Use of min and max functions in C

Question

From C    are min and max preferable over fmin and fmax   For comparing two integers  do they provide basically the same functionality    Do you tend to use one of these sets of functions or do you prefer to write your own  perhaps to improve efficiency  portability  flexibility  etc     Notes    The C   Standard Template Library  STL  declares the min and max functions in the standard C   algorithm header    The C standard  C99  provides the fmin and fmax function in the standard C math h header    Thanks in advance

User · Answer

I always use the min and max macros for ints. I'm not sure why anyone would use fmin or fmax for integer values.

The big gotcha with min and max is that they're not functions, even if they look like them. If you do something like:

min (10, BigExpensiveFunctionCall())

That function call may get called twice depending on the implementation of the macro. As such, its best practice in my org to never call min or max with things that aren't a literal or variable.

User · Answer

You re missing the entire point of fmin and fmax  It was included in C99 so that modern CPUs could use their native  read SSE  instructions for floating point min and max and avoid a test and branch  and thus a possibly mis-predicted branch   I ve re-written code that used std  min and std  max to use SSE intrinsics for min and max in inner loops instead and the speed-up was significant

User · Answer

std  min and std  max are templates  So  they can be used on a variety of types that provide the less than operator  including floats  doubles  long doubles  So  if you wanted to write generic C   code you d do something like this   template lt typename T gt  T const amp  max3 T const amp  a  T const amp  b  T const amp  c       using std  max     return max max a b  c      non-qualified max allows ADL     As for performance  I don t think fmin and fmax differ from their C   counterparts

User · Answer

fmin and fmax are specifically for use with floating point numbers  hence the  f    If you use it for ints  you may suffer performance or precision losses due to conversion  function call overhead  etc  depending on your compiler platform   std  min and std  max are template functions  defined in header  lt algorithm gt   which work on any type with a less-than   lt   operator  so they can operate on any data type that allows such a comparison  You can also provide your own comparison function if you don t want it to work off  lt    This is safer since you have to explicitly convert arguments to match when they have different types  The compiler won t let you accidentally convert a 64-bit int into a 64-bit float  for example  This reason alone should make the templates your default choice   Credit to Matthieu M  amp  bk1e   Even when used with floats the template may win in performance  A compiler always has the option of inlining calls to template functions since the source code is part of the compilation unit  Sometimes it s impossible to inline a call to a library function  on the other hand  shared libraries  absence of link-time optimization  etc

User · Answer

fmin and fmax are only for floating point and double variables   min and max are template functions that allow comparison of any types  given a binary predicate  They can also be used with other algorithms to provide complex functionality

User · Answer

Use std  min and std  max   If the other versions are faster then your implementation can add overloads for these and you ll get the benefit of performance and portability   template  lt typename T gt  T min  T  T             default    inline float min  float f1  float f2     return fmin  f1  f2

User · Answer

As you noted yourself  fmin and fmax were introduced in C99  Standard C   library doesn t have fmin and fmax functions  Until C99 standard library gets incorporated into C    if ever   the application areas of these functions are cleanly separated  There s no situation where you might have to  prefer  one over the other   You just use templated std  min std  max in C    and use whatever is available in C

User · Answer

fmin and fmax  of fminl and fmaxl could be preferred when comparing signed and unsigned integers - you can take advantage of the fact that the entire range of signed and unsigned numbers and you don t have to worry about integer ranges and promotions   unsigned int x   4000000000  int y   -1   int z   min x  y   z    int fmin x  y

User · Answer

Couldn t a C   implementation targeted for processors with SSE instructions provide specializations of std  min and std  max for types float  double  and long double which do the equivalent of fminf  fmin  and fminl  respectively   The specializations would provide better performance for floating-point types while the general template would handle non-floating-point types without attempting to coerce floating-point types into floating-point types that way the fmins and fmaxes would

User · Answer

I would prefer the C   min max functions  if you are using C    because they are type-specific   fmin fmax will force everything to be converted to from floating point   Also  the C   min max functions will work with user-defined types as long as you have defined operator lt  for those types   HTH

User · Answer

As Richard Corden pointed  use C   functions min and max defined in std namespace  They provide type safety  and help to avoid comparing mixed types  i e  float point vs integer  what sometimes may be undesirable    If you find that C   library you use defines min max as macros as well  it may cause conflicts  then you can prevent unwanted macro substitution calling the min max functions this way  notice extra brackets     std  min  x  y   std  max  x  y    Remember  this will effectively disable Argument Dependant Lookup  ADL  also called Koenig lookup   in case you want to rely on ADL

User · Answer

There is an important difference between std  min  std  max and fmin and fmax   std  min -0 0 0 0    -0 0 std  max -0 0 0 0    -0 0   whereas  fmin -0 0  0 0    -0 0 fmax -0 0  0 0     0 0   So std  min is not a 1-1 substitute for fmin   The functions std  min and std  max are not commutative  To get the same result with doubles with fmin and fmax one should swap the arguments  fmin -0 0  0 0    std  min -0 0   0 0  fmax -0 0  0 0    std  max  0 0  -0 0    But as far as I can tell all these functions are implementation defined anyway in this case so to be 100  sure you have to test how they are implemented     There is another important difference  For x     NaN   std  max Nan x    NaN std  max x NaN    x std  min Nan x    NaN std  min x NaN    x   whereas  fmax Nan x    x fmax x NaN    x fmin Nan x    x fmin x NaN    x     fmax can be emulated with the following code  double myfmax double x  double y          z  gt  nan for z    nan is required by C the standard    int xnan   isnan x   ynan   isnan y      if xnan    ynan            if xnan  amp  amp   ynan  return y          if  xnan  amp  amp  ynan  return x          return x              0  gt  -0 is preferred by C the standard     if x  0  amp  amp  y  0           int xs   signbit x   ys   signbit y          if xs  amp  amp   ys  return y         if  xs  amp  amp  ys  return x         return x          return std  max x y       This shows that std  max is a subset of fmax   Looking at the assembly shows that Clang uses builtin code for fmax and fmin whereas GCC calls them from a math library   The assembly for clang for fmax with -O3 is  movapd  xmm2  xmm0 cmpunordsd      xmm2  xmm2 movapd  xmm3  xmm2 andpd   xmm3  xmm1 maxsd   xmm1  xmm0 andnpd  xmm2  xmm1 orpd    xmm2  xmm3 movapd  xmm0  xmm2   whereas for std  max double  double  it is simply  maxsd   xmm0  xmm1   However  for GCC and Clang using -Ofast fmax becomes simply  maxsd   xmm0  xmm1   So this shows once again that std  max is a subset of fmax and that when you use a looser floating point model which does not have nan or signed zero then fmax and std  max are the same   The same argument obviously applies to fmin and std  min

User · Answer

If your implementation provides a 64-bit integer type  you may get a different  incorrect  answer by using fmin or fmax  Your 64-bit integers will be converted to doubles  which will  at least usually  have a significand that s smaller than 64-bits  When you convert such a number to a double  some of the least significant bits can will be lost completely   This means that two numbers that were really different could end up equal when converted to double -- and the result will be that incorrect number  that s not necessarily equal to either of the original inputs

User · Answer

By the way  in cstdlib there are   min and   max you can use   For more   http   msdn microsoft com zh-cn library btkhtd8d aspx

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