Mathematical operations between two strings [duplicate]

Question

I'm trying to add subtract and multiply numbers with huge decimals C++.

Example:

4125487821547.87879845215584844588 - 354556689.899455132265468

What I figured so far is that I need to save inputs as a string, but not sure how to proceed after that.

Appreciate the help in advance Thanks

Answer 1

You need a big integer class or library. There are several implementations available, just to give you an overview on how to use such an external dependency, here is a solution based on Boost:

#include <boost/multiprecision/cpp_int.hpp>

using BigInt = boost::multiprecision::cpp_int;

You can now construct instances by passing string or integral literals to the constructor and do all standard arithmetic operations with these objects, e.g.

const BigInt i("8787984521558484092344588");
const BigInt j("32308942039402934");

std::cout << i - j << "\n";

One nice detail of such classes is that they usually demonstrate one of the few justified scenarios for non-explicit constructors with one argument, i.e., for the sake of smooth interoperability with builtin integral types. Example:

int n = 42;

// Use builtin ints like BigInts via implicit BigInt(int) ctor:
std::cout << (i + n)/(j % 3) << "\n";

You only need the Boost headers for these snippets, no linkage is required. Check out the docs when proceeding with this library.