Cannot exit While loop - c++

Question

I want to exit While loop, when variable "duration" reaches 5 sec.
Variable "duration" always =0
#include <iostream>
#include <Windows.h>
#include <chrono>
using namespace std;
DOUBLE duration;
int main()
{
using clock = std::chrono::system_clock;
using sec = std::chrono::duration <double>;
const auto before = clock::now();
while (duration < 5.0)
{
const auto after = clock::now();
const sec duration = after - before;
std::cout << "It took " << duration.count() << "s" << std::endl;
}
std::cout << "After While Loop "; //Never reaches this line!!!!!
return 0;
}
Actual output:.
.
it took 9.50618s
it tool 9.50642s
.
I expected while loop to exit at 5.0 or higher.
display variable always shows 0.

Answer 1

You are using two separate variables named duration.
using sec = std::chrono::duration <double>; // number 1
const auto before = clock::now();
while (duration < 5.0) // checking number 1, which is not changed during looping
{
const auto after = clock::now();
const sec duration = after - before; // number 2, created afresh in each iteration
std::cout << "It took " << duration.count() << "s" << std::endl;
}
So, what you are checking in the loop condition is not what gets changed in the loop body.

Answer 2

You have two variables called duration:
DOUBLE duration;
and in while loop:
const sec duration = after - before;
During evaluation of while statement you are checking the first one that is never set or changed.
Inside while loop body you're creating a new variable that shadows global variable. See https://en.wikipedia.org/wiki/Variable_shadowing
Insead you shouldn't declare a new variable but use one declared before entering to the while loop.

Answer 3

There are two issues here:
Declaring a local variable with the same name that of the global variable makes them two separate entities, which may get quiet confusing. This phenomenon is called as Variable shadowing.
You are comparing the duration variable which is actually an object of class sec and not an integer or float. So, for comparing the value in duration just do duration.count() to make it comparable to integer or float values.
Here, in your code, the global duration variable is never changed because you are declaring a new variable inside the while loop. The duration variable that is been checked in the while condition is the global duration variable but you expect it to be the duration variable inside the loop.
There are two possible solutions:
Either make the two duration variables the same.
Or break the while(1) loop when the duration variable hold value >= 5,
Solution 1 code looks like this:
#include <iostream>
#include <Windows.h>
#include <chrono>
using namespace std;
int main()
{
using clock = std::chrono::system_clock;
using sec = std::chrono::duration <double>;
sec duration;
const auto before = clock::now();
while (duration.count() < 5.0)
{
const auto after = clock::now();
duration = after - before;
std::cout << "It took " << duration.count() << "s" << std::endl;
}
std::cout << "After While Loop "; //Never reaches this line!!!!!
return 0;
}
Solution 2 code looks like this:
#include <iostream>
#include <Windows.h>
#include <chrono>
using namespace std;
int main()
{
using clock = std::chrono::system_clock;
using sec = std::chrono::duration <double>;
const auto before = clock::now();
while (1) // Soln 1: while(1)
{
const auto after = clock::now();
const sec duration = after - before;
std::cout << "It took " << duration.count() << "s" << std::endl;
if(duration.count() >= 5)
break;
}
std::cout << "After While Loop "; //Never reaches this line!!!!!
return 0;
}