Re: STL container question

From:
Ioannis Vranos <ivranos@no.spam.nospamfreemail.gr>
Newsgroups:
comp.lang.c++
Date:
Thu, 02 Oct 2008 18:54:44 +0300
Message-ID:
<gc2qs4$26om$2@ulysses.noc.ntua.gr>
Lars Tetzlaff wrote:

Ioannis Vranos schrieb:

The program had a serious bug.

corrected:

Ioannis Vranos wrote:

Hendrik Schober wrote:

Ioannis Vranos wrote:

[...]
We must think generally. In general, sorting a list is faster than
sorting a vector, because the list sorting does not involve the
construction or destruction of any object.

Regarding ints, I think sorting a vector of ints and as list of
ints, both have about the same efficiency.

 Why don't you just measure before you doubt the statements
 of those who already went and did this?

 On my platform, this


[ Non-portable code...]

 and thus again disagrees with you.

 Eagerly awaiting your counter example,


#include <iostream>
#include <ctime>
#include <vector>
#include <list>
#include <cstddef>
#include <algorithm>

class SomeClass
{
    typedef std::vector<int> TypeVector;

    TypeVector vec;

    enum { VectorSize= 1000 };

    public:

    SomeClass();

    bool operator<(const SomeClass &argSomeClass) const
    {
        return vec[0]< argSomeClass.vec[0];
    }
};

int main()
{
    using namespace std;

    srand(time(0));

    const size_t SIZE=10000;

    typedef vector<SomeClass> Vector;
    typedef list<SomeClass> List;

    cout<< "\nCreating vector with "<< SIZE<< " elements..."<< flush;
    Vector vec(SIZE);

    cout<<" Done!\n\n"<< flush;

===> List lis;

    cout<< "Filling list with vector elements..."<< flush;

    for(Vector::size_type i= 0; i< vec.size(); ++i)
        lis.push_back(vec[i]);

    cout<< " Done!\n\n"<< flush;

    clock_t timeBeginVector, timeEndVector, timeBeginList, timeEndList;

    cout<< "Timing the sorting of the vector..."<< flush;

    timeBeginVector= clock();

    sort(vec.begin(), vec.end());

    timeEndVector= clock();

    cout<< " Done!\n\n"<< flush;

    cout<< "Timing the sorting of the list..."<< flush;

    timeBeginList= clock();

    lis.sort();

    timeEndList= clock();

    cout<< " Done!\n\n"<< flush;

    cout<< "The sorting of the vector took "
        << static_cast<double>((timeEndVector- timeBeginVector))/
CLOCKS_PER_SEC
        << " seconds\n\n";

    cout<< "The sorting of the list took "
        << static_cast<double>((timeEndList- timeBeginList))/
CLOCKS_PER_SEC
        << " seconds\n\n";
}

SomeClass::SomeClass():vec(VectorSize)
{
    using namespace std;

    for(TypeVector::size_type i= 0; i< vec.size(); ++i)
        vec[i]= rand();

    sort(vec.begin(), vec.end());
}


This program doesn't sort at all, because the vec is initialized with
one constant!


? Yes it is initalised with its number of elements, which are SomeClass
objects created with their default constructor.

Change the list filling code to

    for(Vector::size_type i= 0; i< vec.size(); ++i) {
    vec[i]= SomeClass();
        lis.push_back(vec[i]);
    }


Why?

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