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Extension to the STL "find_if" and "for_each"

Posted by Michael Scherotter on August 6th, 1998
The stl algorithms for_each and find_if are inadequate when you need more data for the predicate. Here are two new algorithms TDfind_if and TDfor_each which solve that problem. 
To use:

std::list<OBJECT> theList;

class DATA
{
public:
    bool IsValid(const OBJECT& rObject) const;
    void OperateOn(OBJECT& rObject) const;
};

bool IsValid(OBJECT theObject, DATA theData)
{
    return theData.IsValid(theObject);
}

void DoOperation(OBJECT theObject, DATA theData)
{
    theData.OperateOn(theObject);
}

DATA theData;

OBJECT theObject = ::TDFind_if(theList.begin(), theList.end(), IsValid, theData);
or
::TDfor_each(theList.begin(), theList.end(), DoOperation, theData);

// CODE STARTS HERE:
/////////////////////////////////////////////////////////////////
// This is like the Standard c++ algorithm find_if, but it takes a second parameter
template<class _II, class _Pr, class T_DATA> inline 
_II
TDfind_if(
  _II _F, // First iterator
  _II _L, // Last iterator
  _Pr _P, // Predicate: a static function that returns bool having two
paramters
  T_DATA dwData)
{
    for (; _F != _L; ++_F)
    {
        if (_P(*_F, dwData))
        {
            break;
        }
    }

    return (_F);
}


template<class _II, class _Fn, class T_DATA> inline
void
TDfor_each(
  _II _F,
  _II _L,
  _Fn _Op,
  T_DATA dData)
{
    for (; _F != _L; ++_F)
    {
        _Op(*_F, dData);
    }
}

Date Posted: 5/4/98
Posted by: Pat Laplante.

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Comments

  • mayyouhelpme

    Posted by Legacy on 11/03/2002 12:00am

    Originally posted by: tangsilai

    hello
    I am a new man in this aspect, And I want some comments of your how to use c++ freely.I am too sad to found it is to hard to learn.Help me please!!

    Reply

  • Even plainer STL

    Posted by Legacy on 08/13/1999 12:00am

    Originally posted by: Dana P'Simer 

    The following code shows how to use just STL to implement

    the exact same functionality without the use of custom

    implementations of find_if and for_each, custom function

    objects, or custom adapter objects.  It uses std::bind1st

    and std::mem_fun1 to call the memeber functions of DATA

    with the OBJECT as an argument.


    -----------------------

    #include <list>

    #include <algorithm>

    #include <iostream>


    typedef int OBJECT;


    std::list<OBJECT> theList;


    class DATA

    {

    public:

        bool IsValid(OBJECT rObject) { return rObject == 5; }

        int OperateOn(OBJECT rObject) { std::cout << rObject << std::endl; return 0; }

    };


    int main(int argc, char* argv[])

    {

    	DATA theData;


    	// fill the list

    	for( OBJECT i = 0; i < 10; i++ )

    		theList.push_back(i);


    	// find the 5

    	std::cout << "The results of find_if:" << std::endl;

    	OBJECT theObject = *std::find_if(theList.begin(), theList.end(), std::bind1st(std::mem_fun1(DATA::IsValid),&theData) );

    	std::cout << theObject << std::endl;


    	// print the list

    	std::cout << "The results of for_each:" << std::endl;

    	std::for_each(theList.begin(), theList.end(), std::bind1st(std::mem_fun1(DATA::OperateOn),&theData));


    	char c;

    	std::cin >> c;

    	return 0;

    }

    Reply

  • Plain STL

    Posted by Legacy on 10/27/1998 12:00am

    Originally posted by: Ithier de Lestrange 

    You do not need to rewrite the find_if and for_each algorithms to have more arguments. You just need to use a
struct (and the operator() function) as shown below:


    

    struct ex : public std::unary_function<OBJECT&, void>

    {

    	ex  (short Param1, bool Param2) : 

    	n_Param1(Param1), b_Param2 (Param2)

    	{

    	}

    	void operator() (OBJECT& x)

    	{

    	x.Function1 (m_Param1, b_param2);

    	}


        int		n_Param1;

    	bool	b_Param2;

    };


    std::list<OBJECT> TheList;


    std::for_each (TheList.begin(), TheList.end(), ex(3, false));


    

    At http://www.sgi.com/Technology/STL/ you can find a very good documentation about STL.

    Reply
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