2

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;
}
Pat. ANDRIA
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Bryson Fernades
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    Just assign any negative value to n before the loop – Martheen Feb 19 '21 at 06:43
  • Does this answer your question? [Equivalent using for-loop instead do-while-loop](https://stackoverflow.com/questions/20735358/equivalent-using-for-loop-instead-do-while-loop) – Ch3steR Feb 19 '21 at 06:44
  • @Ch3steR the problem is the output is not the same since I followed the tutorial from google and youtube. – Bryson Fernades Feb 19 '21 at 06:48
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    In the second code example, since `n` is uninitialized, it may coincidentally contain a negative number, or it may not. The behavior, in particular whether the `while` loop condition will be `true` when first executed, is undefined. – Bungo Feb 19 '21 at 06:56

2 Answers2

1

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

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;
}
Pat. ANDRIA
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  • Welcome. I added an attempt to make the same loop with a `while`. Of course, it is not the only solution but it is easy to understand – Pat. ANDRIA Feb 19 '21 at 07:13
0

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;
}
artunc
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