Question

I wrote this code to find optimal warping path of Dynamic time warping , it returns a struct .

static inline double min (double x, double y,double z )
{
    if(x < y && x < z)
    return x;
    else if(y < x && y < z)
    return y;
    else if(z < x && z < y)
    return z;
}

struct backtracking_result
{
   vector<int> i;
   vector<int> j;
};

backtracking_result backtracking(vector<vector<double> > mGamma)
{
    backtracking_result br;
    cout<<"test"<<mGamma.size()<<endl;
    int i=mGamma.size();
    cout<<"test"<<endl;
    int j=mGamma.at(0).size();
    cout<<"test"<<endl;
    cout<<i<<j<<endl;   
    do
    {
         cout<<"test"<<endl;
         if(i == 1)
            j=j-1;
         else if(j == 1)
            i=i-1;
         else
         {
            if(mGamma[i-1][j] == min(mGamma[i-1][j],mGamma[i][j-1],mGamma[i-1][j-1]))
             i=i-1;
            else if(mGamma[i][j-1] == min(mGamma[i-1][j],mGamma[i][j-1],mGamma[i-1][j-1]))
                 j=j-1;
            else
            {
                i=i-1;
                j=j-1;
            }
         }
    br.i.push_back(i);
    br.j.push_back(j);
  }while((i > 1)&&(j > 1));

  return br;
}

When i call this function , get this error from gdb debugger

Program received signal SIGSEGV, Segmentation fault.
0x000000000041eafb in VectorDTW::backtracking(std::vector<std::vector<double, std::allocator<double> >, std::allocator<std::vector<double, std::allocator<double> > > >) ()

Where is the error ?

Stacktrace :

#0  0x000000000041eb17 in VectorDTW::backtracking(std::vector<std::vector<double, std::allocator<double> >, std::allocator<std::vector<double, std::allocator<double> > > >) ()
#1  0x0000000000420195 in VectorDTW::fastdynamic(std::vector<DollarRecognizer::Point2D, std::allocator<DollarRecognizer::Point2D> >&, std::vector<DollarRecognizer::Point2D, std::allocator<DollarRecognizer::Point2D> >&) ()
#2  0x000000000041d18b in main ()

The answers helped me figure out the problem , I added these two lines after calculating i & j at from the matrix ,

i=i-1;
j=j-1;

Now the program doesn’t try to access the last element . Thanks .
But i still cannot understand the reason for down-votes .

Answer 1

Since j is the size of the internal vector<double>, mGamma[i-1][j] does not exist. You have the typical off-by-one error.