How to get item from multiple consecutive arrays

Question

I am having a little difficuly coming up with a solution to my problem.

I am trying to get the item at an index of a collection of arrays. I cannot actually concatenate the arrays, they need to stay seperate.

Normally, to get the 3rd item from an array, you would do:

function getItemFromJustOneArray(index){
  var my_array = [1,2,3,4,5,6];
  return my_array[index];
}

getItemFromJustOneArray(2); // returns 3

However, I have a bunch of arrays (could be any amount of arrays) and these arrays cannot be merged into one.

function getItemFromMultipleArrays(index){

  var array1 = [1,2];
  var array2 = [3,4,5];
  var array3 = [6];

  // I cannot use concat (or similar) to merge the arrays,
  // they need to stay seperate
  // also, could be 3 arrays, but could also be 1, or 5...

  // return 3;

}

getItemFromMultipleArrays(2); // SHOULD RETURN 3

I have tried a bunch of lines that loops over the array, but I cannot really get a working solution.

Does someone know an elegant solution to this problem?

If there can be an arbitrary number of arrays, it should be a 2-dimensional array, not separate variables for each. — Barmar
– Barmar, Commented Jun 3, 2021 at 16:08
On what basis you want the return value to be 3? What if you have a nested array or an associative array or a multi-dimensional array? — Vasuki Dileep
– Vasuki Dileep, Commented Jun 3, 2021 at 16:10
the rationale for why you want to do this is missing, but if you want a solution you need to check the index against the length of the array(s) to find which array it is in and adjust the index based on skipping arrays. e.g. if (index < array1.length) return array1[index]; else if (index - array1.length < array2.length) return array2[index - array1.length] else return array3[index - array1.length - array2.length] or something like that... — akaphenom
– akaphenom, Commented Jun 3, 2021 at 16:13

Barmar · Accepted Answer · 2021-06-03 16:14:25Z

1

Nest all the arrays in another array. Then loop over that array, decrementing index by each array's length until it's within the length of the current element. Then you can return the appropriate element of that nested array.

function getItemFromMultipleArrays(index) {

  var array1 = [1, 2];
  var array2 = [3, 4, 5];
  var array3 = [6];
  var all_arrays = [array1, array2, array3];
  var i;
  for (i = 0; i < all_arrays.length && index >= all_arrays[i].length; i++) {
    index -= all_arrays[i].length;
  }
  if (i < all_arrays.length) {
    return all_arrays[i][index];
  }
}

console.log(getItemFromMultipleArrays(2)); // SHOULD RETURN 3

answered Jun 3, 2021 at 16:14

Barmar

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3 Comments

akaphenom Over a year ago

could do this without the nesting too and more explicit coding agaist the arrays. this solution works better if there is an arbitrary number of arrays. Explicitly coding checks against arrays might be better if there is a limited number of explicit arrays

Barmar Over a year ago

The question say "could be 3 arrays, but could also be 1, or 5...". So it seems like it needs to work for an arbitrary number.

Robbert Over a year ago

Thank you Barmar, your answer has been very helpful! And yes, it needed to work with an arbitrary amount. Thanks again :-)

Nina Scholz · Accepted Answer · 2021-06-03 16:31:38Z

1

Why not spread the arrays to a new one and use the index for the value?

function getItemFromMultipleArrays(index) {
    const
        array1 = [1, 2],
        array2 = [3, 4, 5],
        array3 = [6];

    return [...array1, ...array2, ...array3][index];
}

console.log(getItemFromMultipleArrays(2)); // 3

Another approach by using an offset for iterating arrays.

function getItemFromMultipleArrays(index) {
    const
        array1 = [1, 2],
        array2 = [3, 4, 5],
        array3 = [6],
        temp = [array1, array2, array3];

    let j = 0;

    while (index >= temp[j].length) index -= temp[j++].length;

    return temp[j][index];
}

console.log(getItemFromMultipleArrays(2)); // 3
console.log(getItemFromMultipleArrays(5)); // 6

edited Jun 3, 2021 at 16:31

answered Jun 3, 2021 at 16:21

Nina Scholz

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3 Comments

Barmar Over a year ago

He wrote "these arrays cannot be merged into one.". That's what you're doing with the ellipsis.

Barmar Over a year ago

Maybe this is a coding challenge, where creating the combined array will be too large.

Robbert Over a year ago

Hi, your second anwser was helpful to me! Thanks a lot. If I could mark two correct answers, I would. (It's just that I prefer for-loops, that's all). Thanks again!

Dharman · Accepted Answer · 2021-06-03 16:32:00Z

0

this should do it, just copying all the arrays into a "big" one and accessing it (added a helping function)

// this function takes any amount of arrays and returns a new
// "concatted" array without affecting the original ones.
function connectArrays(...arrays) {
  return [...arrays.flat()];
}

function getItemFromMultipleArrays(index) {
  var array1 = [1,2];
  var array2 = [3,4,5];
  var array3 = [6];
  var allArrays = connectArrays(array1, array2, array3);
  
  return allArrays[index];
}

getItemFromMultipleArrays(2); // SHOULD RETURN 3

edited Jun 3, 2021 at 16:32

Dharman♦

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answered Jun 3, 2021 at 16:25

Liad

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