how to change length of array in C++ code example

Example 1: length of array c++

// array::size
#include <iostream>
#include <array>

int main ()
{
  std::array<int,5> myints;
  std::cout << "size of myints: " << myints.size() << std::endl;
  std::cout << "sizeof(myints): " << sizeof(myints) << std::endl;

  return 0;
}

Example 2: length of array c++

#include <iostream>
using namespace std;
int main() {
   int arr[5] = {4, 1, 8, 2, 9};
   int len = sizeof(arr);
   cout << "The length of the array is: " << len;
   return 0;
}