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std::setprecision

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When used in an expression out << setprecision(n) or in >> setprecision(n), sets the precision parameter of the stream out or in to exactly n. Parameter of this function is integer, which is new value for precision.

Example: 

#include <iostream>
#include <iomanip>
#include <cmath>
#include <limits>
int main()
{
    const long double pi = std::acos(-1.L);
    std::cout << "default precision (6): " << pi << '\n'    
              << "std::precision(10):    " << std::setprecision(10) << pi << '\n'
              << "max precision:         "
              << std::setprecision(std::numeric_limits<long double>::digits10 + 1)
              << pi << '\n';
}
//Output
//default precision (6): 3.14159
//std::precision(10):    3.141592654
//max precision:         3.141592653589793239
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std::iomanip:
* std::setprecision
* std::setfill
* std::setiosflags
* std::setw
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0 Getting started
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136 Debugging
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138 More undefined behaviors
139 Mutexes
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141 Unit testing
142 decltype
143 Digit separators
144 C++ Containers
145 Arithmetic meta-programming
146 Contributors