C++, scientific notation, format number

Question

Is it possible to format string in scientific notation in the following ways:

• set fixed places in exponent: 1

• set fixed decimal places in mantissa: 0

   double number = 123456.789
  

So the number should be formated

  1e+5

I am not able to set 0 decimal points for mantissa:

cout.precision(0);
cout << scientific << number;

result:

1.234568e+005

Answer 1

You can easily do this with the {fmt} formatting library:

#include <fmt/core.h>

double number = 123456.789;
auto s = fmt::format("{:.0e}\n", number); // s == "1e+05"
s.erase(s.size() - 2, 1); // s == "1e+5"

A formatting facility based on this library is proposed for standardization in C++20: P0645.

Disclaimer: I'm the author of {fmt}.

Answer 2

I can't figure out how to get a single digit in the exponent field but the following matches all your other requirements.

#include <iostream>
#include <iomanip>

int main()
{
  const double number = 123456.789;

  std::cout << std::setprecision(0) << std::scientific << number << std::endl;
}

Output:

1e+05

EDIT:
Did a quick search through the standard (N3291) and couldn't find anything that talked about the number of digits in the exponent field when using scientific notation. This might be implementation defined.

Answer 3

You can actualy format anything once you have a string.. more c++ code would look like:

const double number = 123456.789;
const int expSize = 1;
std::ostringstream oss;
std::string output;
oss << std::scientific << number;
unsigned int ePos = oss.str().find("e");
unsigned int dPos = oss.str().find(".");
if(ePos == 0){
    //no exponent
}
else if(dPos == 0){
    //not decimal
}
else{
    output = oss.str().substr(0, dPos) + oss.str().substr(ePos, 2);
    if(oss.str().size()-expSize > ePos+1)
        output += oss.str().substr(oss.str().size()-expSize, oss.str().size());
    else{
        //expSize too big (or bug -> e used but no exponent?)
    }
    std::cout << output;
}

Output:

1e+5

You can set exponent size in expSize and this works for arbitrary large exponent.

Hope it helps!