---
id: 5a23c84252665b21eecc801c
title: Spiral matrix
challengeType: 5
---

## Description
<section id='description'>

Produce a spiral array.
A  <i>spiral array</i>  is a square arrangement of the first  N<sup>2</sup> natural numbers, where the numbers increase sequentially as you go around the edges of the array spiraling inwards.
For example, given <b>5</b>,  produce this array:
<pre>
0  1  2  3  4
15 16 17 18 5
14 23 24 19 6
13 22 21 20 7
12 11 10  9 8
</pre>
</section>

## Instructions
<section id='instructions'>

</section>

## Tests
<section id='tests'>

``` yml
tests:
  - text: <code>spiralArray</code> should be a function.
    testString: assert(typeof spiralArray=='function','<code>spiralArray</code> should be a function.');
  - text: <code>spiralArray(3)</code> should return an array.
    testString: assert(Array.isArray(spiralArray(3)), '<code>spiralArray(3)</code> should return an array.');
  - text: <code>spiralArray(3)</code> should return <code>[[0, 1, 2],[7, 8, 3],[6, 5, 4]]</code>.
    testString: assert.deepEqual(spiralArray(3), [[0, 1, 2], [7, 8, 3], [6, 5, 4]], '<code>spiralArray(3)</code> should return <code>[[0, 1, 2],[7, 8, 3],[6, 5, 4]]</code>.');
  - text: <code>spiralArray(4)</code> should return <code>[[0, 1, 2, 3],[11, 12, 13, 4],[10, 15, 14, 5],[9, 8, 7, 6]]</code>.
    testString: assert.deepEqual(spiralArray(4), [[0, 1, 2, 3], [11, 12, 13, 4], [10, 15, 14, 5], [9, 8, 7, 6]], '<code>spiralArray(4)</code> should return <code>[[0, 1, 2, 3],[11, 12, 13, 4],[10, 15, 14, 5],[9, 8, 7, 6]]</code>.');
  - text: <code>spiralArray(5)</code> should return <code>[[0, 1, 2, 3, 4],[15, 16, 17, 18, 5],[14, 23, 24, 19, 6],[13, 22, 21, 20, 7],[12, 11, 10, 9, 8]]</code>.
    testString: assert.deepEqual(spiralArray(5), [[0, 1, 2, 3, 4], [15, 16, 17, 18, 5], [14, 23, 24, 19, 6], [13, 22, 21, 20, 7], [12, 11, 10, 9, 8]], '<code>spiralArray(5)</code> should return <code>[[0, 1, 2, 3, 4],[15, 16, 17, 18, 5],[14, 23, 24, 19, 6],[13, 22, 21, 20, 7],[12, 11, 10, 9, 8]]</code>.');
```

</section>

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

<div id='js-seed'>

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

</div>
</section>

## Solution
<section id='solution'>

```js
function spiralArray(n) {
    var arr = Array(n),
        x = 0, y = n,
        total = n * n--,
        dx = 1, dy = 0,
        i = 0, j = 0;
    while (y) arr[--y] = [];
    while (i < total) {
        arr[y][x] = i++;
        x += dx; y += dy;
        if (++j == n) {
            if (dy < 0) {x++; y++; n -= 2}
            j = dx; dx = -dy; dy = j; j = 0;
       }
    }
    return arr;
}
```

</section>