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darenking

SOLVED, C++ vector of vectors hell

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I've done something quite clever with vectors (created a vector of vectors) but I'm a bit out of my depth, and don't know how to pass one of the inner vectors to another object. In fact, I'm not 100% sure I've created my vectors correctly. I have a sequence of values called a Cycle, which is used for animation. These values are read in from a text file. The text file currently contains this: 2 3 4 5 4 3 2 6 7 8 7 6 -1 2 11 12 13 12 11 -1 5 6 5 6 -1 This is a list of three cycles, terminated with -1. They are read in and stored in the World object like this, in a loop which knows that there are three cycles:
//in World.h
	std::vector<int> m_Values;
	std::vector< std::vector<int> > m_Cycles;

//in World.cpp
	int data1;
	file >> data1;
	while( data1 != -1 )
	{
		m_Values.push_back(data1);
		file >> data1;
	}
	m_Cycles.push_back(m_Values);
	m_Values.clear();



Before I ask for help with passing one of the inner vectors, perhaps I should check I've got things right so far. Do I actually have three inner vectors (m_Values vectors) contained in one outer vector called m_Cycles? Or perhaps I have just deleted the data in the inner vector each time, and in fact have just one inner vector? Maybe I have to declare more than one m_Values vector in .h? Or maybe just declare the m_Cycles vector in .h and declare m_Values in the loop in .cpp? [Edited by - darenking on August 2, 2005 4:40:33 AM]

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Unless you loop the code you show from World.cpp three times, m_Cycles will only have one vector. Edit: Otherwise I think it works as you want it to.

Edit: Edit: When pushing m_Values into m_Cycles, what is added to m_Cycles is a copy of m_Values, so you won't need more m_Values vectors.

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Quote:
Original post by darenking
Do I actually have three inner vectors (m_Values vectors) contained in one outer vector called m_Cycles? Or perhaps I have just deleted the data in the inner vector each time, and in fact have just one inner vector?

When you push an object into a vector its value is copied, which means the answer to your first question is that every time you push m_Values into m_Cycles m_Cycles stores a copy of the m_Values you passed. So you aren't removing any data from inside m_Cycles when you change the values in m_Values after its been copyed to m_Cycles.
Quote:
Original post by darenking
Maybe I have to declare more than one m_Values vector in .h? Or maybe just declare the m_Cycles vector in .h and declare m_Values in the loop in .cpp?

declaring m_Cycles in the header file is enough. Then to access one of the pushed vectors you either use m_Cycles[1 (or whatever)] or iterate through them. If you want to get on of the values you can just do m_Cycles[1][1]. Essentially you've made a 2d array out of std::vectors.

Hope that helps.

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OK, I've got that working. Thanks for that.

Now my next problem is that I need to be able to pass a pointer or reference to one of these three inner vectors to another object, my Character object (sprites, essentially). The reason I want to pass by pointer or reference rather than by copy is that I may have hundreds of Characters and don't want to waste lots of memory.

How do I access one of the inner vectors so that I am able to pass it?

And how do I store it the other end?

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To get a pointer to one of the inner vectors you can just do the following:
std::vector<int>* MyPointer = &m_Cycles[x];

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OK, how do I pass this pointer to another object?

Can I just pass it as basically a memory location, as though it were a pointer to an int rather than a pointer to a vector of ints? That way, I would surely be able to access the integers with squared brackets [] as when you access an array.

If so, what's the syntax? That's the really tricky bit for me, even if I get the concept I can never do the syntax.

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Quote:
Original post by darenking
OK, how do I pass this pointer to another object?

Can I just pass it as basically a memory location, as though it were a pointer to an int rather than a pointer to a vector of ints? That way, I would surely be able to access the integers with squared brackets [] as when you access an array.


You can pass it as a vector (as it is) and still be able to access it with square brackets. Just define your function or method to take a vector of that type (the one you want to pass).

If it's the inner one, then you can just index the outer one when you're passing it.

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Note the return type of the subscript operator on the first dimension is of type (const) std::vector<int>&.

i haven't read the entire thread but use a reference or constant reference instead otherwise you'll get confused on the syntax with pointers i have seen it happen many times [grin], just remember with either pointer/reference it has the same issues as pointer/reference to any other type.

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Quote:
Original post by Lotus
Quote:
Original post by darenking
OK, how do I pass this pointer to another object?


You can pass it as a vector (as it is) and still be able to access it with square brackets.


See what i mean [grin], if you have pointer to std::vector to use its subscript operator you first need to dereference the pointer i.e. (*vec_ptr)[index] not vec_ptr[index] which is also compilable but wrong logic/behaviour etc.

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example -



#include <iostream>
#include <vector>
#include <iterator>
#include <algorithm>
using namespace std;


void printVector(vector<int>&rvec){
copy(rvec.begin(),rvec.end(),ostream_iterator<int>(cout,"\n"));
}

int main(){

vector< vector<int> > vec;
vector<int>veci;

for(int x =0; x < 10; x++){
veci.push_back(x);
}
vec.push_back(veci);
veci.clear();

for(int x =10; x < 20; x++){
veci.push_back(x);
}

vec.push_back(veci);
veci.clear();

for(int x =20; x < 30; x++){
veci.push_back(x);
}

vec.push_back(veci);
veci.clear();

for(int x = 30; x < 40; x++){
veci.push_back(x);
}

vec.push_back(veci);
veci.clear();


vector<int> *pVec = &vec[1];
printVector(*pVec);

pVec = &vec[2];
printVector(*pVec);

}

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