Is div function useful (stdlib.h)? [duplicate]

问题

There is a function called div in C,C++ (stdlib.h)

div_t div(int numer, int denom);

typedef struct _div_t
{
  int quot;
  int rem;
} div_t;

But C,C++ have / and % operators.

My question is: "When there are / and % operators, Is div function useful?"

回答1:

The div() function returns a structure which contains the quotient and remainder of the division of the first parameter (the numerator) by the second (the denominator). There are four variants:

div_t div(int, int)
ldiv_t ldiv(long, long)
lldiv_t lldiv(long long, long long)
imaxdiv_t imaxdiv(intmax_t, intmax_t (intmax_t represents the biggest integer type available on the system)

The div_t structure looks like this:

typedef struct
  {
    int quot;           /* Quotient.  */
    int rem;            /* Remainder.  */
  } div_t;

The implementation does simply use the / and % operators, so it's not exactly a very complicated or necessary function, but it is part of the C standard (as defined by [ISO 9899:201x][1]).

See the implementation in GNU libc:

/* Return the `div_t' representation of NUMER over DENOM.  */
div_t
div (numer, denom)
     int numer, denom;
{
  div_t result;

  result.quot = numer / denom;
  result.rem = numer % denom;

  /* The ANSI standard says that |QUOT| <= |NUMER / DENOM|, where
     NUMER / DENOM is to be computed in infinite precision.  In
     other words, we should always truncate the quotient towards
     zero, never -infinity.  Machine division and remainer may
     work either way when one or both of NUMER or DENOM is
     negative.  If only one is negative and QUOT has been
     truncated towards -infinity, REM will have the same sign as
     DENOM and the opposite sign of NUMER; if both are negative
     and QUOT has been truncated towards -infinity, REM will be
     positive (will have the opposite sign of NUMER).  These are
     considered `wrong'.  If both are NUM and DENOM are positive,
     RESULT will always be positive.  This all boils down to: if
     NUMER >= 0, but REM < 0, we got the wrong answer.  In that
     case, to get the right answer, add 1 to QUOT and subtract
     DENOM from REM.  */

  if (numer >= 0 && result.rem < 0)
    {
      ++result.quot;
      result.rem -= denom;
    }

  return result;
}

回答2:

Yes, it is: it calculates the quotient and remainder in one operation.

Aside from that, the same behaviour can be achieved with /+% (and a decent optimizer will optimize them into a single div anyway).

In order to sum it up: if you care about squeezing out last bits of performance, this may be your function of choice, especially if the optimizer on your platform is not so advanced. This is often the case for embedded platforms. Otherwise, use whatever way you find more readable.

回答3:

The semantics of div() is different than the semantics of % and /, which is important in some cases. That is why the following code is in the implementation shown in psYchotic's answer:

if (numer >= 0 && result.rem < 0)
    {
      ++result.quot;
      result.rem -= denom;
    }

% may return a negative answer, whereas div() always returns a non-negative remainder.

Check the WikiPedia entry, particularly "div always rounds towards 0, unlike ordinary integer division in C, where rounding for negative numbers is implementation-dependent."

回答4:

div() filled a pre-C99 need: portability

Pre C99, the rounding direction of the quotient of a / b with a negative operand was implementation dependent. With div(), the rounding direction is not optional but specified to be toward 0. div() provided uniform portable division. A secondary use was the potential efficiency when code needed to calculate both the quotient and remainder.

With C99 and later, div() and / specifying the same round direction and with better compilers optimizing nearby a/b and a%b code, the need has diminished.

This was the compelling reason for div() and it explains the absence of udiv_t udiv(unsigned numer, unsigned denom) in the C spec: The issues of implementation dependent results of a/b with negative operands are non-existent for unsigned even in pre-C99.

回答5:

Probably because on many processors the div instruction produces both values and you can always count on the compiler to recognize that adjacent / and % operators on the same inputs could be coalesced into one operation.

回答6:

It costs less time if you need both value. CPU always calculate both remainder and quotient when performing division. If use "/" once and "%" once, cpu will calculate twice both number.

(forgive my poor English, I'm not native)

来源：https://stackoverflow.com/questions/6718217/is-div-function-useful-stdlib-h

标签

c++

integer-division