7 || num == 11 || num == 13
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); }
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// cull multiples of 2, 3, 5 or 7
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if (num % 2 == 0 || num % 3 == 0 || num % 5 == 0 || num % 7 == 0)
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{ return false; }
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// cull square numbers ending in 1, 3, 7 or 9
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for (let i = 10; i * i <= num; i += 10) {
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if (num % (i + 1) == 0) return false;
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if (num % (i + 3) == 0) return false;
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if (num % (i + 7) == 0) return false;
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if (num % (i + 9) == 0) return false;
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}
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return true;
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}
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if (typeof num === 'number') {
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for (i = 0; arr.length < num; i++) if (isPrime(i)) arr.push(i);
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// first x primes
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if (showPrimes) return arr;
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// xth prime
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return arr.pop();
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}
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if (Array.isArray(num)) {
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for (i = num[0]; i <= num[1]; i++) if (isPrime(i)) arr.push(i);
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// primes between x .. y
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if (showPrimes) return arr;
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// number of primes between x .. y
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return arr.length;
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}
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}
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```
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</section>
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