CODEWITHSUNDEEP
c++scientific-notation
user3.141592653
user3.141592653

Reputation: 63

c++ scientific notation, change the digits of exponent

I am using this code to output a scientific notation number to a file: 

file << scientific << setprecision(10) << num << endl;

For example, if the number is 3.0, I get 3.0000000000E+00 from the code. How do I make the digits of the exponent part to be 3 digits? I want to get 3.0000000000E+000.

Upvotes: 3

Views: 3174

Answers (1)

darclander
darclander

Reputation: 1811

It seems like as @Bob__ stated that visual studio used to have a function called: _set_output_format which allowed you to change the exponent BUT this was removed in the Visual Studio 2015 version I believe.

As quoted in Portable printing of exponent of a double to C++ iostreams.

The %e and %E format specifiers format a floating point number as a decimal mantissa and exponent. The %g and %G format specifiers also format numbers in this form in some cases. In previous versions, the CRT would always generate strings with three-digit exponents. For example, printf("%e\n", 1.0)would print 1.000000e+000. This was incorrect: C requires that if the exponent is representable using only one or two digits, then only two digits are to be printed.

In Visual Studio 2005 a global conformance switch was added: _set_output_format. A program could call this function with the argument _TWO_DIGIT_EXPONENT, to enable conforming exponent printing. The default behavior has been changed to the standards-conforming exponent printing mode.

So in order to resolve your issue if you really want to write with a 3 digit exponent would be to modify the output. You can do so by transforming the double into a string, example: 

#include <iostream>
#include <string>
#include <sstream>
#include <iomanip>


std::string getStrCpy(double dbl) {
    std::ostringstream str;
    str << std::scientific << std::setprecision(10) << dbl;
    std::string s = str.str();
    std::cout << s; // Prints 3.0000000000e+00
    return s;
}

int main() {

    double d = 3.0;
    std::cout.precision(10);
    std::cout << std::scientific;

    std::cout << d << '\n'; // Prints 3.0000000000e+00
    getStrCpy(d); // Will also print 3.0000000000e+00

    return 0;
}

Now getStrCpy() will return a string of the double you were trying to print which you can modify in order to add an extra number to your exponent.

Upvotes: 1

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