Trying to use qsort with vector

Question

I'm trying to learn c++ and was trying using sort and qsort. sort() works just fine but qsort doesn't, I don't know why, so can you help me please this is the code I was trying to compile

#include<iostream>
#include<vector>
#include<cstdlib>
#include<ctime>
#include<algorithm>


using namespace std;

int compvar(const void *one, const void *two)
{
    int a = *((int*)one);
    int b = *((int*)two);
    if (a<b)
       return -1;
    if (a == b)
       return 0;
    return 1;   

}

void bvect(vector<int> &vec, int num)
{
     srand(time(NULL));
     for(int i=0; i<num; ++i)
             vec.push_back(rand()%1000 + 1);
}

void showvec(vector<int> vec)
{
     for (int i=0; i<vec.size(); ++i)
         cout<<vec[i]<<endl;
}


int main()
{
    vector<int>numbers;
    bvect(numbers, 1000);
    showvec(numbers);
    qsort(numbers.begin(), numbers.size(), sizeof(int), compvar);
    showvec(numbers);

    return 0;
}

Dioecious

Allow me to be the first to advise that you "Just say NO!". Using qsort on a vector is just plain nuts. – Ani

Why do you even want to use qsort?! – Ricker

You seem to be making the assumption that your implementation uses raw pointers as vector iterators. Does it? Regardless, your code should not assume it does. Use &numbers[0] instead of numbers.begin(). – Gubernatorial

if you are learning C++, forget about qsort and other C-functions – Echoechoic

use comparator as here cplusplus.com/reference/clibrary/cstdlib/qsort and also you should call srand only once so it would be better if you call it in main function – Echoechoic

int a = *((int*)one); the outermost parentheses aren't needed. – Electuary

@Jupiter - the comparator looks just fine. What do you think should be different? – Electuary

@Pete Becker "if" statement looks like overhead) – Echoechoic

@Jupiter - how would you change it? I don't see much extra there. – Electuary

@PeteBecker: The final if (a == b) return 0; return 1; could be replaced by return (a > b); but that is minor compared to the horror of not using std::sort in the first place. – Koodoo

@Koodoo - the optimizer will make good sense of all of this. – Electuary

Answer 1

First of all, DON'T.

If you just want to muck about, you can replace iterators with actual pointers:

qsort(&numbers[0], numbers.size(), sizeof(int), compvar);

Apart from not doing all the work std::sort does, there is one unexpected thing about qsort. It is slower.

1. sort (myvector1.begin(), myvector1.end());

2. sort (myvector2.begin(), myvector2.end(), myfunction);

3. sort (myvector3.begin(), myvector3.end(), myobject);

4. qsort(&myvector4[0], myvector4.size(), sizeof(int), cmyfunction);

4 is the slowest, followed by 2 (function pointer passed to std::sort). 1 and 3 (default and functor) are the fastest (compiled with gnu's g++ with -O3 flag).

Locarno

Thanks for the answers.It worked!. I know qsort() is slower than sort() and that's what I was trying to test, I'm taking a programming course and the professor told us to test them both.again thanks – Dioecious