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]]

• Nina Scholz: please add an example of the input array and the wanted output array.
• AJ_: concat would give you a lot cleaner code. Check out this stackoverflow.com/questions/14824283/…

Answer 1

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; 
}

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

Answer 2

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));

Answer 3

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]
// ]

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