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std::trunc, std::truncf, std::truncl

From cppreference.com
< cpp‎ | numeric‎ | math
 
 
 
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trunc
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Defined in header <cmath>
float       trunc ( float arg );
(1) (since C++11)
(constexpr since C++23)
float       truncf( float arg );
(2) (since C++11)
(constexpr since C++23)
double      trunc ( double arg );
(3) (since C++11)
(constexpr since C++23)
long double trunc ( long double arg );
(4) (since C++11)
(constexpr since C++23)
long double truncl( long double arg );
(5) (since C++11)
(constexpr since C++23)
double      trunc ( IntegralType arg );
(6) (since C++11)
(constexpr since C++23)
1-5) Computes the nearest integer not greater in magnitude than arg.
6) A set of overloads or a function template accepting an argument of any integral type. Equivalent to (3) (the argument is cast to double).

Contents

[edit] Parameters

arg - floating point value

[edit] Return value

If no errors occur, the nearest integer value not greater in magnitude than arg (in other words, arg rounded towards zero) is returned.

Return value
math-trunc.svg
Argument


[edit] Error handling

Errors are reported as specified in math_errhandling.

If the implementation supports IEEE floating-point arithmetic (IEC 60559),

  • The current rounding mode has no effect.
  • If arg is ±∞, it is returned, unmodified
  • If arg is ±0, it is returned, unmodified
  • If arg is NaN, NaN is returned

[edit] Notes

FE_INEXACT may be (but isn't required to be) raised when truncating a non-integer finite value.

The largest representable floating-point values are exact integers in all standard floating-point formats, so this function never overflows on its own; however the result may overflow any integer type (including std::intmax_t), when stored in an integer variable.

The implicit conversion from floating-point to integral types also rounds towards zero, but is limited to the values that can be represented by the target type.

[edit] Example

#include <cmath>
#include <iostream>
#include <initializer_list>
 
int main()
{
    const auto data = std::initializer_list<double>{
        +2.7, -2.9, +0.7, -0.9, +0.0, 0.0, -INFINITY, +INFINITY, -NAN, +NAN
    };
 
    std::cout << std::showpos;
    for (double const x : data) {
        std::cout << "trunc(" << x << ") == " << std::trunc(x) << '\n';
    }
}

Possible output:

trunc(+2.7) == +2
trunc(-2.9) == -2
trunc(+0.7) == +0
trunc(-0.9) == -0
trunc(+0) == +0
trunc(+0) == +0
trunc(-inf) == -inf
trunc(+inf) == +inf
trunc(-nan) == -nan
trunc(+nan) == +nan

[edit] See also

(C++11)(C++11)
nearest integer not greater than the given value
(function) [edit]
(C++11)(C++11)
nearest integer not less than the given value
(function) [edit]
(C++11)(C++11)(C++11)(C++11)(C++11)(C++11)(C++11)(C++11)(C++11)
nearest integer, rounding away from zero in halfway cases
(function) [edit]
C documentation for trunc