如何在 C++ 中将 do while 循环转换为 while 循环？

Question

I have written a C++ program to let the users entering positive numbers using a do while loop. Notwithstanding, when I try to convert the do while loop into a while loop, the expected output is not the same as do while loop. The code is as below:

#include <iostream>
using namespace std;

int main()
{
    int n;

    do
    {
        cout << "Enter a non-negative integer: ";
        cin >> n;
        if (n < 0)
        {
            cout << "The integer you entered is negative. " << endl;
        }

    }
    while (n < 0);

    return 0;
}

The terminal requires the user to reenter the number until it is positive for the above code that I have written. However, I try to convert the do while loop to while loop as shown below, there is no output at all.

May I know which part I have written wrongly? Thank you.

#include <iostream>
using namespace std;

int main()
{
    int n;

    while (n < 0)
    {
        cout << "Enter a non-negative integer: ";
        cin >> n;
        if (n < 0){
            cout << "The integer you entered is negative. " << endl;
         }
    }

    return 0;
}

Answer 1

In the while version of your code, you do not know what is n a it is not initialized before you use it in the while (n<0) .

int main()
{
    int n;
    // HERE YOU DO NOT KNOW IF n is NEGATIVE OR POSITIVE, YOU EITHER NEED TO INITIALIZE IT OR
    // ENTER IT by cin >> n
    while (n < 0)
    {
        cout << "Enter a non-negative integer: ";
        cin >> n;
        if (n < 0){
            cout << "The integer you entered is negative. " << endl;
         }
    }

    return 0;
}

You then need to rearrange a little bit to get the same ouput as well as to use the first input of n which is out of the loop.

For instance, this will provide the same output:

#include <iostream>
using namespace std;

int main()
{
    int n=-1;
    cout << "Enter a non-negative integer: ";
    cin >> n;
    while (n < 0)
    {

        if (n < 0)
        {
            cout << "The integer you entered is negative. " << endl;
            cout << "Enter a non-negative integer: ";
            cin >> n;
        }
        else
        {
            // if positive you get out of the loop
            break;
        }
 
    }

    return 0;
}

Answer 2

You have to take the n before while(n<0) statement:

#include <iostream>
using namespace std;

int main()
{
    int n;
    cout << "Enter a non-negative integer: ";
    cin >> n;
    while (n < 0)
    {
        cout << "Enter a non-negative integer: ";
        cin >> n;
    if (n < 0)
            cout << "The integer you entered is "
                 << "negative. " << endl;
    }
    
    return 0;
}

Or you may initialize it with negative number:

#include <iostream>
using namespace std;

int main()
{
    int n=-1;
    
    while (n < 0)
    {
        cout << "Enter a non-negative integer: ";
        cin >> n;
    if (n < 0)
            cout << "The integer you entered is "
                 << "negative. " << endl;
    }
    
    return 0;
}