freeCodeCamp/curriculum/challenges/english/10-coding-interview-prep/project-euler/problem-40-champernownes-constant.md

id	challengeType	title	forumTopicId
5900f3941000cf542c50fea7	5	Problem 40: Champernowne's constant	302066

Description

An irrational decimal fraction is created by concatenating the positive integers:

0.123456789101112131415161718192021...

It can be seen that the 12^th digit of the fractional part is 1.

If d_n represents the n^th digit of the fractional part, find the value of the following expression.

d₁ × d₁₀ × d₁₀₀ × d₁₀₀₀ × d₁₀₀₀₀ × d₁₀₀₀₀₀ × d_1000000

Instructions

Tests

tests:
  - text: <code>champernownesConstant(100)</code> should return a number.
    testString: assert(typeof champernownesConstant(100) === 'number');
  - text: <code>champernownesConstant(100)</code> should return 5.
    testString: assert.strictEqual(champernownesConstant(100), 5);
  - text: <code>champernownesConstant(1000)</code> should return 15.
    testString: assert.strictEqual(champernownesConstant(1000), 15);
  - text: <code>champernownesConstant(1000000)</code> should return 210.
    testString: assert.strictEqual(champernownesConstant(1000000), 210);

Challenge Seed

function champernownesConstant(n) {

  return true;
}

champernownesConstant(100);

Solution

function champernownesConstant(n) {
  let fractionalPart = '';
  for (let i = 0; fractionalPart.length <= n; i++) {
    fractionalPart += i.toString();
  }

  let product = 1;
  for (let i = 0; i < n.toString().length; i++) {
    const index = 10 ** i;
    product *= parseInt(fractionalPart[index], 10);
  }

  return product;
}