2634. Filter Elements from Array

Description

Given an integer array arr and a filtering function fn, return a filtered array filteredArr.

The fn function takes one or two arguments:

• arr[i] - number from the arr
• i - index of arr[i]

filteredArr should only contain the elements from the arr for which the expression fn(arr[i], i) evaluates to a truthy value. A truthy value is a value where Boolean(value) returns true.

Please solve it without the built-in Array.filter method.

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

Solutions

Solution 1: Traversal

We traverse the array \(arr\) and for each element \(arr[i]\), if \(fn(arr[i], i)\) is true, we add it to the answer array. Finally, we return the answer array.

The time complexity is \(O(n)\), where \(n\) is the length of the array \(arr\). Ignoring the space consumption of the answer, the space complexity is \(O(1)\).

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function filter(arr: number[], fn: (n: number, i: number) => any): number[] {
    const ans: number[] = [];
    for (let i = 0; i < arr.length; ++i) {
        if (fn(arr[i], i)) {
            ans.push(arr[i]);
        }
    }
    return ans;
}

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