cpp / latest / numeric / math / trunc.html /

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

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).

Parameters

arg - floating point value

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

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

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.

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

See also

(C++11)(C++11)
nearest integer not greater than the given value
(function)
(C++11)(C++11)
nearest integer not less than the given value
(function)
(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)
C documentation for trunc

© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.
https://en.cppreference.com/w/cpp/numeric/math/trunc