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Difference between revisions of "cpp/io/manip/setprecision"

From cppreference.com
< cpp‎ | io‎ | manip
(Added LWG issue #183 DR (part 5).)
m (→‎Example: modernized with <numbers>::pi, though verbose, it also exposes a rounding effect)
Line 21: Line 21:
 
#include <iostream>
 
#include <iostream>
 
#include <iomanip>
 
#include <iomanip>
−
#include <cmath>
 
 
#include <limits>
 
#include <limits>
 +
#include <numbers>
  
 
int main()
 
int main()
 
{
 
{
−
     const long double pi = std::acos(-1.L);
+
     constexpr long double pi {std::numbers::pi_v<long double>};
−
     std::cout << "default precision (6): " << pi << '\n'
+
 
−
               << "std::setprecision(10): " << std::setprecision(10) << pi << '\n'
+
    const auto default_precision {std::cout.precision()};
−
              << "max precision:        "
+
    constexpr auto max_precision {std::numeric_limits<long double>::digits10 + 1};
−
              << std::setprecision(std::numeric_limits<long double>::digits10 + 1)
+
 
−
              << pi << '\n';
+
     std::cout << "default precision: " << default_precision << '\n'
 +
               << "maximum precision: " << max_precision << "\n\n"
 +
                "precision: pi:\n";
 +
 
 +
    for (int p{0}; p <= max_precision; ++p)
 +
    {
 +
        std::cout << std::setw(2) << p << " " << std::setprecision(p) << pi << '\n';
 +
    }
 +
 
 +
    std::cout << std::setprecision(default_precision); // restore defaults
 
}
 
}
 
|output=
 
|output=
−
default precision (6): 3.14159
+
default precision: 6
−
std::setprecision(10): 3.141592654
+
maximum precision: 19
−
max precision:        3.141592653589793239
+
 
 +
precision: pi:
 +
0  3
 +
1  3
 +
2  3.1
 +
3  3.14
 +
4  3.142
 +
5  3.1416
 +
6  3.14159
 +
7  3.141593
 +
8  3.1415927
 +
9  3.14159265
 +
10 3.141592654
 +
11  3.1415926536
 +
12  3.14159265359
 +
13  3.14159265359
 +
14  3.1415926535898
 +
15  3.14159265358979
 +
16  3.141592653589793
 +
17  3.1415926535897932
 +
18  3.14159265358979324
 +
19  3.141592653589793239
 
}}
 
}}
  

Revision as of 13:56, 25 September 2022

 
C++
 
Input/output library
 
Input/output manipulators
Floating-point formatting
setprecision
Integer formatting
Boolean formatting
Field width and fill control
Other formatting
Whitespace processing
Output flushing
(C++20)  

Status flags manipulation
Time and money I/O
(C++11)
(C++11)
(C++11)
(C++11)
Quoted manipulator
(C++14)
 
Defined in header <iomanip>
/*unspecified*/ setprecision( int n );

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.

Contents

Parameters

n - new value for precision

Return value

An object of unspecified type such that

  • if out is an object of type std::basic_ostream<CharT, Traits>, the expression out << setprecision(n)
    • has type std::basic_ostream<CharT, Traits>&
    • has value out
    • behaves as if it called f(out, n)
  • if in is an object of type std::basic_istream<CharT, Traits>, the expression in >> setprecision(n)
    • has type std::basic_istream<CharT, Traits>&
    • has value in
    • behaves as if it called f(in, n)

where the function f is defined as: 

void f(std::ios_base& str, int n)
{
    // set precision
    str.precision(n);
}

Example

Run this code
#include <iostream>
#include <iomanip>
#include <limits>
#include <numbers>
 
int main()
{
    constexpr long double pi {std::numbers::pi_v<long double>};
 
    const auto default_precision {std::cout.precision()};
    constexpr auto max_precision {std::numeric_limits<long double>::digits10 + 1}; 
 
    std::cout << "default precision: " << default_precision << '\n'
              << "maximum precision: " << max_precision << "\n\n"
                 "precision: pi:\n";
 
    for (int p{0}; p <= max_precision; ++p)
    {
        std::cout << std::setw(2) << p << "  " << std::setprecision(p) << pi << '\n';
    }
 
    std::cout << std::setprecision(default_precision); // restore defaults
}

Output: 

default precision: 6
maximum precision: 19
 
precision: pi:
 0  3
 1  3
 2  3.1
 3  3.14
 4  3.142
 5  3.1416
 6  3.14159
 7  3.141593
 8  3.1415927
 9  3.14159265
10  3.141592654
11  3.1415926536
12  3.14159265359
13  3.14159265359
14  3.1415926535898
15  3.14159265358979
16  3.141592653589793
17  3.1415926535897932
18  3.14159265358979324
19  3.141592653589793239

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 183 C++98 setprecision could only be used with streams
of type std::ostream or std::istream 		usable with any
character stream

See also

(C++11)(C++11)
 changes formatting used for floating-point I/O
(function) [edit]
manages decimal precision of floating point operations
(public member function of std::ios_base) [edit]
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