Is there a trunc function in C++? - c++

Question

I searched around and couldn't find the trunc function for C++. I know I can do this:
int main()
{
double a = 12.566789;
cout << setprecision(2) << fixed << (int)(a * 100) / 100.0 << endl;
return 0;
}
but I'm not sure it's the best way to do this. Thank you.

Answer 1

If your C library is so old that it lacks a trunc function (specified in C99), you can easily implement one based on floor and ceil (specified in C89)
double trunc(double d){ return (d>0) ? floor(d) : ceil(d) ; }

Answer 2

trunc is there, in <cmath>:
#include <iostream>
#include <cmath>
int main() {
std::cout << trunc(3.141516) << std::endl;
}
I suppose you're looking for something else?

Answer 3

There's a trunc function in C that you can use in C++
trunc(a*100)/100
Keep in mind that you still have to specify formatting requests, because floating point can't represent all real numbers exactly, and you could get output like 12.5600000001 or 12.55999999 if you don't tell the output code the precision you want.
TL;DR
Use the following for output:
cout << setprecision(2) << fixed << a<< endl;
And the following if you need a truncated result somewhere during a mathematical calculation:
trunc(a*100)/100
(Or better yet, use fixed-point math.)

Answer 4

Sure. Use the trunc() function from math.h. It's a C function, but it works as well in C++ as it does in C. If you want to keep a couple digits, you can always:
double a = 12.566789;
double b = trunc(a * 100) / 100.0;

Answer 5

If you're using an ancient C or C++ library that doesn't implement trunc, use boost::math::trunc.

Answer 6

I've developed a very fast trunc-function:
double ftrunc( double d )
{
static_assert(sizeof(double) == sizeof(uint64_t), "sizeof(double) not equal to sizeof(uint64_t)");
static_assert(numeric_limits<double>::is_iec559, "double must be IEEE-754");
// assume size_t is our CPU's native register-width
static_assert(sizeof(size_t) == sizeof(uint64_t) || sizeof(size_t) == sizeof(uint32_t), "register-width must be 32 or 64 bit");
if constexpr( sizeof(size_t) == sizeof(uint64_t) )
// we have 64 bit registers
{
unsigned const MANTISSA_BITS = 52,
EXP_BIAS = 0x3FF,
INF_NAN_BASE = 0x7FF;
uint64_t const EXP_MASK = (uint64_t)0x7FF << MANTISSA_BITS,
SIGN_MASK = (uint64_t)0x800 << MANTISSA_BITS ,
MIN_INTEGRAL_DIGITS_EXP = (uint64_t) EXP_BIAS << MANTISSA_BITS,
MIN_INTEGRAL_ONLY_EXP = (uint64_t)(EXP_BIAS + MANTISSA_BITS) << MANTISSA_BITS,
INF_NAN_EXP = (uint64_t)INF_NAN_BASE << MANTISSA_BITS,
NEG_MANTISSA_MASK = 0x000FFFFFFFFFFFFFu;
union
{
double du;
uint64_t dx;
};
du = d;
uint64_t exp = dx & EXP_MASK;
if( exp >= MIN_INTEGRAL_DIGITS_EXP )
// value has integral digits
if( exp < MIN_INTEGRAL_ONLY_EXP )
{
// there are fraction-digits to mask out, mask them
unsigned shift = (unsigned)(exp >> MANTISSA_BITS) - EXP_BIAS;
dx &= ~(NEG_MANTISSA_MASK >> shift);
return du;
}
else
if( exp < INF_NAN_EXP )
// value is integral
return du;
else
// infinite, NaN, SNaN
// raise exception on SNaN if necessary
return du + du;
else
{
// below +/-1.0
// return +/-0.0
dx &= SIGN_MASK;
return du;
}
}
else if constexpr( sizeof(size_t) == sizeof(uint32_t) )
// we have 32 bit registers
{
unsigned const MANTISSA_BITS = 52,
HI_MANTISSA_BITS = 20,
EXP_BIAS = 0x3FF,
INF_NAN_BASE = 0x7FF;
uint32_t const EXP_MASK = (uint32_t)0x7FFu << HI_MANTISSA_BITS,
SIGN_MASK = (uint32_t)0x800u << HI_MANTISSA_BITS,
MIN_INTEGRAL_DIGITS_EXP = (uint32_t) EXP_BIAS << HI_MANTISSA_BITS,
MAX_INTEGRAL32_EXP = (uint32_t)(EXP_BIAS + HI_MANTISSA_BITS) << HI_MANTISSA_BITS,
MIN_INTEGRAL_ONLY_EXP = (uint32_t)(EXP_BIAS + MANTISSA_BITS) << HI_MANTISSA_BITS,
INF_NAN_EXP = (uint32_t)INF_NAN_BASE << HI_MANTISSA_BITS,
NEG_HI_MANTISSA_MASK = 0x000FFFFFu,
NEG_LO_MANTISSA_MASK = 0xFFFFFFFFu;
union
{
double du;
struct
{
uint32_t dxLo;
uint32_t dxHi;
};
};
du = d;
uint32_t exp = dxHi & EXP_MASK;
if( exp >= MIN_INTEGRAL_DIGITS_EXP )
// value has integral digits
if( exp < MIN_INTEGRAL_ONLY_EXP )
// there are fraction-digits to mask out
if( exp <= MAX_INTEGRAL32_EXP )
{
// the fraction digits are in the upper dword, mask them and zero the lower dword
unsigned shift = (unsigned)(exp >> HI_MANTISSA_BITS) - EXP_BIAS;
dxHi &= ~(NEG_HI_MANTISSA_MASK >> shift);
dxLo = 0;
return du;
}
else
{
// the fraction digits are in the lower dword, mask them
unsigned shift = (unsigned)(exp >> HI_MANTISSA_BITS) - EXP_BIAS - HI_MANTISSA_BITS;
dxLo &= ~(NEG_LO_MANTISSA_MASK >> shift);
return du;
}
else
if( exp < INF_NAN_EXP )
// value is integral
return du;
else
// infinite, NaN, SNaN
// raise exception on SNaN if necessary
return du + du;
else
{
// below +/-1.0
// return +/-0.0
dxHi &= SIGN_MASK;
dxLo = 0;
return du;
}
}
}
It's faster than most implemementations. On my Ryzen 7 1800X the average execution-time of values >= 2^0 and <= 2^54 is 12 clock cycles.

Answer 7

use ceil or floor from cmath