Vector of structure

[Vahagn8]

Suppose we have a structure:

struct Position {
	unsigned int i;
	unsigned int j; };

and we have a vector: vector<Position> cp;

The aim of the program is that the vector<Position> cp gets values, such as i and j which are some coordinates. Then as the iteration goes on i and j get new values. What we needed is that the values of i and j be deleted before the new vales come, so that vector<Position> cp becomes an empty vector.

The question is how to do this.

I tried : cp.clear();
and I tried : cp.erase(cp.begin(),cp.begin()+2);
but it did not work.

Thanks in advance.

[gunnerfunner]

http://coliru.stacked-crooked.com/a/907ad0a06833f94a

[Aaron Vienneau]

Well, cp.clear() would clear all of the Position objects out of the vector, not the position object's data members i and j.

If you're looking to simply clear the vector of it's position objects, cp.clear() will work fine, as shown in this sample program below:

#include <iostream>
#include <vector>

int main()
{
	std::vector<int> myVector { 1, 2, 3, 4, 5};

	std::cout << "Before Clear: " << std::endl;
	for(auto x : myVector)
		std::cout << x << ", ";

	myVector.clear();

	std::cout << "\nAfter Clear: " << std::endl;
	for(auto x : myVector)
		std::cout << x << ", ";
}

Before Clear: 
1, 2, 3, 4, 5, 
After Clear: 

If you're looking to clear out the data members of the position objects in the vector, that would be done slightly differently.

Erase should also work, but you should use cp.erase(std::begin(cp), std::end(cp));

If cp.clear() is not working, it's more likely that your program is repopulating the vector erroneously, or there is some other logic error. If you supply contextual code to your question, I may be able to help more.

[Aaron Vienneau]

If, on the other hand, you'd like to delete the data members out of the position objects themselves, you'd have to iterate over each position object and call a clear method on each one to clear out their 'i' and 'j' values.

#include <iostream>
#include <vector>

struct Position
{
	int i;
	int j;

	void clear()
	{
		i = 0;
		j = 0;
	}
};

std::ostream& operator<<(std::ostream& io, Position& p)
{
	io << "(" << p.i << ", " << p.j << ")";
        return io;
}

int main()
{
	std::vector<Position> myVector {
		Position{1, 2},
		Position{4, 6},
		Position{2, 9}
	};

	std::cout << "Before Clear: " << std::endl;
	for(auto x : myVector)
		std::cout << x << std::endl;

	for(auto& x : myVector)
		x.clear();

	std::cout << "After Clear: " << std::endl;
	for(auto x : myVector)
		std::cout << x << std::endl;
}

[Vahagn8]

Aaron Vienneau :
I want to delete the i and j values or as you mentioned the data members of the position objects in the vector. So, is there a way to do this? Otherwise, I have tried cp.clear(); and cp.erase(cp.begin(),cp.begin()+2); and that does not work as expected.

I also tried cp.erase(std::begin(cp), std::end(cp)); but it did not work.

gunnerfunner :
Thank you for the link you posted, however, cp.pop_back(); did not do what I want. It reacts as cp.clear(); .

[Aaron Vienneau]

Hi @Vahagn8,

Please see my second response above :)

[Vahagn8]

Hi Aaron Vienneau :
By the time I posted I did not have seen your previous post :)
Thank you, I will look and write, in my country it is already night, I have to leave, so I will probably answer back tomorrow, as soon as I will look through the code.

[Vahagn8]

Aaron Vienneau :
Thank you again for your help !
The method you provided works, so here is the sum up :

To delete the i and j values or the data members of the position objects in a vector consisting of e.g. structures we should iterate over each position object and call a clear method on each one. Below the key components of the code are presented :

struct Position
{
	unsigned int i;
	unsigned int j;

	void clear()
	{
		i = 0;
		j = 0;
	}
};

vector<Position> cp;

 for (auto& x : cp) {
	x.clear();
	} 

So, by doing this we delete the values of i and j by making them to be 0. However, this does not make vector<Position> cp; an empty vector.

On the other hand, if we want to delete the Position objects of the vector, we can simply use cp.clear(); . And this makes vector<Position> cp; an empty vector.

So, if we want to both delete the i and j values or the data members of the position objects in a vector and to transform the vector<Position> cp; into an empty vector we can use the two, one after another, which I did.

[Aaron Vienneau]

Hi Vahagn8,

What you've said sounds right, but you have a small error in your code segment.

struct Position
{
	unsigned int i;
	unsigned int j;

	void clear()
	{cp.clear(); // This is a scoping error. Position doesn't know about CP unless it's global.
		i = 0;
		j = 0;
	}
};

If you'd like to clear both the 'cp' vector and the 'i' and 'j' elemnents from the Positions, you'd do something like:

 for (auto& x : cp) { // This is saying, "For each Position in the vector 'cp'...
	x.clear(); // call 'clear()' on that position. Clear() as we've defined it, sets i and j to zero.
	} 

cp.clear() // Clears the vector of all of the position objects

I don't know if this will help, but think of 'std::vector<Position>cp' as a container of containers.

[Vahagn8]

I edited. I do not know how it (cp.clear(); ) got in to the struct Position . Actually I did not used it in my original code. It happens sometimes. I mean I have noticed that recently my computer reacts in a strange manner, it copies and pastes things I do not have any clue. I think the problem is connected with the touch pad.
That is a good comparison. I was thinking of vector<Position> cp; like some object integrated into some other. Thanks for the help.