Get first array of numbers in array of variable depth

Question

I am using a function in a JavaScript framework where the return value can be ANY of the following

a single xy coordinate pair
```
[x,y]
```
an array of xy coordinate pairs
```
[[x,y],[x,y],...]
```
an array of arrays of xy coordinate pairs
```
[[[x,y],[x,y]],[[x,y],[x,y]],...]
```

The return value depends on the geometry of the object (single point, line, or multiple lines). Regardless of the return value and its array depth, I want to grab the first xy coordinate pair. What is an efficient way to do this?

Here is my code to achieve the objective so far:

//here is the magic method that can return one of three things :)
var mysteryCoordinates = geometry.getCoordinates();
var firstCoord;

if(typeof mysteryCoordinates[0] === 'number') {
    firstCoord = mysteryCoordinates;
} else if (typeof mysteryCoordinates[0][0] === 'number') {
    firstCoord = mysteryCoordinates[0];
} else if (typeof mysteryCoordinates[0][0][0] === 'number') {
    firstCoord = mysteryCoordinates[0][0];
}

I really hate this solution and am looking for something a bit more elegant.

Does the geometry object not have some property telling you what type of geometry it represents, ie point,line,lines? — Patrick Evans
– Patrick Evans, Commented Sep 28, 2016 at 18:46
I want to grab the first xy coordinate pair are you interested only on the first pair ?? Or later you might want some other index data as well? — Rajshekar Reddy
– Rajshekar Reddy, Commented Sep 28, 2016 at 18:47
@PatrickEvans It does, but I would have to do three separate typeof checks on the geometry object to know the proper depth to access the first coordinate pair. — JellyRaptor
– JellyRaptor, Commented Sep 28, 2016 at 18:59

Nina Scholz · Accepted Answer · 2016-09-28 19:37:28Z

4

I guess in pure JS this should do it;

var    arr = [[[1,2],[1,3]],[[4,8],[3,9]]],
getFirstXY = a => Array.isArray(a[0]) ? getFirstXY(a[0]) : a;

console.log(getFirstXY(arr));

edited Sep 28, 2016 at 19:37

Nina Scholz

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answered Sep 28, 2016 at 19:35

Redu

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5 Comments

Nina Scholz Over a year ago

you need no concat.

Redu Over a year ago

@Nina Scholz Cool..! Thanks :)

JellyRaptor Over a year ago

@Redu This works for all 3 scenarios. Care to explain what is going on?

Redu Over a year ago

@JellyRaptor getFirstXY is a recursive function. It checks for the first item of the supplied array and if it is an array calls getFirstXY again with a[0] being the argument up until it is not and then returns the argument array.

JellyRaptor Over a year ago

@Redu This is pretty awesome. Answer accepted. Thanks!

Ben · Accepted Answer · 2016-09-28 20:08:42Z

2

A less efficient, but more elegant solution would be to use _.flatten (http://underscorejs.org/#flatten):

let firstCoord = _.flatten(mysteryCoordinates).slice(0, 2);

You could make it a little more efficient on average by slicing off the first two elements up-front as well:

let firstCoord = _.flatten(mysteryCoordinates.slice(0, 2)).slice(0, 2);

console.log(_.flatten([1,2]).slice(0, 2));
console.log(_.flatten([[1,2],[1,3],[4,8],[3,9]]).slice(0, 2));
console.log(_.flatten([[[1,2],[1,3]],[[4,8],[3,9]]]).slice(0, 2));

<script src="http://underscorejs.org/underscore-min.js"></script>

edited Sep 28, 2016 at 20:08

answered Sep 28, 2016 at 18:51

Ben

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3 Comments

JellyRaptor Over a year ago

Upon further review, this solution only works for scenarios 1 and 2, but not 3. It spits out an array of pairs when the input is the array of arrays of pairs.

Ben Over a year ago

Seems to work for me with the latest version of underscore. Maybe you accidentally passed a truthy value for shallow?

JellyRaptor Over a year ago

+1 for demonstrating functionality. I initially checked results using the flatten() function from the latest lodash version. I presumed it would be equivalent to underscore's flatten, but evidently it is not.

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