为什么在尝试使 i = i*i 时出现错误“使用未初始化的 memory 'i'”和“使用未初始化的局部变量 'i'”

Question

I am new to C++ and was testing out while loops and the sheer speed of C++ and its toll on my CPU and I got the following errors:

Severity Code Description Project File Line Suppression State Warning C6001 Using uninitialized memory 'i'
Severity Code Description Project File Line Suppression State Error 4700 uninitialized local variable 'i' used

I have no idea how to read an error message and haven't even encountered initializing in C++ yet so I don't have any clue about what to do.

#include <iostream>
using namespace std;
    
int main() {
    long long i = 0;
    while (i < 10000000000000000) {
        long long i = i*i;
        cout << i ;
    }
    cout << i;
    return 0;
}

Answer 1

In the body of the while loop

while (i < 10000000000000000) {
    long long i = i*i;
    cout << i ;
}

you declared the variable i that is not initialized and has an indeterminate value and you are trying to use this indeterminate value to initialize the variable itself.

That is in this declaration

    long long i = i*i;

in the initializer there is used the same declared variable i that hides the declaration of the variable with the same name that appears before the loop

Substitute the declaration for the statement

   i = i*i;

But initially you should set the variable i to some value unequal to 0 for example to 10.

long long i = 10;

Otherwise the result of the expression i * i always will be equal to 0.

Something like

#include <iostream>
using namespace std;

int main() {
    long long i = 10;
    while (i < 10'000'000'000'000'000) {
        i = i*i;
        cout << i ;
    }
    cout << i;
    return 0;
}

Though you should be caution selecting the initial value of the variable i because an overflow can occur in the expression i * i and you can get an infinite loop.

Answer 2

long long i = i * i; shadows the long long i = 0; declared outside the while loop. It's a totally separate variable and you're attempting to initialize it to its own value squared. If you want to use the variable i you declared outside the loop, drop the redeclaration:

#include <iostream>
using namespace std;

int main() {
    long long i = 0;
    while (i < 10000000000000000) {
        i = i*i;
        cout << i ;
    }
    cout << i;
    return 0;
}

Note that this will be an endless loop that just prints 0 over and over again.