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STL find

Started by MagTDK May 22, 2002 at 10:46 PM 3 replies 200+ views

Original Post

May 22, 2002 at 10:46 PM

I found this example at http://www.sgi.com/tech/stl/find.html

      
list<int> L;
L.push_back(3);
L.push_back(1);
L.push_back(7);

list<int>::iterator result = find(L.begin(), L.end(), 7);

I understand this, but what if I have a list like so:

        
class CTest
{
  int idata;
  int idata2;
  float idata3;
};

list<int> mylist;

CTest myTest;

myTest.idata = 5;
myTest.idata2 = 10;
myTest.idata3 = 20.00f

mylist.push_back(myTest);

myTest.idata = 2;
myTest.idata2 = 4;
myTest.idata3 = 8.00f

mylist.push_back(myTest);

list<CTest>::iterator result = find(mylist.begin(), mylist.end(), ?);

How do I fill the last parameter of the find function? For example if I wanted to find if idata2 equals 4? [edited by - MagTDK on May 22, 2002 11:49:59 PM]

Shannon Barber

May 22, 2002 at 11:26 PM

You have to write a functor, "Effective STL" by Scott Meyers helped me quite a bit on working with the STL

  
struct search_for_idata2
{
const int sought;
search_for_idata2(int seek) : sought(seek){}
bool operator()(const Test& test)
	{
	return test.idata == sought;
	}
};


//This is what needs to happen

search_for_idata2 search_functor(4);
std::find_if(.begin(), .end(),  search_functor);

//Typically it''s written like this:

std::find_if(.begin(), .end(),  search_for_idata2(4) );
//This create a temporary instance of search_for_idata2, the

// ending () does that, and then the 4 is passed as the ctor arguement

There''s a couple of slicker ways to do it, but that''s the most straight forward.

Oluseyi

May 22, 2002 at 11:33 PM

IIRC, std::list has its own find member method for specifically this reason. But for complex list elements, you''ll still need a search functor.

Stoffel

May 23, 2002 at 1:05 AM

Containers that specialize find are map, set, multimap and multiset. This is because std::find is O(N), and indeed can be no better for list, deque or vector. But the ordered containers can implement find O(lgN), so they specialize it.

MagTDK

May 23, 2002 at 9:36 AM

Thanks for your help.

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