formatted output for doubles [duplicate] - c++

Question

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C++ setprecision(2) printing one decimal?
(2 answers)
Closed 8 years ago.
I want to know how I can, in c++, print for example 10 as 10.0000, or 10.1 as 10.1000.
I want the total number of digits to be the same and pad with '0' the empty spaces on the right.
I read about 'setprecision', but it doesn't add '0'.
'fixed' is the numbers of digits after the floating point, and I want the total number of digits to be fixed.
is there a way to do this, not using printf?

Answer 1

You can do something like this: (considering you want a fixed length of 5)
int precision(double num){
int p;
if (num <1)
p = 5;
else if (num <10)
p = 4;
else if (num <100)
p = 3;
else if (num <1000)
p = 2;
else if (num <10000)
p = 1;
else
p = 0;
return p;
}
int main(){
double num;
std::cin>>num;
std::cout <<std::fixed <<std::setprecision(precision(num)) << num <<std::endl;
return 0;
}
As you required, if the number of digits After decimal is more than 5 it will be truncated.
Now, implement your own logic for what you want to do if number of digits Before decimal point is more than 5. :)

Answer 2

You will ultimately need to find out the length in digits of the double. I'm not entirely sure how to go about that in a safe way but fortunately I can show you an example where we can do this with integers.
Basically I would recommend creating a new facet class that implements the custom formatting inside do_put(). Then imbue this class into your stream. The exact same thing as below can be done for the overload of do_put() that takes a double as its last argument, plus a few changes to the for loop.
#include <iostream>
#include <iomanip>
class num_put : public std::num_put<char>
{
iter_type do_put( iter_type out, std::ios_base& str, char, long v ) const
{
std::streamsize width = str.width();
int digits = num_digits(v);
if (digits > width)
for (std::streamsize i(0); i < digits - width; ++i)
v /= 10;
/* v = static_cast<int>(v * 10) / 10.; // use this instead for
*/ // floating-point
str.flags(std::ios_base::left);
out = std::num_put<char>::do_put(out, str, '0', v);
str.width(width);
return out;
}
private:
template<class T>
static int num_digits(T num) // only works for integral types
{
int length = 1;
while (num /= 10)
++length;
return length;
}
};
int main()
{
std::cout.imbue(std::locale(std::cout.getloc(), new num_put));
std::cout << std::setw(5) << 123; // 12300
std::cout << std::setw(5) << 123456789; // 12345
}