Re: Optimize integer program in speed

=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <>
Thu, 17 Jun 2010 06:08:41 CST
On 17 Jun., 00:02, Keith H Duggar <> wrote:

On Jun 7, 11:09 am, Saeed Amrollahi <> wrote:


given that Daniel and Alf have already explained the
reasons I will just mention that you can also work around this
issue and (usually) achieve the same performance by wrapping a
free function with a unique type viz:

bool bigger ( int i ) { return i >= D || i <= -D ; }

template < bool ( & func ) ( int i ) >
struct FuncWrap
    bool operator ( ) ( int i ) const { return func(i) ; }
} ;


    count += count_if(diff + 1, diff + N, FuncWrap<bigger>()) ;

The function reference is now a static argument and thus more
likely to be optimized as a direct call.

Unfortunately (AFAIK) you will need C++0x support in order to
do away with the non-type parameter explicit type declaration
(using auto and decltype).

Or just use a normal wrapper functor:

struct BiggerWrap
    bool operator()(int i) const { return bigger(i); }


count += count_if(diff + 1, diff + N, BiggerWrap()) ;

The pro is, that it works independent on the language
linkage of bigger,[1] the con is, that it scales badly
because I need a separate class for each function.

In C++ we can eat the sweet part of the fruit without
the sour part, if we take advantage of lambda

count += count_if(diff + 1, diff + N, [](int i){ return bigger(i); });

Greetings from Bremen,

Daniel Kr?gler

[1] This is especially important, if you need to wrap
a standard C library function like int abs(int), where
it is implementation-defined, whether the linkage is
extern "C" or extern "C++".

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