freeCodeCamp/curriculum/challenges/english/02-javascript-algorithms-and-data-structures/basic-javascript/replace-loops-using-recursion.english.md

id, title, challengeType, videoUrl, forumTopicId

id	title	challengeType	videoUrl	forumTopicId
5cfa3679138e7d9595b9d9d4	Replace Loops using Recursion	1	https://www.freecodecamp.org/news/how-recursion-works-explained-with-flowcharts-and-a-video-de61f40cb7f9/	301175

Description

Recursion is the concept that a function can be expressed in terms of itself. To help understand this, start by thinking about the following task: multiply the first n elements of an array to create the product of those elements. Using a for loop, you could do this:

  function multiply(arr, n) {
    var product = 1;
    for (var i = 0; i < n; i++) {
        product *= arr[i];
    }
    return product;
  }

However, notice that multiply(arr, n) == multiply(arr, n - 1) * arr[n - 1]. That means you can rewrite multiply in terms of itself and never need to use a loop.

  function multiply(arr, n) {
    if (n <= 0) {
      return 1;
    } else {
      return multiply(arr, n - 1) * arr[n - 1];
    }
  }

The recursive version of multiply breaks down like this. In the base case, where n <= 0, it returns 1. For larger values of n, it calls itself, but with n - 1. That function call is evaluated in the same way, calling multiply again until n <= 0. At this point, all the functions can return and the original multiply returns the answer.

Note: Recursive functions must have a base case when they return without calling the function again (in this example, when n <= 0), otherwise they can never finish executing.

Instructions

Write a recursive function, sum(arr, n), that returns the sum of the first n elements of an array arr.

Tests

tests:
  - text: <code>sum([1], 0)</code> should equal 0.
    testString: assert.equal(sum([1], 0), 0);
  - text: <code>sum([2, 3, 4], 1)</code> should equal 2.
    testString: assert.equal(sum([2, 3, 4], 1), 2);
  - text: <code>sum([2, 3, 4, 5], 3)</code> should equal 9.
    testString: assert.equal(sum([2, 3, 4, 5], 3), 9);
  - text: Your code should not rely on any kind of loops (<code>for</code> or <code>while</code> or higher order functions such as <code>forEach</code>, <code>map</code>, <code>filter</code>, or <code>reduce</code>.).
    testString: assert(!removeJSComments(code).match(/for|while|forEach|map|filter|reduce/g));
  - text: You should use recursion to solve this problem.
    testString: assert(removeJSComments(sum.toString()).match(/sum\(.*\)/g).length > 1);

Challenge Seed

function sum(arr, n) {
  // Only change code below this line

  // Only change code above this line
}

After Test

const removeJSComments = str => str.replace(/\/\*[\s\S]*?\*\/|\/\/.*$/gm, '');

Solution

function sum(arr, n) {
  // Only change code below this line
  if(n <= 0) {
    return 0;
  } else {
    return sum(arr, n - 1) + arr[n - 1];
  }
  // Only change code above this line
}