Creating 2d-array from 1d array

Question

Im new to programming, I have an assignment that asks to create a 2d-array from a 1d array. I came up with this (no help from any outside sources because it takes away the learning experience). It works for our professors test inputs, I was just wondering is this an ugly/inefficient solution.

function twoDArray(arr, lenSubArray) {
    var newArr = []; 
    var placeHolder = 0; 
    var leftOver = 0; 
    for (var i = 1; i < arr.length + 1; i++) {
        /* if i is a multiple of the specified sub-array size 
           then add the elements from placeHolder to i
        */
        if (i % lenSubArray === 0) {
            newArr.push(arr.slice(placeHolder, i)); 
            placeHolder += lenSubArray; 
            leftOver++; // tells us how many sub-arrays were created
        }
    }
    /* if original array is not divisible by the length of the specified sub-array
       then there will be left over values. Retrieve these values and create an 
       array out of them and add them to the 2d array.
    */
    if (!(arr.length % lenSubArray === 0)) {
         /* sub-array count multiplied by the length of each 
            sub-array gives us the right index to retrieve remaining values
        */
        leftOver = (leftOver * lenSubArray);
        newArr.push(arr.slice(leftOver))
    }

    return newArr; 
}

Test input: twoDArray([1, 2, 3, 4, 5], 3) output would be: [[1, 2, 3], [4, 5]]

please add an example of the input array and the wanted output array. — Nina Scholz
– Nina Scholz, Commented Aug 20, 2016 at 18:23
concat would give you a lot cleaner code. Check out this stackoverflow.com/questions/14824283/… — AJ_
– AJ_, Commented Aug 20, 2016 at 18:25

Thomas · Accepted Answer · 2016-08-20 18:31:14Z

2

you're way too complicated:

create a result-array
push slices from i to i+lenSubArray
increment i by lenSubArray

and slice is smart enough to properly handle the end of the Array

function twoDArray(arr, lenSubArray) {
  var i = 0, result = [];
  while(i < arr.length)
    result.push( arr.slice(i, i+=lenSubArray) );
  return result;  
}

answered Aug 20, 2016 at 18:31

Thomas

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1 Comment

Obito Over a year ago

Thanks a ton, I had no idea slice() could handle the end of the array.

Nina Scholz · Accepted Answer · 2016-08-20 19:02:09Z

1

You could use Array#reduce and build new arrays based on the index.

function twoDArray(array, length) {
    return array.reduce(function (r, a, i) {
        i % length ? r[r.length - 1].push(a) : r.push([a]);
        return r;
    }, []);
}

console.log(twoDArray([1, 2, 3, 4, 5], 3));

answered Aug 20, 2016 at 19:02

Nina Scholz

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Comments

score 0 · Accepted Answer · 2016-08-20 19:08:40Z

0

You do not need to iterate over an array. You can use Array.prototype.slice function instead.

function twoDArray(a, b){
    return (Array(Math.ceil(a.length / b)) + '').split(',').map(function(c, d){
        return a.slice(b * d, b * (d + 1));
    });
}

Here is how you call it

var a = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12];
console.log(twoDArray(a, 3));
// Output:
// [
//   [1, 2, 3],
//   [4, 5, 6],
//   [7, 8, 9],
//   [10, 11, 12]
// ]

edited Aug 20, 2016 at 19:08

answered Aug 20, 2016 at 18:31

user6586783

4 Comments

Thomas Over a year ago

oh, that's not nice: if you calculate the length as Math.ceil(a.length/b) you don't have to patch the result by filtering, and instead of (Array(len)+'').split(',').map(fn) you can use Array(len).fill(0).map(fn) or Array.from({ length: len }, fn)

user6586783 Over a year ago

@Thomas. Using Math.ceil is bad practice, it is better to convert second argument to integer and then filter array is necessary. Also, Array.prototype.fill is not implemented in some versions of Node.js.

Thomas Over a year ago

why do you consider Math.ceil as bad, even worse, than creating two unnecessary Arrays and calling a filter-function on every element of the result?

user6586783 Over a year ago

@Thomas. Because Math.ceil can produce NaN values if some error uccurs which can cause problems later. But anyway, I edited answer.

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