008.js

/*
Source: https://projecteuler.net/problem=8

Problem:
"The four adjacent digits in the 1000-digit number that have the greatest product are 9 × 9 × 8 × 9 = 5832.

73167176531330624919225119674426574742355349194934
96983520312774506326239578318016984801869478851843
85861560789112949495459501737958331952853208805511
12540698747158523863050715693290963295227443043557
66896648950445244523161731856403098711121722383113
62229893423380308135336276614282806444486645238749
30358907296290491560440772390713810515859307960866
70172427121883998797908792274921901699720888093776
65727333001053367881220235421809751254540594752243
52584907711670556013604839586446706324415722155397
53697817977846174064955149290862569321978468622482
83972241375657056057490261407972968652414535100474
82166370484403199890008895243450658541227588666881
16427171479924442928230863465674813919123162824586
17866458359124566529476545682848912883142607690042
24219022671055626321111109370544217506941658960408
07198403850962455444362981230987879927244284909188
84580156166097919133875499200524063689912560717606
05886116467109405077541002256983155200055935729725
71636269561882670428252483600823257530420752963450

Find the thirteen adjacent digits in the 1000-digit number that have the greatest product. What is the value of this product?"


My Algorithm:

1. Define an array to hold adjacent-13-digit parts of the 1000-digit number
2. Define the 1000-digit number as a string and then parse 13-digits at a time to add to the array
3. Loop through the array and find the products
*/


//For example, digitProduct(234) = 2*3*4 = 24
function digitProduct(x) {
  x = x.toString(); 
  x = x.split(""); //separate the string into an array of different numbers
  
  let product = 1;
  for (let i = 0; i < x.length; i++) {
    product *= parseInt(x[i]); //multiply the product by each of the numbers
  }
  
  return product; 
}

function makeAdjacentThirteenDigitsArray(string) {
  let arr = [];
  let i = 0;
  while (i < string.length) {
    let thirteenDigits = "";
    for (let j = i; j < i+13; j++) { //make a string starting at i and containing the next 12 letters
      thirteenDigits += string.charAt(j);
    }
    if (thirteenDigits.length === 13) { //checking that it's 13 digits just so it doesn't make arrays of less than 13 chars at the end 
      arr.push(thirteenDigits);
    }
    i++;
  }
  return arr;
}

function solution(string) {
  let arr = makeAdjacentThirteenDigitsArray(string);
  let largestProductSum = 0;
  
  arr.forEach(element => { //for each element in the array,
    if (digitProduct(element) > largestProductSum) { //if the digit product of those 13 numbers is bigger than the largest product sum so far,
      largestProductSum = digitProduct(element); //update the largest product sum
    }
  });

  return largestProductSum;
}

//ouput the answer!
console.log(solution("7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450"));