Re: help with vector<vector<double>>

On Aug 10, 2:47 pm, "Bo Persson" <b...@gmb.dk> wrote:


So I was curious to see which is faster -- pushing back onto a vector
or using direct index access of the elements. I declare a
std::vector<int> v, setting the number of elements to vSize. I set
the values using a for-loop. Then, I declare two
std::vector<vector<int> > objects, m1 & m2. The first of these I
initialize with mSize elements, and the second I use the
std::vector::reserve method to claim mSize space. The using two for-
loops I populate m1 & m2 using direct index accessing of the elements
to set them equal to v, and then I use push_back to fill m2. I time
these two for-loops as well as the whole function. What I find
(perhaps not suprisingly) is that I fill m1 much, much faster, i.e.
push_back() with reserve() are SLOW. Anyway, here's the code.
Nothing to special.

#include <vector>
#include <iostream>
#include <iomanip>
#include <time.h>

using namespace std;

int main(){
    time_t t0_0,t1_0,t2_0;
    time_t t0_f,t1_f,t2_f;

         t0_0 = time(NULL);
    int vSize = 10000000;
    int mSize = 10;

         vector<int> v(vSize);

         for (int i=0;i<vSize;i++){v.at(i) = i;}

    vector<vector<int> > m1(mSize);
    vector<vector<int> > m2;
    m2.reserve(mSize);
    t1_0 = time (NULL);
    for (int j=0;j<mSize;j++){m1.at(j) = v;}

    t1_f = time(NULL);
    t2_0 = time(NULL);

    for (int i=0; i<mSize;i++){ m2.push_back(v);}

    t2_f = time(NULL);
    t0_f = time(NULL);
    cout << "testTime0 = " << t0_f-t0_0 << endl;
    cout << "testTime1 = " << t1_f-t1_0 << endl;
    cout << "testTime2 = " << t2_f-t2_0 << endl;

    return 0;
}