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curriculum/challenges/english/10-coding-interview-prep/rosetta-code/emirp-primes.md

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+---
+title: Emirp primes
+id: 599d0ba974141b0f508b37d5
+challengeType: 5
+forumTopicId: 302253
+---
+
+## Description
+<section id='description'>
+An  emirp (<strong>prime</strong> spelled backwards) are primes that when reversed (in their decimal representation) are a different prime.
+</section>
+
+## Instructions
+<section id='instructions'>
+Write a function that: 
+<ul>
+  <li>Shows the first <code>n</code> emirp numbers.</li>
+  <li>Shows the emirp numbers in a range.</li>
+  <li>Shows the number of emirps in a range.</li>
+  <li>Shows the <code>n<sup>th</sup></code> emirp number.</li>
+</ul>
+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.
+</section>
+
+## Tests
+<section id='tests'>
+
+```yml
+tests:
+  - text: <code>emirps</code> should be a function.
+    testString: assert(typeof emirps === 'function');
+  - 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>
+    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]);
+  - text: <code>emirps(1000)</code> should return <code>70529</code>
+    testString: assert.deepEqual(emirps(1000), 70529);
+  - text: <code>emirps([7700,8000],true)</code> should return <code>[7717,7757,7817,7841,7867,7879,7901,7927,7949,7951,7963]</code>
+    testString: assert.deepEqual(emirps([7700, 8000], true), [7717, 7757, 7817, 7841, 7867, 7879, 7901, 7927, 7949, 7951, 7963]);
+  - text: <code>emirps([7700,8000],true)</code> should return <code>11</code>
+    testString: assert.deepEqual(emirps([7700, 8000], false), 11);
+
+```
+
+</section>
+
+## Challenge Seed
+<section id='challengeSeed'>
+
+<div id='js-seed'>
+
+```js
+function emirps(n) {
+
+}
+```
+
+</div>
+
+
+
+</section>
+
+## Solution
+<section id='solution'>
+
+
+```js
+function emirps(num, showEmirps)
+{
+  const is_prime = function(n)
+	{
+    if (!(n % 2) || !(n % 3)) return false;
+    let p = 1;
+    while (p * p < n)
+			        { if (n % (p += 4) == 0 || n % (p += 2) == 0)
+			                { return false; } }
+    return true;
+  };
+  const is_emirp = function(n) {
+    const r = parseInt(n.toString().split('').reverse().join(''));
+    return r != n && is_prime(n) && is_prime(r);
+  };
+
+  let i,
+    arr = [];
+  if (typeof num === 'number') {
+    for (i = 0; arr.length < num; i++) if (is_emirp(i)) arr.push(i);
+    // first x emirps
+    if (showEmirps) return arr;
+    // xth emirp
+    return arr.pop();
+  }
+
+  if (Array.isArray(num)) {
+    for (i = num[0]; i <= num[1]; i++) if (is_emirp(i)) arr.push(i);
+    // emirps between x .. y
+    if (showEmirps) return arr;
+    // number of emirps between x .. y
+    return arr.length;
+  }
+}
+
+```
+
+</section>