Extremely large numbers in javascript

Question

I'm working on the Project Euler problems (currently question 13).

For this question I have to find the first 10 digits of the sum of 100 numbers all of a size similar to this:

91,942,213,363,574,161,572,522,430,563,301,811,072,406,154,908,250

I think I could use something like Java's BigInteger, but I started solving the problems in JavaScript (I'm trying to boost my js abilities for work), and I would like to continue using it, even to solve this problem.

I'd like to stick to pure JS if possible.

Answer 1

You are going to need a javascript based BigInteger library. There are many to choose from. Here is one https://github.com/peterolson/BigInteger.js

You can use it like this

var n = bigInt("91942213363574161572522430563301811072406154908250")
    .plus("91942213363574161572522430563301811072406154908250");

Answer 2

Javascript recently got a new primitive data type BigInt. https://github.com/tc39/proposal-bigint

Only Chrome has released this feature now, while other browsers are still implementing it. https://developers.google.com/web/updates/2018/05/bigint

Basically it can be declared using either literals like

var a = 1n;

or

var b = BigInt('22222222222222222222222222222222');

Math operators don't do auto conversion between BigInt and Number, so 1 + 1n will throw an error.

Answer 3

You could always convert your sum to a string, rip out the . and grab the result - something like this:

var sum = 2384762348723648237462348;
sum = sum.toString(); // "2.3847623487236483e+24"

// Rip out the "."
sum = sum.substr(0, 1) + sum.substr(2);

// Grab the first 10 characters
var firstTen = sum.substr(0, 10);

Answer 4

Surprisingly, sticking all the values in an array and adding them all together and just taking the first 10 digits worked. I must have had a typo somewhere in my code when it didn't work before.

I'm sure that doing something this simple wouldn't work in all cases (like those @AlexMcmillan and @zerkms have been debating about). I think the safest bet is the BigInteger library mentioned by @bhspencer, but it seems like adding the first x significant digits with y digits as a buffer might also be worth a shot in some cases.

Answer 5

I did this using an array and updating all entries with a function.

function f(a) {
  for (let i = 0; i < a.length - 1; i++) {
      a[i + 1] = a[i + 1] + parseInt(a[i] / 10);
      a[i] = a[i] % 10;
  }
  return a;
}
// remember to init the array with enough elements for all digits
var a = Array(200);
a.fill(0);
a[0] = 1;

Here is a JSFiddle with the code for problem 20.