Advice on Sentinel loops C++ - c++

Question

so i get how sentinel loops work, they use the expression at the beginning of the the loop declaration to determine where the loop stops. However i don't understand how to connect variables in the scope of the loop and outside the scope. For example
int i;
int sum = 0;
cout << "Enter a number" << endl;
cin>> i;
while (i > 0)
{
sum = sum + i;
cout << "Enter a number" << endl;
cin>> i;
}
cout << "The sum of the numbers entered is" << endl;
cout << sum;
return 0;
So the sum in the loop is correct since its within scope but the i or number entered to define i out of the scope in the initial i . How do you go about this, can you connect the variables inside and outside the loop? Or can you use uninitialized memory to make a sentinel loop? Do you have to add a separate variable outside the loop and connect the sum of i outside and inside the loop? So just trying to understand how you'd connect the variables inside and outside the scope. If sentinels need to have the variable defined prior to the loop.
Thank you for any light you can shed on this.

Answer 1

Variables are defined in a particular scope and become visible to all scopes within the outer scope unless overwritten (which most programmers tell you is a very, very bad idea). For instance:
#include
using std::cout;
using std::endl;
int main(int, char **) {
int i = 5;
int k = 10;
if (true) {
int j = 15;
int i = 20;
cout << "i inside if: " << i << ". J: " << j << endl;
}
cout << "i outside if: " << i << ". K: " << k << endl;
}
Running it produces:
i inside if: 20. J: 15
i outside if: 5. K: 10
In this case, the scope of method main() contains two variables, i and k. Those variables are also available inside the if-statement. The if also defines two variables, j and i. They are only available inside the if-statement.
The i inside the if-statement hides the larger i (which is dangerous / confusing), but I wanted to demonstrate it. As you can see, i outside the if retains it's original value -- it wasn't modified when we declared the inner copy.
In your code, i and sum are defined outside the if-statement, so they're available inside it, and changes you make to them persist. That is, because you haven't defined new variables, everything just works.