freeCodeCamp/curriculum/challenges/english/08-coding-interview-prep/project-euler/problem-40-champernownes-constant.english.md

id, challengeType, title

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

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

Challenge Seed

function champernownesConstant(n) {
  // Good luck!
  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;
}