创建带有子子数组的函数

Question

I have a function which creates an array of subarrays. It takes three parameters: the number of subarries to create within the array ( numSub ), the number of times the desired value occurs within each subarray ( numValue ), and the desired value ( value ).

For example, if I were to call the function with (3, 2, test) , I would want the following output:

Array [Array ["test", "test"], Array ["test", "test"], Array ["test", "test"]]

Here is the function I have written:

 function subarrs(numSub, numValue, value) { let arr = []; for (let i = 0; i < numSub; i++) { arr.push([]); } arr.forEach(function(sub) { sub.fill(value, 0, numValue - 1); }); return arr; } console.log(subarrs(3, 2, 'test')); 

I have looped through the numSub variable and inserted an array into arr for each iteration. This was successful. ( Array [Array [], Array [], Array []] )

I then use a forEach loop to fill each sub-array with value beginning at index 0 and ending at index numValue - 1 (because the second occurrence of the value would actually be at index 1.)

The function does not work as intended, however. Rather than the aforementioned desired output, I receive this:

Array [Array [], Array [], Array []]

Answer 1

You can use fill on an array that has received the right length, like Array(numValue).fill(numValue) . Here is how you could do it:

 function subarrs(numSub, numValue, value) { return Array.from({length: numSub}, () => Array(numValue).fill(value)); } console.log(subarrs(3, 2, 'test')); 

Answer 2

You are filling an empty array. It's still an empty array (nothing to fill).

You should construct the array of some length:

arr.push(new Array(numValue));

Complete:

function subarrs(numSub, numValue, value) {
  let arr = [];
  for (let i = 0; i < numSub; i++) {
    arr.push(new Array(numValue));
  }
  arr.forEach(function(sub) {
    sub.fill(value);
  });
  return arr;
}

console.log(subarrs(3, 2, 'test'));

Answer 3

Array.fill() only modifies array values. It does not add new ones. Use push again instead

 function subarrs(numSub, numValue, value) { let arr = []; for (let i = 0; i < numSub; i++) { arr.push([]); } arr.forEach(function(sub) { for (let j = 0; j < numValue; j++) { sub.push(value); } }); return arr; } console.log(subarrs(3, 2, 'test')); 

Answer 4

fill will only work on indexes that already exist and its second parameter in your case should be the length of the array (which is the default value) and not length - 1 . You can see it here:

 let myEmptyArray = []; let myFullArray = [1, 2, 3, 4]; myEmptyArray.fill(0, 0, 4); myFullArray.fill(0, 0, 4); console.log(myEmptyArray, myFullArray) 

You could push an Array with the necessary slots already in place with new Array(numValue) . Something like this:

 function subarrs(numSub, numValue, value) { let arr = []; for (let i = 0; i < numSub; i++) { arr.push(new Array(numValue)); } arr.forEach(function(sub) { sub.fill(value); }); return arr; } console.log(subarrs(3, 2, 'test')); 

Answer 5

You can use .push method to add value into your array instead .fill , see working demo :

 function subarrs(numSub, numValue, value) { let arr = []; for (let i = 0; i < numSub; i++) { arr.push([]); for (let j = 0; j < numValue; j++) { arr[i].push(value); } } return arr; } console.log(subarrs(3, 2, 'test')); 

Answer 6

If you have es2015+ You can do it easily :

const subarrs = (length, subLength, value) => 
    [...Array(length)].map(() => [...Array(subLength)].fill(value));

subarrs(3, 2, 'test');

(Edited) after the first comment