Difference between revisions of "cpp/io/manip/setprecision"
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Andreas Krug (Talk | contribs) m (headers sorted, fmt) |
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===Example=== | ===Example=== | ||
{{example | {{example | ||
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|code= | |code= | ||
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#include <iomanip> | #include <iomanip> | ||
| + | #include <iostream> | ||
#include <limits> | #include <limits> | ||
#include <numbers> | #include <numbers> | ||
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int main() | int main() | ||
{ | { | ||
| − | + | constexpr long double pi{std::numbers::pi_v<long double>}; | |
| − | const auto default_precision {std::cout.precision()}; | + | 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' | std::cout << "default precision: " << default_precision << '\n' | ||
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for (int p{0}; p <= max_precision; ++p) | for (int p{0}; p <= max_precision; ++p) | ||
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std::cout << std::setw(2) << p << " " << std::setprecision(p) << pi << '\n'; | std::cout << std::setw(2) << p << " " << std::setprecision(p) << pi << '\n'; | ||
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std::cout << std::setprecision(default_precision); // restore defaults | std::cout << std::setprecision(default_precision); // restore defaults | ||
Latest revision as of 23:06, 15 September 2023
| Defined in header <iomanip>
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| /*unspecified*/ setprecision( int n ); |
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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.
Contents |
[edit] Parameters
| n | - | new value for precision |
[edit] 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); }
[edit] Example
Run this code
#include <iomanip> #include <iostream> #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
[edit] 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 streamsof type std::ostream or std::istream |
usable with any character stream |
[edit] See also
| (C++11)(C++11) |
changes formatting used for floating-point I/O (function) |
| manages decimal precision of floating point operations (public member function of std::ios_base)
|
