NAME¶

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

Synopsis¶

Defined in header <cmath>
float trunc ( float num );

double trunc ( double num ); (until C++23)

long double trunc ( long double num );
constexpr /* floating-point-type */ (since C++23)
trunc ( /* floating-point-type */ num );
float truncf( float num ); (1) (2) (since C++11)
(constexpr since C++23)
long double truncl( long double num ); (3) (since C++11)
(constexpr since C++23)
Additional overloads (since C++11)
Defined in header <cmath>
template< class Integer > (A) (constexpr since C++23)
double trunc ( Integer num );

1-3) Computes the nearest integer not greater in magnitude than num.
The library provides overloads of std::trunc for all cv-unqualified floating-point
types as the type of the parameter.
(since C++23)

A) Additional overloads are provided for all integer types, which are (since C++11)
treated as double.

Parameters¶

num - floating-point or integer value

Return value¶

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

Return value¶

math-trunc.svg
num

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 num is ±∞, it is returned, unmodified.
* If num is ±0, it is returned, unmodified.
* If num 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.

The additional overloads are not required to be provided exactly as (A). They only
need to be sufficient to ensure that for their argument num of integer type,
std::trunc(num) has the same effect as std::trunc(static_cast<double>(num)).

Example¶

// Run this code

#include <cmath>
#include <initializer_list>
#include <iostream>

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¶

floor
floorf nearest integer not greater than the given value
floorl (function)
(C++11)
(C++11)
ceil
ceilf nearest integer not less than the given value
ceill (function)
(C++11)
(C++11)
round
roundf
roundl
lround
lroundf
lroundl
llround
llroundf
llroundl nearest integer, rounding away from zero in halfway cases
(C++11) (function)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
C documentation for
trunc