double recursion segmentation fault c++ - c++

Question

I've recently written this quicksort sorting algorithm. After compiling I get a "segmentation fault (core dumped)". I debugged it and it turned out that line 25 causes the problem:
v2 = quicksort(v2);
However, I do not know why I get a the "core dumped", as I have no idea what's the problem with this line.
Here's my code:
#include <iostream>
#include <vector>
#include <random>
using namespace std;
vector <float> quicksort(vector <float> Vec)
{
if(Vec.size() > 1)
{
float pivot = Vec[(Vec.size())/2-1];
vector <float> v1, v2;
vector <float> V;
for(unsigned int i = 0; i < Vec.size(); i++)
{
if(Vec[i] >= pivot)
v2.push_back(Vec[i]);
else
v1.push_back(Vec[i]);
}
v1 = quicksort(v1);
v2 = quicksort(v2);
//after debuggung, I found out that the line above causes the "segmentation fault (core dumped)" (line 25)
for(unsigned int i = 0; i < v1.size(); i++)
V.push_back(v1[i]);
for(unsigned int i = 0; i < v2.size(); i++)
V.push_back(v2[i]);
return V;
}
else
{
return Vec;
}
}
int main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
vector <float> v;
for(int i = 0; i < 100; i++)
{
v.push_back(rand() % 100);
cout << v[i] << " ";
}
v = quicksort(v);
for(int i = 0; i < 100; i++)
{
cout << v[i] << " ";
}
return 0;
}
Thanks for the help.

Answer 1

First of all, to get a totally random number using rand() you need to seed the number generator. To do this you include the "time.h" library and then write: srand (time(NULL));
Secondly, your quicksort takes two parameters, a vector named Vec and an int f, that isn't used for anything. Take int f of the function's parameters.
Thirdly, the problem is that an infinite loop is happening in this part of the code (lines 17 to 23):
for(unsigned int i = 0; i < Vec.size(); i++){
if(Vec[i] >= pivot)
v2.push_back(Vec[i]);
else
v1.push_back(Vec[i]);
}
Imagine that our Vec vector is {2, 3} (this were the actual values, because we didn't seed the random number generation).
What's happening is that we have our pivot = 2, and then we are saying that if Vec[0], which is 2, is bigger or equal than the pivot, we add Vec[0] to v2, and then the same for Vec[1], which is 3.
Then this loops infinitely, because then you say v2 = quicksort(v2);. That will make Vec = v2. And that means that it will never get smaller, because, again, Vec is {2, 3}, and therefore our pivot = 2.

Answer 2

It goes in stack overflow.
If the pivot is the maximum number of the vector Vec it has to be dragged from Vinf into Vsup.
#include <iostream>
#include <vector>
#include <random>
using namespace std;
vector <float> quicksort(vector <float> Vec) { //the int f was never used un the function
if (Vec.size() > 1){
vector <float> Vinf, Vsup, Vtmp;
Vinf.push_back(Vec[(Vec.size()) / 2 - 1]);
float pivot = Vinf[0];
Vec.erase(Vec.begin() + Vec.size() / 2 - 1);
for (unsigned int i = 0; i < Vec.size(); i++)
Vec[i] > pivot ? Vsup.push_back(Vec[i]) : Vinf.push_back(Vec[i]);
if (Vinf.size() == Vec.size() + 1) {
Vsup.push_back(pivot);
Vinf.erase(Vinf.begin());
}
Vinf = quicksort(Vinf);
Vsup = quicksort(Vsup);
for (unsigned int i = 0; i < Vinf.size(); Vtmp.push_back(Vinf[i++]));
for (unsigned int i = 0; i < Vsup.size(); Vtmp.push_back(Vsup[i++]));
return Vtmp;
}
else
return Vec;
}
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
vector <float> v;
for (int i = 0; i < 100; cout << v[i++] << " ")
v.push_back(rand() % 100);
cout << endl;
v = quicksort(v);
for (int i = 0; i < v.size(); cout << v[i++] << " ");
return 0;
}