std::setprecision sets the number of significant figures. How do I use iomanip to set the precision?

I have always found iomanip confusing and counter intuitive. I need help.

A quick internet search finds (https://www.vedantu.com/maths/precision) "We thus consider precision as the maximum number of significant digits after the decimal point in a decimal number" (the emphasis is mine). That matches my understanding too. However I wrote a test program and:

stm << std::setprecision(3) << 5.12345678;
std::cout << "5.12345678: " << stm.str() << std::endl;
stm.str("");

stm << std::setprecision(3) << 25.12345678;
std::cout << "25.12345678: " << stm.str() << std::endl;
stm.str("");

stm << std::setprecision(3) << 5.1;
std::cout << "5.1: " << stm.str() << std::endl;
stm.str("");

outputs:

5.12345678: 5.12
25.12345678: 25.1
5.1: 5.1

If the precision is 3 then the output should be:

5.12345678: 5.123
25.12345678: 25.123
5.1: 5.1

Clearly the C++ standard has a different interpretation of the meaning of "precision" as relates to floating point numbers.

If I do:

stm.setf(std::ios::fixed, std::ios::floatfield);

then the first two values are formatted correctly, but the last comes out as 5.100.

How do I set the precision without padding?

Solution 1

You can try using this workaround:

decltype(std::setprecision(1)) setp(double number, int p) {
    int e = static_cast<int>(std::abs(number));
    e = e != 0? static_cast<int>(std::log10(e)) + 1 + p : p;
    while(number != 0.0 && static_cast<int>(number*=10) == 0 && e > 1) 
        e--; // for numbers like 0.001: those zeros are not treated as digits by setprecision.
    return std::setprecision(e);
}

And then:

auto v = 5.12345678;
stm << setp(v, 3) << v;

Solution 2

Another more verbose and elegant solution is to create a struct like this:

struct __setp {
    double number;
    bool fixed = false;
    int prec;
};

std::ostream& operator<<(std::ostream& os, const __setp& obj)
{
    if(obj.fixed)
        os << std::fixed;
    else os << std::defaultfloat;
    os.precision(obj.prec);
    os << obj.number; // comment this if you do not want to print immediately the number
    return os;
}

__setp setp(double number, int p) {
     __setp setter;
     setter.number = number;
     
    int e = static_cast<int>(std::abs(number));
    e = e != 0? static_cast<int>(std::log10(e)) + 1 + p : p;
    while(number != 0.0 && static_cast<int>(number*=10) == 0)
        e--; // for numbers like 0.001: those zeros are not treated as digits by setprecision.

    if(e <= 0) {
        setter.fixed = true;
        setter.prec = 1;
    } else
        setter.prec = e;
    return setter;
}

Using it like this:

auto v = 5.12345678;
stm << setp(v, 3);