How to convert decimal scientific notation float/double to normal string

Question

I know there are many questions solving this problem when the number is not very large, but when it comes to large numbers (while still in range of float/double), the output will lose precision :

#include <stdio.h>
#include <iostream>
#include <sstream>

int main() {
    double d = -1e307, d2 = 1e307;
    std::cout << std::fixed << d << std::endl;

    printf("d2 is %lf\n", d2);
    return 0;
}

This code will get output like this:

-9999999999999999860310597602564577717002641838126363875249660735883565852672743849064846414228960666786379280392654615393353172850252103336275952370615397010730691664689375178569039851073146339641623266071126720011020169553304018596457812688561947201171488461172921822139066929851282122002676667750021070848.000000

d2 is 9999999999999999860310597602564577717002641838126363875249660735883565852672743849064846414228960666786379280392654615393353172850252103336275952370615397010730691664689375178569039851073146339641623266071126720011020169553304018596457812688561947201171488461172921822139066929851282122002676667750021070848.000000

But this is obviously not correct! How to make real conversion?!

Answer 1

Actually I handled this my self, here is my code, the function translates arbitrary float string to a normal one.

This function inspired by this question.

// DW: remove e
std::string trim_real(std::string str) {
    // DW: all to lowercase
    std::transform(str.begin(), str.end(), str.begin(), ::tolower);

    // DW: find "e" and "."
    size_t pe = str.find_first_of("e");
    std::string se = str.substr(pe + 1, str.size() - (pe + 1));
    int de = std::stoi(se);

    // DW: get string without "e"
    std::string sc = str.substr(0, pe);
    std::cout << "sc is " << sc << std::endl;

    // DW: remove uneccessary 0
    size_t pf0 = sc.find_first_not_of("0");
    size_t pl0 = sc.find_last_not_of("0");
    std::string sc_trim = sc.substr(pf0, pl0 - pf0 + 1);
    std::cout << "sc_trim is " << sc_trim << std::endl;

    // DW: move "." according to e
    size_t pp = sc_trim.find_first_of(".");
    if (pp == std::string::npos) {
        // DW: this means there is no "."
        pp = 0;
    }
    int pp_tobe = pp + de;

    sc_trim.erase(pp, 1);
    if (de > 0) {
        // DW: should move "." to the right
        if (pp_tobe < sc_trim.size()) {
            sc_trim.insert(pp_tobe, ".");
        } else {
            sc_trim += std::string(pp_tobe - sc_trim.size() + 1, '0');
            sc_trim += ".0";
        }
    } else {
        // DW: should move "." to the left
        if (pp_tobe >= 0) {
            sc_trim.insert(pp_tobe, ".");
        } else {
            sc_trim = "." + std::string(0 - pp_tobe, '0') + sc_trim;
        }
    }

    if (sc_trim == ".") {
        // DW: this means sc_trim is actually 0 now
        sc_trim = "0.0";
    }
    return sc_trim;
}