Trouble while writing an resiseable vector with std::vector - c++

Question

i'm a Brazilian beginner in c++ coding (apologize my poor knowledge in both things). I'm trying to write an .txt output file containing the positions of pixels i click with the mouse. I'm making use of opencv library, so thats a functional part of the code:
#include "opencv2/highgui/highgui.hpp"
#include <iostream>
#include <vector>
#include <fstream>
using namespace std;
using namespace cv;
//declaration of vector and counter
int i = 1;
std::vector<int>vet_x(i);
std::vector<int>vet_y(i);
//the callback function
void CallBackFunc(int event, int x, int y, int flags, void* userdata)
{
if (event == EVENT_LBUTTONDOWN)
{
vet_x.resize(i);
vet_y.resize(i);
vet_x[i] = x;
vet_y[i] = y;
i++;
cout << "Left button of the mouse is clicked - position (" << x << ", " << y << ")" << endl;
}
}
int main(int argc, char** argv)
{
Mat img = imread("lena.jpg");
//Create a window
namedWindow("Mouse Track Test", 1);
//set the callback function for mouse event
setMouseCallback("Mouse Track Test", CallBackFunc, NULL);
//show the image
imshow("Mouse Track Test", img);
// Wait until user press some key
waitKey(0);
//the writing begins after the press of the key
ofstream myfile;
myfile.open("points.txt");
for (int j = 1; j <= vet_x.size(); j++)
{
cout << vet_x[j] << "," << vet_y[j] << endl;
myfile << vet_x[j] << "," << vet_y[j] << endl;
}
myfile.close();
return 0;
}
The problem is: the file write only the last clicked position!
But if i turn the "vet_x.reserve(1024);" line, it works well, but only for the y coordinates...
So, what is my mistake?

Answer 1

C++ array indexing is 0-based. Thus when you resize a vector v to size 1, and assign to v[1], you're assigning to a non-existing item. This is Undefined Behavior.
To catch this kind of out-of-bounds indexing you can use the at method, which guarantees an exception. I.e., writing v.at(i) instead of v[i].
However, you should simply use the push_back member function to add items to a vector. I.e., v.push_back( x ), where x is the value that you want to add. It can also be a good idea to use a single vector of 2D points, instead of one vector for x and one vector for y.

Answer 2

vet_x.resize(i);
vet_y.resize(i);
vet_x[i]=x;
vet_y[i]=y;
You are assigning elements out of range, which is an undefined behavior. After resize(i) the last valid index is i-1. std::vector operator [] never insert elements in the container.
Rather just do
vet_x.push_back(x);
vet_y.push_back(y);

Answer 3

Your method of adding variables into vectors is wrong. I suggest this:
struct Point
{
int x, y;
Point(int sx, int sy)
:x(sx),y(sy)
{
}
};
std::vector<Point> clickedPositions;
//the callback function
void CallBackFunc(int event, int x, int y, int flags, void* userdata)
{
if ( event == EVENT_LBUTTONDOWN )
{
clickedPositions.push_back(Point(x,y));
}
}
and while writing it into the file:
for(int j=0; j<clickedPositions.size(); j++)
{
myfile << clickedPositions[j].x < <","<< clickedPositions[j].y <<endl;
}