Filter Elements from Array

Problem Statement

In this task, you are provided with an integer array named arr and a function fn which serves as a filter. Your goal is to create a new array, filteredArr, that only includes elements from arr that satisfy a certain condition set by fn. The function fn might take one or two parameters—the element arr[i] and optionally its index i.

You are to populate filteredArr with those elements from arr for which the function call fn(arr[i], i) returns a truthy value. A truthy value in this context means any value that, when converted to a Boolean, yields true.

This problem restricts the use of JavaScript's built-in Array.filter method. Therefore, you will need to implement the filtering logic manually.

Examples

Example 1

Input:

arr = [0,10,20,30], fn = function greaterThan10(n) { return n > 10; }

Output:

[20,30]

Explanation:

const newArray = filter(arr, fn); // [20, 30]
The function filters out values that are not greater than 10

Example 2

Input:

arr = [1,2,3], fn = function firstIndex(n, i) { return i === 0; }

Output:

[1]

Explanation:

fn can also accept the index of each element
In this case, the function removes elements not at index 0

Example 3

Input:

arr = [-2,-1,0,1,2], fn = function plusOne(n) { return n + 1 }

Output:

[-2,0,1,2]

Explanation:

Falsey values such as 0 should be filtered out

Constraints

• 0 <= arr.length <= 1000
• -109 <= arr[i] <= 109

Approach and Intuition

The task is to filter the array based on a custom function without using the built-in filter method. Here's a streamlined approach based on the examples provided:

1. Initialize an empty array filteredArr to hold the results.
2. Iterate through each element arr[i] of array arr.
3. For each element, invoke the filter function fn. If fn takes two parameters, pass both the element and its index i.e., fn(arr[i], i). If it takes one parameter, provide only the element i.e., fn(arr[i]).
4. If the result of fn(arr[i], i) or fn(arr[i]) is truthy, append arr[i] to filteredArr.
5. Continue this process for the entire array.
6. Return filteredArr.

For a better understanding, let’s look at specific examples:

• Example 1: For each element, the function greaterThan10 checks if it's greater than 10. This function requires only one parameter. Filtered results are [20, 30].

• Example 2: Involves a function firstIndex which checks if the current index i is 0. This requires both the element and its index. The filtered result is [1], as only the first element meets this condition.

• Example 3: Uses a function plusOne that increments each number by one and filters out any result that isn't truthy (e.g., if the incremented value is 0). This results in [ -2, 0, 1, 2].

With these guidelines, the problem can be solved by simulating the filter process manually, ensuring that only elements fulfilling the custom criteria are returned. This method directly mirrors the built-in filter logic but is applied manually, allowing for detailed control of the filtering condition and adherence to constraints.

Solutions

var applyFilter = function(data, condition) {
    return data.filter(condition);
};

let numbers = [1, 2, 3, 4, 5];
let evenNumbers = applyFilter(numbers, num => num % 2 === 0);