map, filter, reduceについて Leetcodeを使って調べる!!!

leetcode : 2635. Apply Transform Over Each Element in Array

mapとは？

const arr: number[] = [1, 2, 3, 4];


const mapped: number[] = arr.map(num => num * 2);

console.log(mapped);  // result: [2, 4, 6, 8]

leetcode : 2635

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

};


const plusOne = (n: number): number => n + 1;
console.log(map([1, 2, 3], plusOne)); // result: [2, 3, 4]

const plusIndex = (n: number, i: number): number => n + i;
console.log(map([1, 2, 3], plusIndex)); // result: [1, 3, 5]

const constant = (): number => 42;
console.log(map([1, 2, 3], constant)); // result: [42, 42, 42]

pythonの場合

def map_function(arr, fn):
    return [fn(x, i) for i, x in enumerate(arr)]

def plus_one(n, i):
    return n + 1

result = map_function([1, 2, 3], plus_one)
print(result)  # result: [2, 3, 4]

study!

関数を引数で使う場合は今までなかったので知らなかっだが、
この問題通して関数も引数使うことができると知りました。
なお、ラムダ式使うことで短くコード作成することができます。

[参考資料]
https://github.com/vgratsilev/LeetCode/tree/main/Problems/2635.%20Apply%20Transform%20Over%20Each%20Element%20in%20Array
https://leetcode.com/problems/apply-transform-over-each-element-in-array/?envType=study-plan-v2&envId=30-days-of-javascript

leetcode : 2634. Filter Elements from Array

filterとは?

const arr: number[] = [1, 2, 3, 4, 5];

const filtered: number[] = arr.filter(num => num > 3);

console.log(filtered);  // result: [4, 5]

leetcode : 2634

type Fn = (n: number, i: number) => any

function filter(arr: number[], fn: Fn): number[] {

    const result: number[] = [];
    for(let i=0; i<arr.length; i++){
        if (fn(arr[i],i)){
            result.push(arr[i]);
        }
    }
    return result;
    
};

const greaterThan10 = (n: number): boolean => n > 10;

const arr = [0, 10, 20, 30];
console.log(filter(arr, greaterThan10)); 

study!

ifでfilterする時引数として関数を使うことが可能です。

[参考資料]
https://leetcode.com/problems/filter-elements-from-array/description/?envType=study-plan-v2&envId=30-days-of-javascript

leetcode : 2626. Array Reduce Transformation

reduceとは?

const arr: number[] = [1, 2, 3, 4];


const sum: number = arr.reduce((accum: number, curr: number) => accum + curr, 0);

console.log(sum);  // result: 10

leetcode 2626

type Fn = (accum: number, curr: number) => number

function reduce(nums: number[], fn: Fn, init: number): number {

    nums.forEach((number, i) => {
        init=fn(init,number)
    });
    return init
    
}

const nums1 = [1, 2, 3, 4];
const fn1 = (accum: number, curr: number): number => accum + curr;
const init1 = 0;

console.log(arrayReduce(nums1, fn1, init1)); 

[参考資料]
https://leetcode.com/problems/array-reduce-transformation/?envType=study-plan-v2&envId=30-days-of-javascript

最後に

日本語の勉強するために作成しておりますので、間違った日本語や内容がありましたらいつでもフィードバックいただければ幸いです。