Detect Array of Arrays

Question

i have a json that may return something like this

"coordinates":
  [
    [[100.0,0.0],[101.0,0.0],[101.0,1.0],[100.0,1.0],[100.0,0.0]],
    [[100.2,0.2],[100.8,0.2],[100.8,0.8],[100.2,0.8],[100.2,0.2]]
  ]

Which will appear as an array of array Which I need to handle differently than something that may look like

"coordinates":[30.0,10.0]

But I was doing different actions based on the length of the coordinates array which in both cases is 2. (2 is a point else polygon or polyline) But I need to make sure that it isn't an array of arrays

Two pass deserializing, maybe? Try to deserialize to an array of arrays and if that fails (because it's not that format) try the "normal" deserialization. — JBC
– JBC, Commented Aug 7, 2014 at 18:50

Rajkumar Madhuram · Accepted Answer · 2014-08-07 19:04:03Z

6

Maybe something like this?

if (Array.isArray(coordinates)) {
  // is an array
  if (Array.isArray(coordinates[0])) {
    // is an array of array
    // process polyline
  } else {
    // process point
  }
}

edited Aug 7, 2014 at 19:04

answered Aug 7, 2014 at 18:55

Rajkumar Madhuram

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4 Comments

Thomas Junk Over a year ago

You point in the right direction. Array.isArray() is right. But you have to check every element in the array.

plalx Over a year ago

@ThomasJunk Why would you? It's probably safe to assume that if the first item in the array is an array, then it's not point coordinates.

Thomas Junk Over a year ago

@plalx »It's probably safe to assume« d'accord. It depends on the source of the JSON and your knowledge about it. In some or maybe all cases you are right ;)

Rajkumar Madhuram Over a year ago

@ThomasJunk, as we are dealing with geometry, I assumed input would be consistent (either some path generators or user actions producing these coordinates). So IMO, a simple check would be sufficient.

Thomas Junk · Accepted Answer · 2014-08-07 19:09:52Z

2

But I need to make sure that it isn't an array of arrays

What about a simple check-Function:

var a1=[1,2,3,4];
var a2=[[1],[2]];

function checkArrayOfArrays(a){
    return a.every(function(x){ return Array.isArray(x); });
}

console.log(checkArrayOfArrays(a1));
console.log(checkArrayOfArrays(a2));

JSBIN to play with. MDN documentation to Array.prototype.every().

Edit: Of course there is the case, that you have a mix-state, which in this case would be recognized as false, which isn't always desireable. Then Array.prototype.some() comes to the rescue.

edited Aug 7, 2014 at 19:09

answered Aug 7, 2014 at 19:04

Thomas Junk

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Comments

sifriday · Accepted Answer · 2014-08-07 18:54:39Z

0

Given:

bar = {"coordinates":[30.0,10.0]}

typeof bar.coordinates[0]
"number"

foo = {"coordinates":
  [
    [[100.0,0.0],[101.0,0.0],[101.0,1.0],[100.0,1.0],[100.0,0.0]],
    [[100.2,0.2],[100.8,0.2],[100.8,0.8],[100.2,0.8],[100.2,0.2]]
  ]}

typeof foo.coordinates[0]
"object"

Then you can do:

foo = (deserialise your JSON here)
if (typeof foo.coordinates[0] == "object") {
   // handle polyline
}
else {
   // handle poinmt
}

answered Aug 7, 2014 at 18:54

sifriday

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Comments

Jeremy J Starcher · Accepted Answer · 2014-08-07 18:54:39Z

0

A function like this:

var array = [30, 31, 32];
var nestedarray = [
  [1, 2, 3]
];

function isArray(obj) {
  // Under ES5 you could use obj.isArray
  return toString.call(obj) === "[object Array]";
}

function isNestedArray(a) {
  // Is the thing itself an array?
  // If not, can't be a nested array
  if (!isArray(a)) {
    return false;
  }

  // Is the first element an array?
  if (isArray(a[0])) {
    return true;
  }

  // Otherwise, nope, not a nested array
  return false;
}

console.log(isNestedArray(array));
console.log(isNestedArray(nestedarray));
console.log(isNestedArray(null));

answered Aug 7, 2014 at 18:54

Jeremy J Starcher

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Comments

plalx · Accepted Answer · 2014-08-07 19:11:27Z

Why can't you simply check whether the item you are processing is an array?

Array.isArray(coordinates[0]); would tell if the first item in the coordinates array is an array (considering that coordinates references the value of the coordinates property).

You could also do the opposite and check wheter the first item is a number.

function arePointCoords(coords) {
    return Array.isArray(coords) && typeof coords[0] === 'number';
}


arePointCoords([30.0,10.0]); //true
arePointCoords([
    [[100.0,0.0],[101.0,0.0],[101.0,1.0],[100.0,1.0],[100.0,0.0]],
    [[100.2,0.2],[100.8,0.2],[100.8,0.8],[100.2,0.8],[100.2,0.2]]
]); //false

Please note that using names closely related to the domain rather than technical terms e.g. arePointCoords(coords) rather than !isArrayOfArrays(coords) also makes your code easier to understand. However, arePointCoords could rely on a more generic function such as isArrayOfArrays internally.

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