Small experiments

vec_init_vs_push_back.cpp

#include <ctime>
#include <iostream>
#include <vector>

using namespace std;

// Assumption: Initializing a vector with size before assignments runs faster
// then filling it by "push_back"s.

int main() {
  const int num_int = 100000000;

  auto previous = clock();

  vector<int> v1;
  for (int i = 0; i < num_int; ++i) {
    v1.push_back(i);
  }

  cout << "Without initialization: "
    << (clock() - previous) / (double)CLOCKS_PER_SEC << "s" << endl;
  previous = clock();

  vector<int> v2(num_int);
  for (int i = 0; i < num_int; ++i) {
    v2[i] = i;
  }

  cout << "With initialization: "
    << (clock() - previous) / (double)CLOCKS_PER_SEC << "s" << endl;

  return 0;
}

// Result: It shows that the larger the vector is, the bigger the speed
// difference is. When "num_int" equals to 100 millions, it achieves 2x speedup
// on my computer.

// Conclusion: Initialize a vector using "Vector(size_t n)" when you know the
// size of the vector beforehand and the size is very large.