139 lines
2.7 KiB
Markdown
139 lines
2.7 KiB
Markdown
---
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id: 599d0ba974141b0f508b37d5
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title: Emirp primes
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challengeType: 5
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forumTopicId: 302253
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dashedName: emirp-primes
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---
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# --description--
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An emirp (**prime** spelled backwards) are primes that when reversed (in their decimal representation) are a different prime.
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# --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 `n` 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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# --hints--
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`emirps` should be a function.
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```js
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assert(typeof emirps === 'function');
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```
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`emirps(20,true)` should return `[13,17,31,37,71,73,79,97,107,113,149,157,167,179,199,311,337,347,359,389]`
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```js
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assert.deepEqual(emirps(20, true), [
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13,
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17,
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31,
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37,
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71,
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73,
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79,
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97,
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107,
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113,
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149,
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157,
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167,
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179,
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199,
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311,
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337,
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347,
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359,
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389
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]);
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```
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`emirps(1000)` should return `70529`
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```js
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assert.deepEqual(emirps(1000), 70529);
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```
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`emirps([7700,8000],true)` should return `[7717,7757,7817,7841,7867,7879,7901,7927,7949,7951,7963]`
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```js
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assert.deepEqual(emirps([7700, 8000], true), [
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7717,
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7757,
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7817,
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7841,
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7867,
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7879,
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7901,
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7927,
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7949,
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7951,
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7963
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]);
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```
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`emirps([7700,8000],false)` should return `11`
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```js
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assert.deepEqual(emirps([7700, 8000], false), 11);
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```
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# --seed--
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## --seed-contents--
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```js
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function emirps(n) {
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}
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```
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# --solutions--
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```js
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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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