JS subset of array of arrays

Question

If I have an array of arrays as follows:

[ [ A0 , B0 , C0 , D0 , E0 ],
  [ A1 , B1 , C1 , D1 , E1 ],
  [ A2 , B2 , C2 , D2 , E2 ]  ] // array of 3x5

and I want to get a new array of subset arrays like so:

[ [ A0 , B0 , C0           ],
  [      B1 , C1 , D1      ],
  [           C2 , D2 , E2 ]  ]  //array of 3x3

This means that the sliceLength = 3, but how can one do this? Is it better to use splice (as in duplicate the original, then remove the unwanted parts), slice, and/or push?

This is what I have thus far, but the syntax looks off:

var newMega = [];
var sliceLength = 3;
for (var i=0; i<originalMatrix.length; i++) {
  var inner = originalMatrix.slice(i, sliceLength);  //  Here is the issue
  newMega.push(inner); 
}

It appears that the syntax would call slice on the entire originalMatrix, without knowing which index of the nested arrays it should be slicing. Should it be originalMatrix[i].slice(i, sliceLength)?

I don't think .slice is the correct method to use here. It looks like you want to end up with 3 arrays of length 5, but without values at some positions. .slice will always return an array of length three in your case. It looks like you have to delete the elements that you don't want. This will preserve the length (5) of each inner array. — Felix Kling
– Felix Kling, Commented Nov 18, 2013 at 16:41
Do you want to end up with arrays if length 3 or length 5? edit: Ok :) Your example looks a bit like you wanted a sparse matrix. — Felix Kling
– Felix Kling, Commented Nov 18, 2013 at 16:46
@FelixKling Well, you just cost me another 10 minutes looking into what a sparse array is! Thanks, learning a lot. — chris Frisina
– chris Frisina, Commented Nov 18, 2013 at 16:57

p.s.w.g · Accepted Answer · 2013-11-18 16:44:12Z

2

Try slicing the inner array (originalMatrix[i]). Also, you probably want to call splice rather than slice:

var newMega = [];
var spliceLength = 3;
for (var i=0; i<originalMatrix.length; i++) {
  var inner = originalMatrix[i].splice(i, spliceLength);
  newMega.push(inner); 
}

Here's a demonstration.

edited Nov 18, 2013 at 16:44

answered Nov 18, 2013 at 16:39

p.s.w.g

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3 Comments

Felix Kling Over a year ago

This would result in each of the inner arrays have a length of three. Based on the OP's example I don't think that's what they want, but I could be just misinterpreting the example.

p.s.w.g Over a year ago

@FelixKling That's exactly the way I read the question. Each sub-array has length 3.

Felix Kling Over a year ago

I read it that each arrays should still have length 5, but has "holes" at some positions. Anyways, the OP clarified and your interpretation is correct.

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