Double precision in C++ (or pow(2, 1000))

Question

I'm working on Project Euler to brush up on my C++ coding skills in preparation for the programming challenge(s) we'll be having this next semester (since they don't let us use Python, boo!).

I'm on #16, and I'm trying to find a way to keep real precision for 2¹°°°

For instance:

int main(){
    double num = pow(2, 1000);
    printf("%.0f", num):
    return 0;
}

prints

10715086071862673209484250490600018105614050000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000

Which is missing most of the numbers (from python):

>>> 2**1000

10715086071862673209484250490600018105614048117055336074437503883703510511249361224931983788156958581275946729175531468251871452856923140435984577574698574803934567774824230985421074605062371141877954182153046474983581941267398767559165543946077062914571196477686542167660429831652624386837205668069376L

Granted, I can write the program with a Python 1 liner

sum(int(_) for _ in str(2**1000))

that gives me the result immediately, but I'm trying to find a way to do it in C++. Any pointers? (haha...)

Edit:

Something outside the standard libs is worthless to me - only dead-tree code is allowed in those contests, and I'm probably not going to print out 10,000 lines of external code...

Answer 1

If you just keep track of each digit in a char array, this is easy. Doubling a digit is trivial, and if the result is greater than 10 you just subtract 10 and add a carry to the next digit. Start with a value of 1, loop over the doubling function 1000 times, and you're done. You can predict the number of digits you'll need with ceil(1000*log(2)/log(10)), or just add them dynamically.

Spoiler alert: it appears I have to show the code before anyone will believe me. This is a simple implementation of a bignum with two functions, Double and Display. I didn't make it a class in the interest of simplicity. The digits are stored in a little-endian format, with the least significant digit first.

typedef std::vector<char> bignum;

void Double(bignum & num)
{
    int carry = 0;
    for (bignum::iterator p = num.begin();  p != num.end();  ++p)
    {
        *p *= 2;
        *p += carry;
        carry = (*p >= 10);
        *p -= carry * 10;
    }
    if (carry != 0)
        num.push_back(carry);
}

void Display(bignum & num)
{
    for (bignum::reverse_iterator p = num.rbegin();  p != num.rend();  ++p)
        std::cout << static_cast<int>(*p);
}

int main(int argc, char* argv[])
{
    bignum num;
    num.push_back(1);
    for (int i = 0;  i < 1000;  ++i)
        Double(num);
    Display(num);
    std::cout << std::endl;
    return 0;
}