copy vector content into an array exercise - arrays

Question

I am currently working through C++ Primer 5th edition. In chapter 3, Exercise 3.42, I am asked to "Write a program to copy a vector of int's into an array of int's."
the below code works. However I have a few questions.
1) on line 16,
int arr[5];
initializes an array with 5 elements. How can I alter the array so that it automatically gets/has the same size as the vector ivec?
2) Is there a simpler way to write the program with what the book has taught so far?
//Exercise 3.42
#include "stdafx.h"
#include <iostream>
#include <vector>
using std::vector;
using std::begin;
using std::endl;
using std::cout;
using std::end;
int main(){
vector<int> ivec = { 1, 2, 3, 4, 5 }; //initializes vector ivec.
int arr[5]; //initializes array "arr" with 5 null elements.
int i1 = 0; // initializes an int named i1.
for (vector<int>::iterator i2 = ivec.begin(); i2 != ivec.end(); ++i2){
arr[i1] = *i2; //assigned the elements in ivec into arr.
++i1; // iterates to the next element in arr.
}
for (int r1 : arr){ //range for to print out all elements in arr.
cout << r1 << " ";
}
cout << endl;
system("pause");
return 0;
}

Answer 1

1) You can't in portable C++ with an array : an array length is fixed at compile time. But you could with a vector
2) Certainly, assuming the target array is large enough, use std::copy :
std::copy(std::begin(ivec), std::end(ivec), arr);
Also remove all those using, they are nothing but noise. A little bit a cleaning gives :
#include <iostream>
#include <algorithm>
#include <vector>
int main(){
std::vector<int> ivec = { 1, 2, 3, 4, 5 };
int arr[5];
std::copy(std::begin(ivec), std::end(ivec), arr);
for (auto r1 : arr){
std::cout << r1 << ' ';
}
std::cout << std::endl;
}
You could even reuse std::copy to print the content of the vector :
int main(){
std::vector<int> ivec = { 1, 2, 3, 4, 5 }; //initializes vector ivec.
int arr[5]; //initializes array "arr" with 5 null elements.
std::copy(std::begin(ivec), std::end(ivec), arr);
std::copy(arr, arr + 5, std::ostream_iterator<int>(std::cout, " "));
}
Live demo
Note:
If you want to keep a hand written loop for the copy, a more canonical/c++11 way of doing it is :
auto i1 = std::begin(arr);
auto i2 = std::begin(ivec);
while ( i2 != std::end(ivec)){
*i1++ = *i2++;
}

Answer 2

1) If you need an array with a length that is not known at compile time, you can create the array dynamically on the heap (in real code you should usually rather use std::vector):
int* arr=new int[ivec.size()];
However, this has some other drawbacks. E.g. begin(arr)/end(arr) and thus range for loops don't work anymore and you'll have to manually delete the array at some point (I don't know whether you've already learned about RAII and smart pointers)
2) quantdev has already provided a very good answer for this part