c++ cout << don't print '0' before decimal point

问题

I didn't find the solution to write decimal number inferior to 1 without the '0' before the decimal point. I want to display numbers at this format : ".1", ".2", etc...

using :

std::cout << std::setw(2) << std::setprecision(1) << std::fixed << number;

always give me formats like "0.1", "0.2", etc...

What do I wrong ? Thanks for your help

回答1:

You need to convert it to a string and use it for printing. There is no way for a stream to print a floatingpoint without a leading zero, if there is one.

std::string getFloatWithoutLeadingZero(float val)
{
    //converting the number to a string
    //with your specified flags

    std::stringstream ss;
    ss << std::setw(2) << std::setprecision(1);
    ss << std::fixed << val;
    std::string str = ss.str();

    if(val > 0.f && val < 1.f)
    {
        //Checking if we have no leading minus sign

        return str.substr(1, str.size()-1);
    }
    else if(val < 0.f && val > -1.f)
    {
        //Checking if we have a leading minus sign

        return "-" + str.substr(2, str.size()-1);
    }

    //The number simply hasn't a leading zero
    return str;
}

Try it online!

EDIT: Some solution you may like more would be a custom float type. e.g.

class MyFloat
{
public:
    MyFloat(float val = 0) : _val(val)
    {}

    friend std::ostream& operator<<(std::ostream& os, const MyFloat& rhs)
    { os << MyFloat::noLeadingZero(rhs._val, os); }

private:
    static std::string noLeadingZero(float val, std::ostream& os)
    {
        std::stringstream ss;
        ss.copyfmt(os);
        ss << val;
        std::string str = ss.str();

        if(val > 0.f && val < 1.f)
            return str.substr(1, str.size()-1);
        else if(val < 0.f && val > -1.f)
            return "-" + str.substr(2, str.size()-1);

        return str;
    }
    float _val;
};

Try it online!

回答2:

In iomanip library, it seems no function to trim 0 before cout. You need to convert output to string.

Here is my solution:

double number=3.142, n; //n=3
char s[2];
sprintf (s, ".%d", int(modf(number, &n)*10)); 
                     //modf(number, &n)=0.142 s='.1'
cout << s;

来源：https://stackoverflow.com/questions/26684939/c-cout-dont-print-0-before-decimal-point