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C++ Vectors

Posted on 2010-09-01
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Last Modified: 2012-05-10
I was looking at figuring out what the quickest way of 'finding' a value in vectors are.

For example, when I scan through all my ships in game, I'll do something like:

      const ShipClass *ship = &game->ship[0];
      const ShipClass* end1 = ship + Nobj;
      while (ship != end1)
      {
            if (ship && ship->active) {
                                      //DO STUFF HERE
            }
            ship++;
      }

I'm wondering to 'find', I need to do something similar, going through an array and matching it with what I'm looking for, like:

              int findthis = 30;
      const ShipClass *ship = &game->ship[0];
      const ShipClass* end1 = ship + Nobj;
      while (ship != end1)
      {
            if (ship && ship->id == findthis) {
                                      //DO STUFF HERE
            }
            ship++;
      }


Rather, I am wondering if this is the fastest possible solution for this, or if there is a better/quicker method for finding a value in a vector list.

Any suggestions/tips? Thanks!
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Question by:Valleriani
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4 Comments
 
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Accepted Solution

by:
evilrix earned 1000 total points
ID: 33581603
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Expert Comment

by:w00te
ID: 33581865
This is from memory, but it should work for a simple function.  It's an example of the kind of things evilrix is pointing out.
It uses a template function though, you just pass in the int value you want to search for as a template parameter and it looks for it.  You could adjust this to take in any value (including another Ship object), and you can perform whatever comparisons you want in the function.  Find if will always give the iterator to the first position the said value was found in.
Most of the code is just there to set up the vector, the find if function accomplishes pretty much everything :)
Enjoy,
(PS, I compiled it on linux but i had to type it on windows where there istn a compiler, so if theres a typo just try and fix it, the idea works ;) )
-w00te

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

struct Ship
{
	int id;
	Ship(int x) : id(x) {}
};

int main()
{
	template <int T>
	bool FindCondition(const Ship& ship) {return ship.id == T; }

	vector<Ship> sVec;
	sVec.push_back(Ship(12));
	sVec.push_back(Ship(15));

	vector<Ship>::iterator iter = find_if(sVec.begin(), sVec.end(), FindCondition<15>);

	if (iter != sVec.end())
		cout<<iter->id<<" was found." <<endl;
}

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LVL 12

Assisted Solution

by:w00te
w00te earned 1000 total points
ID: 33582179
Sorry about that, tried to compile now, i pushed the template in main accidentally hehe.  Here's the working code:
 template <int T>
 bool FindCondition(const Ship& ship) {return ship.id == T; }

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

struct Ship
{
	int id;
	Ship(int x) : id(x) {}
};

template <int T>
bool FindCondition(const Ship& ship) {return ship.id == T; }

int main()
{
	vector<Ship> sVec;
	sVec.push_back(Ship(12));
	sVec.push_back(Ship(15));

	vector<Ship>::iterator iter = find_if(sVec.begin(), sVec.end(), FindCondition<15>);

	if (iter != sVec.end())
		cout<<iter->id<<" was found." <<endl;
}

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LVL 7

Author Closing Comment

by:Valleriani
ID: 33584030
Thank you both!
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