freeCodeCamp/curriculum/challenges/english/08-coding-interview-prep/rosetta-code/emirp-primes.english.md

---
title: Emirp primes
id: 599d0ba974141b0f508b37d5
challengeType: 5
---

## 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> is a function.
    testString: assert(typeof emirps === 'function', '<code>emirps</code> is a 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], '<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>');
  - text: <code>emirps(10000)</code> should return <code>948349</code>
    testString: assert.deepEqual(emirps(10000), 948349, '<code>emirps(10000)</code> should return <code>948349</code>');
  - 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], '<code>emirps([7700,8000],true)</code> should return <code>[7717,7757,7817,7841,7867,7879,7901,7927,7949,7951,7963]</code>');
  - text: <code>emirps([7700,8000],true)</code> should return <code>11</code>
    testString: assert.deepEqual(emirps([7700, 8000], false), 11, '<code>emirps([7700,8000],true)</code> should return <code>11</code>');

```

</section>

## Challenge Seed
<section id='challengeSeed'>

<div id='js-seed'>

```js
function emirps(n) {
  // Good luck!
}
```

</div>



</section>

## Solution
<section id='solution'>


```js
// noprotect
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>