Using .Map() on nested arrays

[Mutiplify]

Hello!

Thanks for taking the time to read this.

I’m looking to better understand how to use map on nested arrays.

The following code accesses a nested array and adds one to every single number.

let arr = [[1,2],[3,4],[5,6]];

let arr2 = arr.map(ele=> ele.map((ele) =>ele + 1))

What I want to do, and haven’t been able to figure out is this.

• How can I add one to every n’th number in each array

• How can I filter out non integers while using map? Consider the following.

let arr = [[A,2],[B,4],[C,6]];

let arr2 = arr.map(ele=> ele.filter(?????))

How do I use the filter function to return just an array of numbers? I’ve been messing around in a JavaScript testbed and I can’t seem to figure it out. I’ve read the Mozilla documentation but it still eludes me.

Thanks for the help!

[DanCouper]

Each element in that array is an array of two numbers. You don’t need to map each sub-array, just return an array where the first element is the same and the second one is the same + 1.

If you want to filter it, again, you know each element is an array of a string then a number (I assume that’s what you meant). So just return the second element of that sub-array.

Its slightly unclear what you’re trying to do, but if you have an array of arrays, and those sub-arrays are a mixture of numbers and other things and the sub-arrays are of unknown length, and you want to produce a single array of numbers, then flatMap is what you want (maps and flattens), eg

const arrayOfArrays = [
  [1, 2, "foo"],
  [3, true, "bar"],
  [false, "baz", 4],
];

const isNumber = (value) => typeof value === "number";

const nums = arrayOfArrays.flatMap((subArr) => subArr.filter(isNumber));

console.log(nums)
// [1, 2, 3, 4]

[Mutiplify]

Hi Dan,

I figured it out after a little more digging and from your advice.

What I meant was something like this.

let arr = [[10, 20],[30,40]]

let p = [];
p = arr.map(ele => ele.map((ele, index) => index == 1   ? ele + 1: ele))

This will return

 [[10, 21],[30,41]]

So this will let me manipulate the second value in an array of nested arrays. For some reason I neglected the index argument when reading the .map() documentation, Thanks for pointing me in the correct direction.

I didn’t want to flatten the array because I’d prefer to manipulate it in place to keep count of where the values I want are.

[DanCouper]

You know that each sub array has two elements. You don’t need to map the sub array, you just return the exact values you want. So as an example for a single array:

function addOneToTheSecondElement ([a, b]) {
  return [a, b + 1];
}

[Bekalu_Tilahun]

let n = 5 // the index of the element your want to add 1 to
let arr2 = arr.map(elem=> elem[n]+=1)

[DanCouper]

@Mutiplify

I didn’t want to flatten the array because I’d prefer to manipulate it in place to keep count of where the values I want are

So this

let arr2 = arr.map(elem=> elem[n]+=1)

Is going to do exactly that as well as mutating the arrays.

With that, you’re saying “for every elem in art, mutate the nth element by assigning its value + 1 to it.” That action will also return the new value, so for example:

const arr = [[1,2],[3,4]]
const newArr = arr.map(elem => elem[1] += 1)
console.log(arr) // [[1,3],[3,5]]
console.log(newArr) // [3,5]

You want to return a new array each time

return [elem[0], elem[1] + 1];