freeCodeCamp/curriculum/challenges/english/02-javascript-algorithms-and-data-structures/intermediate-algorithm-scripting/dna-pairing.md

---
id: afd15382cdfb22c9efe8b7de
title: DNA Pairing
challengeType: 5
forumTopicId: 16009
dashedName: dna-pairing
---

# --description--

The DNA strand is missing the pairing element. Take each character, get its pair, and return the results as a 2d array.

[Base pairs](http://en.wikipedia.org/wiki/Base_pair) are a pair of AT and CG. Match the missing element to the provided character.

Return the provided character as the first element in each array.

For example, for the input GCG, return \[\["G", "C"], \["C","G"],\["G", "C"]]

The character and its pair are paired up in an array, and all the arrays are grouped into one encapsulating array.

# --hints--

`pairElement("ATCGA")` should return `[["A","T"],["T","A"],["C","G"],["G","C"],["A","T"]]`.

```js
assert.deepEqual(pairElement('ATCGA'), [
  ['A', 'T'],
  ['T', 'A'],
  ['C', 'G'],
  ['G', 'C'],
  ['A', 'T']
]);
```

`pairElement("TTGAG")` should return `[["T","A"],["T","A"],["G","C"],["A","T"],["G","C"]]`.

```js
assert.deepEqual(pairElement('TTGAG'), [
  ['T', 'A'],
  ['T', 'A'],
  ['G', 'C'],
  ['A', 'T'],
  ['G', 'C']
]);
```

`pairElement("CTCTA")` should return `[["C","G"],["T","A"],["C","G"],["T","A"],["A","T"]]`.

```js
assert.deepEqual(pairElement('CTCTA'), [
  ['C', 'G'],
  ['T', 'A'],
  ['C', 'G'],
  ['T', 'A'],
  ['A', 'T']
]);
```

# --seed--

## --seed-contents--

```js
function pairElement(str) {
  return str;
}

pairElement("GCG");
```

# --solutions--

```js
var lookup = Object.create(null);
lookup.A = 'T';
lookup.T = 'A';
lookup.C = 'G';
lookup.G = 'C';

function pairElement(str) {
 return str.split('').map(function(p) {return [p, lookup[p]];});
}
```