Question

I’m doing a BigInt implementation in C++ and I’m having a hard time figuring out how to create a converter from (and to) string (C string would suffice for now).

I implement the number as an array of unsigned int (so basically putting blocks of bits next to each other). I just can’t figure out how to convert a string to this representation.

For example if usigned int would be 32b and i’d get a string of “4294967296”, or “5000000000” or basically anything larger than what a 32b int can hold, how would I properly convert it to appropriate binary representation?

I know I’m missing something obvious, and I’m only asking for a push to the right direction. Thanks for help and sorry for asking such a silly question!

Answer 1

Well one way (not necessarily the most efficient) is to implement the usual arithmetic operators and then just do the following:

// (pseudo-code)
// String to BigInt

String s = ...;
BigInt x = 0;

while (!s.empty())
{
    x *= 10;
    x += s[0] - '0';
    s.pop_front();
}

Output(x);

// (pseudo-code)
// BigInt to String

BigInt x = ...;
String s;

while (x > 0)
{
    s += '0' + x % 10;
    x /= 10;
}

Reverse(s);
Output(s);

If you wanted to do something trickier than you could try the following:

1. If input I is < 100 use above method.
2. Estimate D number of digits of I by bit length * 3 / 10.
3. Mod and Divide by factor F = 10 ^ (D/2), to get I = X*F + Y;
4. Execute recursively with I=X and I=Y