e4a229187e
Either improved the algorithm or reduced the difficulty for the slowest tests. All these pass with testProtectTimeout = 500, so should be safe on most machines with the current timeout of 2500.
106 lines
3.0 KiB
Markdown
106 lines
3.0 KiB
Markdown
---
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title: Emirp primes
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id: 599d0ba974141b0f508b37d5
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challengeType: 5
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forumTopicId: 302253
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---
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## Description
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<section id='description'>
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An emirp (<strong>prime</strong> spelled backwards) are primes that when reversed (in their decimal representation) are a different prime.
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</section>
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## Instructions
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<section id='instructions'>
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Write a function that:
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<ul>
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<li>Shows the first <code>n</code> emirp numbers.</li>
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<li>Shows the emirp numbers in a range.</li>
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<li>Shows the number of emirps in a range.</li>
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<li>Shows the <code>n<sup>th</sup></code> emirp number.</li>
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</ul>
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The function should accept 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 emirps 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 or a number.
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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>emirps</code> should be a function.
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testString: assert(typeof emirps === 'function');
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- text: <code>emirps(20,true)</code> should return <code>[13,17,31,37,71,73,79,97,107,113,149,157,167,179,199,311,337,347,359,389]</code>
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testString: assert.deepEqual(emirps(20, true), [13, 17, 31, 37, 71, 73, 79, 97, 107, 113, 149, 157, 167, 179, 199, 311, 337, 347, 359, 389]);
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- text: <code>emirps(1000)</code> should return <code>70529</code>
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testString: assert.deepEqual(emirps(1000), 70529);
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- text: <code>emirps([7700,8000],true)</code> should return <code>[7717,7757,7817,7841,7867,7879,7901,7927,7949,7951,7963]</code>
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testString: assert.deepEqual(emirps([7700, 8000], true), [7717, 7757, 7817, 7841, 7867, 7879, 7901, 7927, 7949, 7951, 7963]);
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- text: <code>emirps([7700,8000],true)</code> should return <code>11</code>
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testString: assert.deepEqual(emirps([7700, 8000], false), 11);
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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 emirps(n) {
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// Good luck!
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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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// noprotect
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function emirps(num, showEmirps)
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{
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const is_prime = function(n)
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{
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if (!(n % 2) || !(n % 3)) return false;
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let p = 1;
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while (p * p < n)
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{ if (n % (p += 4) == 0 || n % (p += 2) == 0)
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{ return false; } }
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return true;
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};
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const is_emirp = function(n) {
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const r = parseInt(n.toString().split('').reverse().join(''));
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return r != n && is_prime(n) && is_prime(r);
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};
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let i,
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arr = [];
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if (typeof num === 'number') {
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for (i = 0; arr.length < num; i++) if (is_emirp(i)) arr.push(i);
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// first x emirps
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if (showEmirps) return arr;
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// xth emirp
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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 (is_emirp(i)) arr.push(i);
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// emirps between x .. y
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if (showEmirps) return arr;
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// number of emirps 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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