C++ iterator in loop - c++

Question

+++ See update below +++
This is a code for reverse printing the content of an array. I used 3 slightly different methods for doing it: directly putting the dimension of the array in the for loop, using iterator and using reverse_iterator and measured the execution time of printing the for loop.
#include <iostream>
#include <vector>
#include <chrono>
using get_time = std::chrono::high_resolution_clock;
int main() {
std::cout << "Enter the array dimension:";
int N;
std::cin >> N;
//Read the array elements
std::cout << "Enter the array elements:" <<'\n';
std::vector <int> v;
int input;
for(size_t i=0; i<N; i++){
std::cin >> input;
v.push_back(input);
}
auto start = get_time::now();
for(int i=N-1; i>=0; i--){
std::cout << v[i] <<" ";
}
auto finish = get_time::now();
auto time_diff=finish-start;
std::cout << "Elapsed time,non-iterator= " << std::chrono::duration<double>
(time_diff).count() << " Seconds" << '\n';
auto start2 = get_time::now();
std::vector <int>::reverse_iterator ri;
for(ri=v.rbegin(); ri!=v.rend(); ri++){
std::cout << *ri <<" ";
}
auto finish2 = get_time::now();
auto time_diff2=finish2-start2;
std::cout << "Elapsed time, reverse iterator= " << std::chrono::duration<double>
(time_diff2).count() << " Seconds" << '\n';
auto start3 = get_time::now();
std::vector <int>::iterator i;
for(i=v.end()-1; i>=v.begin(); i--){
std::cout << *i <<" ";
}
auto finish3 = get_time::now();
auto time_diff3=finish3-start3;
std::cout << "Elapsed time, iterator= " << std::chrono::duration<double>
(time_diff3).count() << " Seconds" << '\n';
return 0;
}
The output is as follows:
Output:
5 4 3 2 1 Elapsed time,non-iterator= 2.7913e-05 Seconds
5 4 3 2 1 Elapsed time, reverse iterator= 5.57e-06 Seconds
5 4 3 2 1 Elapsed time, iterator= 4.56e-06 Seconds
My question is:
Why the direct method is almost 5 times slower than both iterator and reverse_iterator methods? Also, is this faster execution of iterator machine dependent?
This is a prototype, but I will need to deal with much bigger matrices; that is why I am asking this question. Thank you.
+++ Update +++
I am posting the updated results after incorporating the comments. It was too big for a comment.
I changed the for loop to evaluate the sum of an array with 100000 elements. I evaluated the same sum using the above mentioned methods (compiled with -O3 in clang++) and I have averaged the execution time for 3 methods over 10000 runs. Here are the results:
Average (10000 runs) elapsed time, non-iterator= 2.50183e-05
Average (10000 runs) elapsed time, reverse-iterator= 3.48299e-05
Average (10000 runs) elapsed time, iterator= 7.35307e-05
The results are much more uniform now, and now the non-iterator method is the fastest! Any insights? Or even this result is meaningless and I should do some more test?
the updated code:
#include <iostream>
#include <vector>
#include <chrono>
using get_time = std::chrono::high_resolution_clock;
int main() {
double time1,time2,time3;
int run=10000;
for(int k=0; k<run; k++){
//Read the array elements
std::vector <int> v;
int input,N=100000;
for(size_t i=0; i<N; i++){
v.push_back(i);
}
int sum1{0},sum2{0},sum3{0};
auto start = get_time::now();
for(int i=N-1; i>=0; i--){
sum1+=v[i];
}
auto finish = get_time::now();
auto time_diff=finish-start;
std::cout << "Sum= " << sum1 << " " << "Elapsed time,non-iterator= " << std::chrono::duration<double>
(time_diff).count() << " Seconds" << '\n';
auto start2 = get_time::now();
std::vector <int>::reverse_iterator ri;
for(ri=v.rbegin(); ri!=v.rend(); ri++){
sum2+=*ri;
}
auto finish2 = get_time::now();
auto time_diff2=finish2-start2;
std::cout << "Sum= " << sum2 <<" Elapsed time, reverse iterator= " << std::chrono::duration<double>
(time_diff2).count() << " Seconds" << '\n';
auto start3 = get_time::now();
std::vector <int>::iterator i;
for(i=v.end()-1; i>=v.begin(); i--){
sum3+=*i;
}
auto finish3 = get_time::now();
auto time_diff3=finish3-start3;
std::cout << "Sum= " <<sum3 << " Elapsed time, iterator= " << std::chrono::duration<double>
(time_diff3).count() << " Seconds" << '\n';
time1+=std::chrono::duration<double>(time_diff).count();
time2+=std::chrono::duration<double>(time_diff2).count();
time3+=std::chrono::duration<double>(time_diff3).count();
}
std::cout << "Average (" << run << " runs)" << " elapsed time, non-iterator= " << time1/double(run) <<'\n';
std::cout << "Average (" << run << " runs)" << " elapsed time, reverse-iterator= " << time2/double(run) <<'\n';
std::cout << "Average (" << run << " runs)" << " elapsed time, iterator= " << time3/double(run) <<'\n';
return 0;
}