CLS hitting on performance - c++

Question

This code prints the desired no(4 here) of perfect numbers. It's an old code of mine and checks up every number whether it's perfect or not. It runs normally and finishes in 6-ish seconds, but if I put a cls before checking every number it takes a big hit(about 100 secs). Please tell me the theory behind it(why multiple cls takes a hit on performance).
No normie answers like "Dude obviously it takes a hit, it's refreshing the screen every time."
PS- I know the better way is not to check perfect numbers but to generate them.(By Euclid-Euler theorem)
IDE- DevC++
#include <iostream>
#include <string>
using namespace std;
int choice, quan, i, a[50], number;
string schoice;
void startgame()
{
choice = 3;
quan = 4;
system("cls");
cout << "Hey guys, today we are gonna print different types of numbers\n";
cout << "\nFollowing are some of the special numbers found in the known observable little universe of ours:- \n";
cout << "\n1. Prime numbers\n2. Square numbers\n3. Perfect Numbers\n\n";
cout << "Which ones do you wanna see?\n";
//cin >> choice;
cout << "\nCool! How many of them do you wanna see?\n";
//cin >> quan;
}
void perfect()
{
if (choice == 3)
{
int j = 0, y = 0, f = 0, number = 2;
do
{
//This is the cls in question
//system("cls");
cout << "The number under inspection: " << number << "\n";
f = 0;
for (i = (number - 1); i >= 1; i--)
{
if (number % i == 0)
{
f = f + i;
}
}
if (f == number)
{
//cout<<number<<", ";
a[j] = number;
j = j + 1;
y = y + 1;
}
number++;
}
while (y < quan);
system("cls");
cout << "\nHere are your " << quan << " perfect numbers starting from 1:-\n";
j = 1;
for (i = 0; i < quan; i++)
{
cout << a[i];
if (j != quan)
{
cout << ", ";
}
j++;
}
}
}
int main()
{
do
{
startgame();
perfect();
cout << "\n\nReturn to Main menu?(Y/N)\n";
schoice = "N";
//cin >> schoice;
}
while (schoice == "Y");
return 0;
}

Answer 1

When you run system("cls"), you create a new process, and you do that every time you pick a new number for inspection.
Creating a process is an expensive operation, and more importantly you don't need it, if all you want is to update current number on screen.
It is sufficient to write
cout << "The number under inspection: " << number << "\r";
cout.flush();
The first line will output text and return cursor to the beginning of the same line.
The second line will make sure all the above does show up on the display.
Note: this is also slow operation, so you would probably want to rate limit it.
I suggest you output this for every 1 number in 1000 or so.

Answer 2

Windows CreateProcess() is quite expensive, much more expensive than Unix fork(). Moreover, system("cls") is an inefficient, non-portable hack. You can't use it in non-windows systems. You can use ncurses library that supports console manipulation.
For Windows, via Console API:
void clear() {
COORD topLeft = { 0, 0 };
HANDLE console = GetStdHandle(STD_OUTPUT_HANDLE);
CONSOLE_SCREEN_BUFFER_INFO screen;
DWORD written;
GetConsoleScreenBufferInfo(console, &screen);
FillConsoleOutputCharacterA(
console, ' ', screen.dwSize.X * screen.dwSize.Y, topLeft, &written
);
FillConsoleOutputAttribute(
console, FOREGROUND_GREEN | FOREGROUND_RED | FOREGROUND_BLUE,
screen.dwSize.X * screen.dwSize.Y, topLeft, &written
);
SetConsoleCursorPosition(console, topLeft);
}